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Colour vision and seeing colours
Davies, Will

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10.1093/bjps/axw026

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Davies, W 2018, 'Colour vision and seeing colours', The British Journal for the Philosophy of Science, vol. 69,
no. 3, pp. 657–690. https://doi.org/10.1093/bjps/axw026

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C OLOUR VISION AND S E EING COLOU RS  

WILL DAVIES 

Abstract 

Colour vision plays a foundational explanatory role in the philosophy of colour, and serves as 

perennial quarry in the wider philosophy of perception. I present two contributions to our 

understanding of this notion. The first is to develop a constitutive approach to characterising 

colour vision. This approach seeks to comprehend the nature of colour vision qua 

psychological kind, as contrasted with traditional experiential approaches, which prioritise 

descriptions of our ordinary visual experience of colour. The second contribution is to argue 

that colour vision does not constitutively involve the ability to see colours, in a natural and 

categorically committed sense. I argue that two subjects exactly alike in respect of their 

constitutive colour vision abilities could differ in respect of whether or not they have 

categorical perception of colour. The argument is supported by thought experiment and 

dissociations observed in cognitive neuropsychology. The argument also bears connections to 

recent neo-Whorfian accounts of colour categorisation. 

1. Introduction 

2. Colour Vision, Experience, and Natures 

3. The Central Argument 

3.1. Colour constancy and discrimination 

3.2. Dissociating colour vision and categorical perception for colour 

4. Whorf and the View from Cognitive Neuropsychology 

5. Conclusion 

1 
 



1. Introduction 

What does colour vision, by virtue of its nature, enable us to see? A straightforward answer is 

that colour vision enables us to see colours. What does this mean? Another straightforward 

answer is that ‘seeing colours’, at least in humans, means seeing red, orange, yellow, green, 

blue, and so on. So far, so simple. 

Things are not quite so simple, however. The most natural reading of ‘seeing red, 

orange, and yellow’ is categorically committed, in that it implies the ability to see, in a sense 

not yet explained, the colour categories red, orange, and yellow. Thus understood, the ability 

to see colours implies the possession of categorical perception for colour (CP). CP is a vivid 

feature of our ordinary experience of colour, as canonically manifested in the banded or 

segmented appearance of a rainbow. Colour category terms such as ‘red’, and derivatives 

such as ‘reddish’ and ‘shade of red’, also provide our primary means of communicating about 

colour and colour vision. Despite the experiential vividness and communicative 

pervasiveness of colour categories, however, I believe that colour vision should be 

dissociated from CP for colour. Our normal human visual ability to perceive colour has a 

constitutive nature that is distinct from that of our ability for colour categorisation. To put the 

claim in its most provocative form, colour vision does not, by its nature, confer the ability to 

see colours, in the natural, categorically committed, sense. 

The central argument is as follows. It is constitutive of possessing colour vision that 

an organism possesses the ability to achieve colour constancy; colour constancy is 

characterised by abilities to discriminate visually between both the surface spectral 

reflectance properties of objects and wavelength properties of the ambient illumination; it is 

possible that colour constancy qua SSR and wavelength discrimination, hence colour vision, 

could be completely dissociated from CP for colour; therefore, CP for colour does not form 

2 
 



part of the nature of our colour vision abilities. This argument will be presented in Section 3. 

The rationale behind the argument’s focus on the constitutive nature of colour vision will be 

discussed in Section 2. 

In Section 4, I provide further motivation for the key dissociative premise of the 

argument by discussing some fascinating cases from cognitive neuropsychology. In cases of 

optic aphasia for colours, subjects retain normal visual discriminatory abilities with respect to 

colour while becoming severely impaired in their performance on colour categorisation tasks. 

In the evocative phrase of Goldstein and Gelb ([1918]), such subjects have an ‘impairment of 

the categorical attitude’ towards colour. These cases suggest that the dissociative possibility 

on which the argument rests is not only coherent, but perhaps not even that distant. I shall 

also bring out some connections between my argument and recent neo-Whorfian views of 

colour categorisation in cognitive psychology, on which CP is taken to be partly influenced 

by linguistic colour classification. While the argument is strictly independent of these 

contentious empirical issues, the discussion should present fruitful new points of contact 

between philosophical and scientific work on colour perception. 

 

2. Colour Vision, Experience, and Natures 

My central theses are, first, that colour constancy is constitutive of colour vision, and second, 

that CP for colour does not form part of this constitutive nature. Two preliminary questions 

arise: Why the interest in colour vision? And why the emphasis on the constitutive nature of 

colour vision? As for the first question, colour vision plays a foundational explanatory role in 

the philosophy of colour. Although not always made explicit, colour vision is assumed to 

provide a privileged source of evidence about colour ontology, surely the central issue in this 

area. This assumption has roots in Aristotle’s claim that colour is the ‘special object’ of 

3 
 



vision, by which he meant that it is impossible to acquire knowledge of colour via any other 

sense modality. In contemporary philosophy, a pervasive and related thought is that in order 

to know what colour properties are, we must first understand how colours are represented in 

visual perception.1 Debates about colour ontology therefore have centred on the question as 

to which properties (and relations) are represented in colour vision and whether or not these 

properties (relations) correspond to genuine properties (relations) of the ordinary objects that 

we perceive. Such questions, among other things, have prompted significant interest in colour 

vision within the wider philosophy of perception. Colour vision serves as the go-to source of 

examples in this area, a perennial quarry for philosophers seeking to clarify the nature of 

perception and consciousness. 

The philosophical project of understanding colour vision clearly overlaps significantly 

with work in vision science. Reassuringly, the last twenty five years or so has seen a 

remarkable increase in philosophical engagement with results from physiological, 

psychophysical, and computational studies of colour vision. While increasingly empirically 

informed, however, it nonetheless remains extremely common, if not standard, in 

philosophical contexts to adopt a broadly pre-theoretical, experiential, conception of colour 

vision, at least as a starting point for debate. On this experiential view, colour vision is 

characterised by our having conscious visual experiences of this distinctive type, or by 

objects appearing in these distinctive ways in experience, where the demonstratives are 

intended to pick out aspects or qualitative dimensions of our visual experience that are clearly 

distinguishable from those aspects corresponding to our experience of shape, texture, motion, 

and so on. As a foundation for work in the philosophy of perception, the experiential 

conception licenses an approach to debates about perceptual natures, abilities, constancies, 

and contents that is thoroughly phenomenological and introspective in flavour. As a 

1 See Boghossian and Velleman ([1991], p. 106), Byrne & Hilbert ([2003], p. 5), Hilbert ([1992], p. 359), 
Jackson & Pargetter ([1987], p. 68), and Strawson ([1979], p. 109). 

4 
 

                                                        



foundation for ontology, the experiential view implies that theories of colour ultimately 

should be answerable to features of our ordinary visual experience of colour. A prima facie 

sufficient reason to reject some account of colour ontology, then, would be that it fails to 

recover or respect some aspect of ordinary conscious colour appearances. Such is the current 

and historical orthodoxy in the philosophy of colour.2 

The foregoing characterisation of the experiential view of colour vision probably 

won’t ring true for everyone. Nonetheless, I believe it comes close enough to capturing some 

dominant philosophical tendencies to serve as a target in what follows. The central 

methodological aim of this paper is to develop an alternative to this orthodox experiential 

approach. My proposal is that our philosophical starting point instead should be an account of 

the nature of colour vision qua psychological kind. The kind in question will be a type of 

ability or capacity: colour vision is an ability that subjects possess and that is exercised when 

they visually perceive colours, much as your ability to understand English is a capacity that is 

being exercised while reading this sentence. In seeking an account of the nature of colour 

vision, then, we seek a characterisation of this distinctive psychological ability; an ability 

shared in common between all actual and possible colour perceivers. I call this the 

constitutive approach to theorising about colour vision. 

Thus stated, the constitutive approach is in fact consistent with a certain type of 

experiential view of colour vision. An advocate of the experiential view might endorse the 

constitutive approach and yet persist in claiming that colour vision qua psychological kind 

constitutively involves the ability to undergo this and that type of visual experience.3 For 

reasons that will become clear in the next paragraph, I think that any such constitutive-cum-

2 Some randomly chosen examples of the experiential orthodoxy in action: Armstrong ([1987], pp. 36-37), 
Boghossian & Velleman ([1989], p. 85), McGinn ([1996], pp. 541-542), Tye ([2000], pp. 152-153). Armstrong 
([1978], p. 125) notes that this approach has roots in Hume, for whom colour ‘phenomenology decides the 
question of ontology’. See Akins & Hahn ([2000], p. 233) and Matthen ([2005], p. 167 ff) for discussion. 
3 The approach is likewise consistent with the view that CP for colour is partly constitutive of colour vision. The 
force of my argument lies in its denial of this possibility. 

5 
 

                                                        



experiential view would be extremely implausible. The key point for now is simply that 

unlike mainstream philosophical approaches, the constitutive approach does not force or 

require an experiential conception of colour vision. On the constitutive approach, a 

philosophical account of colour vision is not beholden to, or necessarily constrained by, 

features of our ordinary colour experience. This approach leaves open many differing views 

of colour vision, each of which might have interesting and unexplored philosophical 

consequences. As I discuss later, the view that I find most plausible is that the constitutive 

bedrock for the possession of colour vision is the ability to achieve colour constancy. The 

ensuing argument draws out a surprising consequence of this constitutive thesis: that colour 

vision thus understood does not involve the ability to see colours in the categorically 

committed sense. Even if the reader does not agree with my constancy-centric constitutive 

starting point, however, the argument should still be of interest as an illustration of the 

constitutive approach at work. 

There have been some important steps in the direction of the constitutive approach in 

the recent philosophical literature, (and hence away from the orthodox experiential view 

described above). For example, Thompson and colleagues ([1998]) were among the first to 

bring comparative, interspecies, studies of colour vision to the attention of philosophers. This 

work highlighted the great variety of colour-related capacities encountered throughout the 

animal kingdom, and emphasised the fact that colour vision is a significant psychological 

kind featuring in actual empirical attributions and explanations across a diverse range of 

organisms.4 Such comparative studies in turn provided one of the key motivations for 

Matthen’s ([1999], [2005]) pluralist account of colour ontology. These philosophers bucked 

the major historical trend by deemphasising the evidential role of normal human experience 

in understanding colour vision and, consequently, colour. Such considerations breached the 

4 The significance of comparative studies of colour vision is now more widely appreciated. For example, see 
Allen ([2009]), Byrne & Hilbert ([2003], pp. 15-16), Cohen ([2009], pp. 26-29), and Kalderon ([2007], p. 578). 

6 
 

                                                        



experiential orthodoxy, and furthermore undermine any constitutive-cum-experiential view of 

the sort adumbrated above. While these developments are undoubtedly significant, however, 

in my view their focus on the implications of animal colour vision has obscured a crucial 

point much closer to home. We can all agree that ordinary human colour experience provides 

an unsuitable model for the colour vision capacities of other animals; but I would add that 

ordinary colour experience cannot even be taken as a presumptive guide to the core nature of 

our very own colour vision abilities. It is this last, seemingly redoubtable, vestige of the 

experiential view that I seek to undermine in what follows. 

In voicing scepticism about the informativeness of ordinary colour experience for a 

constitutive account of colour vision, I am not claiming that phenomenology is irrelevant to 

our understanding of this kind. In empirical methodology, the study of visual perception 

obviously couldn’t get off the ground without it. I allow, moreover, that visual experience 

plays an inalienable role in providing our initial identifying characteristics of colour vision 

qua psychological kind. This point is compatible, however, with the claim that colour vision 

is not constitutively associated with features of our ordinary conscious experience of colour. 

This is by now a familiar point in the philosophy of science: explanatory kinds are often, if 

not typically, initially identified via features that ultimately turn out to be merely superficial 

or contingent features of those kinds. Once such an identification has been made, an account 

of a kind’s nature may allow instances to be identified or hypothesised in circumstances in 

which the initial means of identification are partially, or even completely, absent.5 As such, 

whatever the role of experience in acquainting us with colour vision qua psychological kind, 

we should not attempt straightforwardly to infer or ‘read off’ the nature of these visual 

abilities from features of our ordinary experience of colour. That our ordinary experience of 

colour typically or even canonically displays some feature F does not entail that F is a 

5 See Shea ([2012]) for an interesting application of this thought to the study of phenomenal consciousness 
itself. 

7 
 

                                                        



constitutive feature of our colour vision capacities. That is to say, not all functionally 

significant features of colour experience will be constitutively necessary for the possession of 

colour vision. 

These last claims presuppose that colour experience has a latent and underappreciated 

complexity. Although colour is traditionally conceived as a paradigmatic ‘low-level’, 

‘simple’, or ‘basic’ perceptible feature, colour experience in fact reflects the upshot of a wide 

range of perceptual, structural, organisational, recognitory, and broadly conceptual abilities or 

processes. As in other areas of cognition, this composite nature is not transparent to the naïve 

subject, whose experience of colour is monolithic and unified. Patterns of breakdown and 

dissociation among such processes in impaired subjects, while clearly unfortunate, therefore 

provide one of the few keys to unlocking this complexity. Two processes that are ordinarily 

reflected in colour experience, and which will be central in what follows, are colour 

constancy and CP for colour. In the next Section, I argue that it is possible that the processes 

required to maintain colour constancy could become completely dissociated from those 

involved in colour categorisation. I also claim that colour constancy is constitutively both 

necessary and sufficient for the possession of colour vision. It follows that colour vision qua 

constancy can be dissociated from our ability to perceive superordinate colour categories 

such as red and orange. If this is correct, then at least one pervasive and vivid feature of our 

ordinary experience of colour is not, strictly speaking, part of the nature of our ability 

visually to perceive colour. The feature in question is that of seeing colours, in the 

categorically committed sense.6  

 

3. The Central Argument 

6 Compare Akins ([2001], p. 110), who challenges the assumption that ‘what we see consciously provides the 
raison d’être of the spectral system – that the purpose of the colour system is to produce “the colours”’. 

8 
 

                                                        



My central argument is as follows. 

1. Necessarily, for any organism x, x possesses colour vision if and only if x possesses 

the ability to achieve colour constancy. (CC) 

2. Therefore, necessarily, for any organisms x, y, x and y are alike in respect of their 

core, constitutive, abilities visually to perceive colour if and only if x and y are alike 

in respect of their ability to achieve colour constancy. 

3. Necessarily, for any organisms x, y, if x and y are alike in respect of their ability to 

discriminate visually among a) the surface spectral reflectance properties (SSRs) of 

objects in the environment, and b) the wavelength properties of the ambient 

illumination, then x and y are alike in respect of their ability to achieve colour 

constancy.7 (DV) 

4. Therefore, necessarily, for any organisms x, y, if x and y are alike in respect of the 

visual discriminations they are able to make between SSRs and the wavelength 

properties of the ambient illumination, then x and y are alike in respect of their 

constitutive colour vision abilities. (2, 3) 

5. It is possible that an organism with exactly the same visual discriminatory abilities 

as a normal human could completely lack CP for colour. 

6. Therefore, it is possible that two organisms exactly alike in respect of their 

constitutive colour vision abilities could differ in respect of whether they possess CP 

for colour. (4, 5) 

7 Note that in fixing on SSRs and properties of the illuminant, I am not prejudging the question of colour 
ontology. My approach is similar to Matthen ([1999], p. 78), who presents a broadly functional characterisation 
of colour vision in terms of wavelength discrimination, but whose view clearly does not imply that colours are 
wavelength properties. 

9 
 

                                                        



7. Therefore, CP for colour does not form part of the constitutive nature of normal 

human colour vision abilities. 

Premises one and three, CC and DV, are complementary and need to be understood in 

conjunction. CC claims that colour vision constitutively involves the ability to achieve colour 

constancy. This raises the question as to how colour constancy itself should be characterised. 

DV presents a discriminatory view, on which one’s ability to achieve colour constancy 

supervenes on one’s visual discriminatory abilities with respect to SSRs and wavelength 

properties of the illuminant. Together CC and DV imply premise 4, on which an organism’s 

constitutive colour vision abilities supervene on its visual discriminatory capacities. Premise 

5 is the key dissociative claim that it is possible to dissociate an organism’s capacity to 

perceive colour thus understood from its ability for CP for colour. The following subsections 

address these claims in more detail.  

 

3.1. Colour constancy and discrimination 

The constitutive approach holds that our starting point for philosophical theorising about 

colour vision should be to understand the nature of colour vision qua psychological kind. As 

noted above, this general approach is consistent with many differing views of colour vision, 

not all of which can be explored here. My preferred view is CC. The view that perceptual 

constancies mark constitutive bedrock for visual perception has been defended at length in 

recent work by Burge ([2010]) and in Davies ([unpublished]). In the philosophy of colour, 

Hilbert ([1992]) has also defended a thesis close to CC.8 I take this work for granted in what 

8 Smith ([2002]) also argues that perceptual constancy is necessary and sufficient for perception. As Burge 
([2010], p. 409n) notes, Smith’s view of constancy is overtly phenomenological in character and is not aimed at 
understanding the nature of constancy qua psychological kind. Thompson et al. ([1998], p. 380) agree on the 
centrality of colour constancy to colour vision, but claim in addition that colour vision ‘involves’ additive colour 
mixture and colour contrast. They do not explain why additive colour mixture should be necessary for colour 
vision. Akins & Hahn ([2014]) have also stressed the importance of chromatic contrast for colour vision. In my 

10 
 

                                                        



follows, and hence shall say fairly little in defence of CC. Readers who reject CC need not be 

deterred, however, for two reasons. 

First, even without CC my argument warrants the conclusion that colour constancy, if 

not colour vision per se, does not constitutively involve the ability to see colours in the 

categorically committed sense. This weaker thesis still should be of interest to philosophers 

working on colour, and to those interested in the nature of perceptual constancy more 

generally. Second, although it is philosophically revisionary, the view of colour vision 

implied by CC and DV is by some, more empirical, lights reassuringly conservative. Among 

vision scientists, the textbook standard for the possession of colour vision is the ability to 

discriminate differences in wavelength properties of light.9 My view bears some similarities 

in taking colour vision to supervene on capacities visually to discriminate both wavelength 

properties and SSRs of objects in the environment. (As will become clear, however, my view 

employs a representational notion of visual discrimination, which is richer than the 

behavioural, operational, notion assumed in vision science. DV thus implies that colour 

constancy is a resolutely representational phenomenon.) If the reader therefore can stretch to 

accepting premise four, then this together with premise five will suffice for my conclusion.

 In brief, the central motivation for CC is that the achievement of perceptual constancy 

with respect to some feature – shape, motion, size, depth, colour – marks the transition from 

mere sensory registration of, or informational sensitivity to, features of the proximal stimulus, 

to fully fledged representational sensitivities to distal features of the environment.10 In the 

case of colour constancy, the transition is from those organisms that possess wavelength 

sensitive cells or organs, but whose associated abilities and behaviours are subsumable under 

view, there are important but under-theorised connections between colour constancy and colour contrast. I hope 
to discuss this in future work. 
9 For example, see Gordon and Abramov ([2001], p. 93). Compare the aforementioned functional 
characterisation in Matthen ([1999], p. 78). 
10 On the distinction between sensory registration and perceptual representation, see Burge ([2010], pp. 315-
319). 

11 
 

                                                                                                                                                                            



non-cognitive, non-representational, notions of phototaxis, photokinesis, phototropism, and 

so on; to organisms that possess abilities to extract from the proximal stimulus reliably 

veridical representations of distal features of the environment, and whose abilities and 

behaviours thus come to be explicable only via the rich cognitive, representational, 

explanatory resources of visual perceptual psychology.11 Visual representations of distal 

features possess a primary biological and ecological significance in underwriting and guiding 

organisms’ basic activities such as foraging, eating, initiating or avoiding predation, seeking 

out a mate, locating shelter, and so on. For this reason the central preoccupation of much 

vision science has been to explain how perceptual constancies are achieved.12 Palmer and 

colleagues ([2003], p. 314) express a common view, 

 

In our opinion, constancy provides the single most crucial landmark in visual 
processing. It is the set of visual operations whose presumed job it is to convert visual 
representations that encode image-based (retina-based) features into ones that encode 
environment-based (object-based) features... [M]any of the most crucial inferences the 
visual system must make are concerned with the logical leap from 2-D representations 
to some more ecologically useful representations that contain explicit information 
about properties of external, environmental objects. 

 

The ability to achieve constancy thus marks a significant cut – a difference in kind– among 

organisms with sensory capacities. The proposal is that this cut marks the boundary between 

the merely sensory, and the genuinely perceptual. To borrow from Burge ([2010], p. 413), 

‘perceptual constancies are paradigmatic marks of objectification. I think that their presence 

in a sensory system is necessary and sufficient for the system’s being a perceptual system.’ 

11 For discussion of taxes and their relevance to understanding the lower, pre-constancy, borders of visual 
perceptual representation, see Burge ([2010], p. 326 ff). 
12 According to Thompson ([1995], p. 43) the achievement of perceptual constancy is ‘the central information-
processing problem’ studied by computational theories of vision, which have the strongest claim to the title of 
Kuhnian ‘normal science’ in the study of perception. 

12 
 

                                                        



In emphasising constancy as the core constitutive standard for colour vision, I am not 

denying that colour vision as instantiated in humans and other species displays many other 

functionally significant features besides constancy. Others may wish to emphasise these 

different features. In many cases, the descriptive or explanatory context may indeed demand 

a more inclusive approach. The rationale behind CC, however, is to provide a maximally 

general, cross-cutting, characterisation of colour vision; a standard that unifies the many 

varied colour vision systems encountered across the animal kingdom. As such it is telling that 

all animals currently known to possess colour vision seem to exhibit colour constancy to 

some degree. For example, comparative studies have found evidence for colour constancy in 

the honeybee (Neumeyer [1981]; Werner et al. [1988]), goldfish (Dörr and Neumeyer [2000]; 

Neumeyer et al. [2002]), butterfly (Kinoshita and Arikawa [2000]), and diurnal and nocturnal 

hawkmoth (Balkenius and Kelber [2004]), to name a few. Neumeyer ([1998], p. 347) infers 

from these findings that ‘colour constancy... is indeed essential for colour vision’. In contrast, 

although comparative studies have found signs of CP for colour in some non-human species, 

the evidence is far more limited and equivocal. Kelber & Osorio ([2010], p. 1622), for 

example, report that ‘other than in birds, and the limited evidence from fish, there is little 

convincing evidence for colour categorisation in other species, including non-human 

primates...’13 

One possible objection to CC is that colour constancy is largely achieved on the 

cheap, in that it is attributable to very low-level, pre-representational, processes such as 

retinal adaptation. If colour constancy is in fact a predominantly pre-representational process, 

this would undermine the idea that constancy is constitutive of colour vision. In response, I 

take issue with the claim that colour constancy is solely or even predominantly attributable to 

processes of retinal adaptation. Such processes are doubtless important in many viewing 

13 Kelber & Osorio do caution, however, that ‘application of suitable tests may well change this view.’ 

13 
 

                                                        



contexts, but numerous studies indicate that subjects are capable of extremely reliable colour 

constancy judgements across changes in illumination that are too rapid to allow for 

significant adaptation.14 

These points notwithstanding, my main issue with the objection is that there is no 

valid inference from ‘X is largely attributable to non-representational processes’ to ‘X is non-

representational’. This inference slips from a claim about the mechanisms responsible for X, 

to a claim about the nature of X itself. I assume that many paradigmatically representational 

cognitive capacities – such as intentional action, reasoning, and linguistic understanding – are 

realised by a wide range of neural mechanisms, not all of which involve representation. The 

fact that an entire system, module, or process might have component mechanisms that are 

non-representational should not impugn the representational nature of the system, module, or 

process itself. Now as I understand it, perceptual constancy is a resolutely representational 

phenomenon. This reflects mainstream empirical assumptions, as seen in the above quote 

from Palmer and colleagues ([2003], p. 314). Even granting that some – perhaps even the 

majority – of the component mechanisms involved in achieving colour constancy are non-

representational, then, this provides no good reason to doubt the representational nature of 

constancy qua psychological kind.15 

Having proposed CC as a constitutive standard for colour vision, we must now 

confront the question as to how colour constancy itself should be characterised. This is a 

contentious matter, and the subject of ongoing debate. The default characterisation of colour 

constancy in both philosophical and empirical work is known as ‘appearance invariantism’. 

Invariantism characterises colour constancy as the phenomenon whereby objects visually 

appear approximately the same in respect of colour across changes in illumination. ‘Colour 

14 For example, see Foster et al. ([1991]) and Foster et al. ([2001]). I discuss the limitations of adaptation-based 
conceptions of constancy in more detail in Davies ([forthcoming]). 
15 For a survey of the multiplicity of mechanisms involved in colour constancy, see Smithson ([2005]). 

14 
 

                                                        



appearance’ here is to be understood in the quasi-technical sense familiar to vision scientists, 

as pertaining to the aspect of perceptual phenomenology represented by multi-dimensional 

colour appearance spaces, such as the standard three dimensional hue-saturation-lightness 

(HSL) space. On this view, then, a subject’s degree of colour constancy is determined by the 

extent to which these qualitative dimensions of visual experience – say, an object’s hue and 

saturation – remain invariant under changes in illumination. Information about the illuminant 

is not visually represented on this view: the standard line is that such information is 

‘discarded’ or ‘discounted’ in the process of generating illumination-invariant colour 

appearances. 

Against this naïve view, ‘variantists’ have argued that colour constancy cannot be 

grounded in appearance invariance, most notably due to psychophysical evidence that 

changes in illumination in fact produce quite significant variations in an object’s hue and 

saturation.16 On this view, information about the illuminant is not discarded by the visual 

system, but rather retained and reflected in the qualitative dimensions of colour appearance. 

In a broadly Helmholtzian spirit, variantists seek to characterise colour constancy by 

appealing to the representational outputs of putative, quasi-cognitive, processes of perceptual 

‘inference’. On this view, the visual system supposedly ‘infers’ or otherwise derives 

representations of objects’ stable, illumination-independent, surface colours, from patterns of 

illumination-dependent change in objects’ phenomenal colour appearance.17 

Invariantism and variantism mark the traditional poles in philosophical debates about 

colour constancy. I want to consider an alternative characterisation that has arisen in the 

16 The classic empirical study is Arend & Reeves ([1986]). See Cohen ([2008]) for a variantist, neo-
Helmholtzian, interpretation of these data. 
17 Wright ([2013]) discusses such views under the label of ‘projective’ accounts of constancy. 

15 
 

                                                        



empirical literature, but which has not received much philosophical attention. This is the 

operational view (OV) of Craven and Foster ([1992], p. 1360),18 

  

[We present] an alternative and complementary property of colour constancy, as 
follows: The ability of a subject to correctly attribute changes in the colour 
appearance of a scene either to changes in the spectral composition of the illuminant 
or to changes in the reflecting properties of that scene. 

 

According to OV, the key criterion for colour constancy is the possession of abilities to 

‘discriminate’ a change in reflectance from a change in illumination. These abilities can be 

assessed in psychophysical tasks in which the subject is presented with a coloured stimulus 

with reflectance properties R1 under illuminant A, proceeded either by the same stimulus 

under another illuminant B, or by a different stimulus with reflectance R2. The subject is then 

asked to push one of two buttons depending on whether they judge the change that has 

occurred to be in the surface properties of the stimulus, or in the illumination. High levels of 

colour constancy correspond to high degrees of accuracy in identifying changes in 

illumination and changes in stimulus reflectance as such. That is, subjects do well if a change 

from illuminant A to illuminant B is not confused or conflated with a change in the surface 

properties of the stimulus. Similarly, they do well if a change in stimulus reflectance from R1 

to R2 is not conflated with a change in illumination. 

 How does OV contrast with invariantism and variantism? Unlike invariantism, OV 

does not ground colour constancy in the stability of colour appearance across changes in 

illumination. As Craven and Foster ([1992], p. 1364) remark, ‘it may be irrelevant whether 

the colour appearance of an object does or does not change under a change in illuminant, 

providing that any changes that do occur can be correctly attributed’. Insofar as OV allows 

18 Also see Foster & Nascimento ([1994]). 

16 
 

                                                        



for constancy in circumstances of significant illumination-dependent change in colour 

appearance, it clearly has affinities with variantism. Unlike variantism, however, OV does not 

ground constancy in putative processes of perceptual inference from variant colour 

appearance to illumination-independent surface colour properties. Moreover, OV is not 

committed to the claim that colour constancy canonically or typically involves such 

illumination-dependent phenomenology. As I read the view, OV is strictly neutral on this 

issue. So long as the subject is capable of differentiating changes in illumination from 

changes in surface reflectance, then the degree of appearance (in)variance that she encounters 

is simply ‘irrelevant’ in evaluating her constancy capacities. 

OV’s discriminatory characterisation is thus consistent with invariantist and variantist 

views of colour appearance, and indeed with other, more complex, views of the 

phenomenology of colour constancy.19 This phenomenological neutrality is appealing for 

those seeking a maximally general, cross-cutting, characterisation of colour constancy. As 

discussed elsewhere (Davies [forthcoming]), the empirical data suggest that neither 

invariantism nor variantism uniquely characterises the phenomenology of constancy. In 

contrast, it does seem that wherever we are inclined to attribute constancy, we should find 

robust capacities to discriminate changes in illumination from changes in surface reflectance 

properties. Absenting these capacities, an organism would be liable regularly to confuse or 

conflate changes in illumination for changes in an object’s surface properties, and vice versa. 

Such an organism would not display any evidence of having disentangled the respective 

contributions of these variables to its overall perceptual state: such an organism would lack 

colour constancy. 

19 In Davies ([forthcoming]), I argue that OV is consistent with ‘complex invariantism’, a view on which visual 
experience has qualitative dimensions for both material and lighting colour, numbering six dimensions in total. 

17 
 

                                                        



This last point suggests that OV presents at least a necessary condition on colour 

constancy: no capacity to discriminate between illumination and surface changes implies no 

colour constancy. Is the condition also sufficient? This suggestion is problematic, for there 

are strong reasons to think that discrimination, at least in the sense intended by OV, is not 

necessarily sufficient for colour constancy. Recall that OV is an ‘operational’ view of 

constancy, on which capacities to discriminate are explicitly linked to performance in a 

psychophysical task. On this view, a subject discriminates a change in reflectance from a 

change in illumination just in case she provides largely correct behavioural responses – in this 

case, button presses – in tasks of the sort described above. One might reasonably object, 

however, that discriminatory behaviour of this type does not necessarily suffice for the 

attribution of genuine perceptual, representational, abilities with respect to distal features of 

the environment. That is to say, it is conceivable that an organism could display such 

discriminatory behaviour, while altogether lacking genuine visual representational abilities.20 

Now of course we assume that all human subjects actually assessed in these experiments do 

possess such abilities, and that their task performance can be taken as evidence of their 

perceptual capacities. But the mere possibility of discriminatory behaviour in the absence of 

visual representation undermines the proposal that OV can characterise colour constancy qua 

constitutive standard for colour vision. 

The discriminatory view of colour constancy (DV) presented in premise 3 is designed 

to avoid these problems with OV.21 DV is clearly inspired by OV, and shares its empirical 

motivations. DV follows OV in characterising colour constancy via capacities visually to 

20 This echoes Hilbert’s ([1992]) central objection to the sufficiency of the wavelength discrimination criterion 
for colour vision. See also Matthen ([1999], p. 52). 
21 Despite the order of presentation, DV is explanatorily prior to CC: we require a working characterisation of 
colour constancy, prior to claiming that colour constancy is constitutive of colour vision. In practice, however, 
philosophical attempts at characterising colour constancy inevitably draw on ongoing empirical research, much 
of which simultaneously provides motivation for CC. This does not create any vicious circularity, so long as we 
are careful to distinguish the project of extracting a maximally general understanding of ‘colour constancy’ as it 
occurs in such studies, from the project of inferring what these studies tell us about the nature of colour vision. 

18 
 

                                                        



discriminate differences in distal properties. More precisely, an organism’s ability to achieve 

colour constancy supervenes on its capacities to discriminate visually among SSRs of objects 

in the environment, and wavelength properties of the ambient illumination. Necessarily, any 

change in the degree of colour constancy that an organism is able to achieve in a given 

viewing context implies some underlying change in these visual discriminatory abilities. 

Whereas OV adopts a behavioural concept of discrimination, however, DV employs an 

inherently visual representational notion. On this view, S is able to discriminate visually 

between X and Y just in case S is able to form representations, within a perceptual subsystem 

that can be antecedently described as visual, with the veridicality conditions that X and Y are 

distinct, and moreover that these representations are reliably veridical within some suitably 

restricted range of actual and counterfactual cases.22 Here the relevant discriminatory 

responses are not patterns of behaviour, judgement, or report: the discriminatory responses 

are representational states of the visual system. 

According to DV, then, an organism’s colour constancy capacities supervene on its 

abilities to generate discriminatory perceptual representations of both SSRs and wavelength 

properties of the ambient illumination. It is important to note that what are being 

discriminated in this case are not objects, but rather property types themselves; more 

accurately, property types as presented by their property instances. In discriminating between 

the reflectance properties R1 and R2 of objects a and b, then, my visual system does not 

simply represent that a and b differ in respect of reflectance: it (somehow) represents that the 

property types R1 and R2 are distinct. What form would these putative representations take? 

This question requires a paper in its own right, but for present purposes we can adopt an 

inclusive view. For example, one idea would be that the visual system represents something 

22 This model is inspired by Williamson’s ([1990]) cognitive account of discrimination, on which to 
discriminate a from b is to activate the knowledge that a and b are distinct. My characterisation aims to preserve 
Williamson’s insight that discrimination proper implies capacities for reliably veridical representation of 
distinctness, while doing away with the epistemic baggage. 

19 
 

                                                        



of the form ‘the surface property presented by this object differs from the surface property 

presented by that object’. Another possibility is that the visual system employs some as yet 

unknown type of direct representation of tropes or property instances, as in ‘this surface 

property differs from that surface property’. In both cases, the relation that is represented as 

holding between these properties also remains to be clarified. One possibility is that ‘differs 

from’ represents a brute relation of non-identity. Another possibility, which I find more 

plausible, is that this relation will be unpacked in terms of a direction and magnitude of 

difference in some quality space. However these issues turn out, the key overarching claim is 

that these visual discriminatory abilities are grounded in non-atomic, relational, 

representations within the colour vision system. 

These clarificatory remarks bring out another important difference between DV and 

OV. According to OV, colour constancy requires the capacity to discriminate between two 

stimulus conditions or states of affairs: namely, the condition of having undergone a change 

in surface properties, versus the condition of having undergone a change in illumination. DV, 

in contrast, requires the capacity (somehow) to represent the types of surface property or 

illumination instantiated by these stimuli as distinct. By implication, DV requires the subject 

to be capable of visually representing types of surface property and illumination and, 

moreover, to discriminate dimensions of change in respect of these properties. 

In saying that colour constancy requires the visual representation of both surface 

properties and the illuminant, once again I want to remain neutral on the precise types of 

representation involved, and on how these representations influence or determine the 

organism’s visual experience of the scene. I allow, for example, that some organisms may 

explicitly represent both surface and lighting properties via qualitative dimensions of the 

phenomenology of visual experience. Indeed as I argue in Davies ([forthcoming]), this 

plausibly is often the case in normal human subjects. Another possibility, however, is that 

20 
 



one or more type of representation is processed by nonconscious or subpersonal perceptual 

processes. However these issues turn out, the main claim for DV is simply that colour 

constancy requires some sort of visual representation of the illumination conditions, such as 

to undergird the organism’s perceptual discriminations of changes in the illuminant. DV thus 

marks another significant departure from traditional models of constancy, on which 

illumination information is simply ‘discarded’ or ‘discounted’ in the process of generating a 

stable surface colour estimate.23 

DV is clearly hostage to empirical fortune in various ways. First, DV predicts that 

differences in respect of the degree of colour constancy that an organism can achieve in a 

given context should be reflected somewhere, somehow, in the organism’s capacity to 

discriminate both SSRs and properties of the illuminant. The view therefore would be 

disconfirmed, for example, by evidence of significant impairments to colour constancy with 

largely preserved capacities visually to discriminate these properties. Second, eventually we 

should be able to identify plausible neural substrates for DV in humans, and to identify 

functionally similar substrates in all other creatures thought to possess colour constancy. 

These substrates moreover should be capable of sustaining perceptual representations of 

features of both object surfaces and illumination conditions, and these representations should 

be capable of driving the organism’s behaviour in responding to changes in either surface or 

illumination properties as such. Third, DV should prove illuminating and explanatory in 

characterising colour constancy capacities in empirical practice. DV would be disconfirmed, 

to some extent, if it provided little or no help in understanding the nature of colour constancy 

23 A referee raised the following objection. Consider an organism with a visual system very much like ours, but 
situated in a world populated only by coloured lights and no surfaces. Such an organism would be able to 
discriminate between the colours of lights, but on my view would not exhibit colour constancy. It seems 
implausible to suppose, however, that such an organism would lack colour vision. In response, note that an 
organism with a visual system very much like ours would, ipso facto, retain the ability to achieve colour 
constancy, even if situated in a world in which there are no surfaces, and hence where it is not possible to 
discriminate differences in SSRs. On my view, then, despite being unable to exercise its capacity for colour 
constancy (hence colour vision) the organism nonetheless could possess the ability for colour constancy (hence 
colour vision). 

21 
 

                                                        



capacities in both human and non-human cases. It should be noted, however, that DV is not 

intended to provide a complete characterisation of colour constancy in normal humans. The 

aim is rather that of providing a maximally general, all inclusive, characterisation of 

constancy; an account that is suited to our project of identifying a core, basic, and unifying 

feature of colour vision. I believe that DV is well suited to this purpose, and that the view will 

withstand the demands and tests of evolving empirical practice and philosophical theory. 

There are several alternative characterisations of colour constancy that cannot be 

considered here. One particularly pertinent view, however, is the categorical view of colour 

constancy (CVCC) presented by Thompson ([1995], pp. 196-7),24 

 

The key to colour constancy is to be found in this joining of object identification and 
categorical colour perception. Rather than providing constant perceptual indicators of 
surface reflectance, the primary role of colour vision is probably to generate a 
relatively stable set of perceptual categories that can facilitate object identification 
and then guide behaviour accordingly... If colour constancy is an adaptation, it is 
better regarded not as an adaptation for object detection via the recovery of surface 
reflectance, but rather as an adaptation for integrating a physically heterogeneous 
collection of distal stimuli into a small set of visually salient equivalence classes... 

 

Thompson argues that the primary function of colour vision is probably to aid object 

recognition, a task that requires only coarse grained information as to which colour category 

the object belongs to. Colour constancy is then seen primarily as functioning to ensure that 

objects are assigned the same colour category under differing illuminants, for the purposes of 

object (re-)identification. This view clearly differs somewhat from DV. More worryingly, 

however, by tying colour constancy so closely to colour categorisation, CVCC threatens my 

central claim that colour vision qua constancy can be completely dissociated from CP for 

colour. 

24 Thompson here follows Jameson & Hurvich ([1989], p. 195). See also Olkkonen et al ([2010]), Hardin 
([1992]), Hatfield ([1992]), and Troost & De Weert ([1991], p. 596). 

22 
 

                                                        



I offer two responses; the first defensive, the second more concessive. The first 

concerns CVCC’s emphasis on the role of colour constancy in facilitating object recognition. 

Identifying an object as Bruce or as a banana presumably involves the activation of some 

memory representation of the individual or kind in question. Given our limited storage 

capacities, we can predict that much visual information will be stored in memory in 

simplified, discretised, and easily accessible forms. And indeed there is evidence that 

information about colour is represented in visual memory in categorical form.25 Insofar as 

object identification involves the activation of memory representations of colour, then, it is 

plausible that these would be categorical representations of colour. The problem with CVCC 

is that by focusing on the functional associations between constancy and object recognition, it 

risks imputing features of the representations involved in colour memory onto those involved 

in sustaining occurrent colour constancy. These dangers are well recognised in the empirical 

literature. Kay and Regier ([2006], p. 53) note that ‘much of the evidence for categorical 

“perception” of colour comes from tasks that involve memory; hence it could be that the 

category effects stem from memory rather than perception.’26 Boynton and colleagues 

([1989], pp. 229-230) similarly note that ‘common experience suggests that, when colours 

must be remembered, categorisation is required’, and present evidence that categorical effects 

can be induced in simple colour discrimination tasks ‘provided that the stimuli to be 

compared are separated temporally.’27 Even if colour constancy does function in part to aid 

object recognition, the categorical representations implicated in this process might better be 

attributed to memory than colour constancy itself. 

 My second response concedes that DV does offer quite a narrow perspective on the 

nature and role of colour constancy. As noted above, this reflects my central explanatory aim 

25 See Raftapopolous ([2010]), particularly section 4.3 for discussion. 
26 See also Huttenlocher et al. ([2000]). 
27 In contrast, however, Franklin et al. ([2005]) present evidence for CP in visual search tasks, which minimise 
the role of memory. 

23 
 

                                                        



of accounting for colour constancy, hence colour vision, in all its varied guises. In this 

context, narrowness avoids over specificity and affords greater generality. It must be 

acknowledged, however, that some will think it impossible or at least misguided to divorce 

consideration of colour constancy from the downstream functions that it serves. Viewing 

constancy in this more holistic way will likely require a far richer and more complicated view 

than that offered by DV. I have no principled objection to such approaches, and indeed I 

would expect them to advance our overall understanding of perception and its integration 

with other cognitive capacities. My approach is simply different, and finer grained, seeking to 

control for as many variables as possible in accounting for this fundamental perceptual kind. 

 

3.2. Dissociating colour vision and categorical perception for colour 

Premise five states that it is possible that an organism with exactly the same discriminatory 

abilities – which by premise four implies the same colour vision abilities – as a normal 

human could completely lack CP for colour. CP is a vivid and pervasive feature of our 

ordinary visual experience of colour. The colour spectrum appears in experience to contain 

segmented regions or bands of colour corresponding to the ordinary categories red, orange, 

yellow, green, blue, indigo, and violet. If asked to name the colours that we see, most of us 

would say that we see red, orange, yellow, or shades of these categories – various reds, 

oranges, and yellows, and so on. It is undoubtedly central to our ordinary understanding of 

colour vision that it involves seeing colours in this categorically committed sense. 

In what follows, CP is characterised as follows. Take three shades of colour, a, b, and 

c, such that the pairs a and b, and b and c, are separated by exactly the same discriminable 

distance, (a and c will be separated by twice that distance), and where a and b are ‘within-

category’ shades – both shades are within the category green, say – and b and c is an ‘across-

24 
 



category’ pair – such that c lies within the category blue. Now a subject possesses CP for 

green just in case the within-category shades a and b appear more similar to her than the 

across-category shades b and c, despite the fact that they are separated by the same 

discriminable distance. These differential similarity effects reflect the most central feature of 

CP for colour, namely the banding or warping of the colour spectrum at category boundaries. 

The related similarity judgements with respect to triads of suitably chosen shades also 

provide the simplest and most precise way of assessing whether a subject possesses CP for a 

given category. 

Assessing subjects’ differential similarity judgements in this way is an ‘implicit’ test 

for CP, in that it does not require the subject overtly to attend to or report on the category 

membership of the stimuli. Other measures of CP are available, however. A different implicit 

test involves assessing subjects’ reaction times on tasks in which colour category information 

can either aid or hinder the task’s completion. So-called ‘explicit’ tests for CP require either 

the comprehension or production of a colour category name, or overt attention to category 

membership. Examples include asking subjects to select the ‘best’ or ‘focal instance’ of a 

given colour category, or actively to sort colour chips into categories. It is no doubt possible 

that a subject may possess CP by the lights of one test and not another. As such, we should 

leave open the possibility that these different tests probe different characteristics of CP for 

colour. Ultimately our picture of CP therefore might need to include a cluster of 

characteristics, including but not limited to differential similarity effects. I ignore this 

complication in what follows, however, as I believe that my central argument could be 

extended and rerun so as to accommodate a more complicated view of CP. 

Non-categorical perception (NCP) with respect to a category C is defined as the 

failure to meet the above standard for CP for C. A subject with NCP with respect to green, 

then, would not perceive any differential similarities between stimuli b and c straddling the 

25 
 



boundary between green and blue, versus the stimuli a and b that both sit within the category 

green. In other words, the shades a and b, and b and c, would appear equally similar to her. 

The notion of NCP should not seem contentious or surprising: there are obviously far more 

colour categories for which we are non-categorical perceivers than for which we are 

categorical perceivers. Extending this notion, however, we can conceive of a subject who 

fails to exhibit CP with respect to any superordinate colour categories. Let’s call such a 

global absence of CP total NCP. A subject with total NCP would experience the colours of 

the rainbow in a way which is quite unfamiliar. Such a subject would perceive the physical 

continuum of light wavelengths in a rainbow as a non-banded, non-segmented, continuum of 

hues. The spectrum would appear as a pure flux, altogether lacking discrete bands of colour. 

A subject with total NCP would not see all regions of the spectrum as chromatically 

homogenous. They would be able to discriminate between suitably distanced shades all the 

way along the spectrum, and shades that are closer together in this sequence would appear 

more similar than shades which are further apart. What would be lacking is the familiar 

superordinate grouping of these spectral hues which is so vividly apparent in our ordinary 

visual experience of colour. 

My argument for premise five centres on a thought experiment involving two 

subjects: CAT, a normal adult human, and NONCAT, who has exactly the same SSR and 

wavelength discrimination abilities as CAT. Given two colour stimuli a and b of differing 

reflectance properties, CAT can visually discriminate a from b iff NONCAT can visually 

discriminate a from b. By premises three and four, CAT and NONCAT are thus exactly alike 

in respect of their ability to achieve colour constancy, and hence in respect of their colour 

vision abilities. Now imagine that CAT and NONCAT are visually presented with three 

shades a, b, and c, such that a and b, and b and c, (not a and c), are separated by the same 

discriminable distance for CAT and NONCAT: both subjects can just discriminate a from b, 

26 
 



and b from c, say.28 All other contextually relevant variables such as the ambient illumination 

and viewing angle are held fixed. The shades a and b belong to the category red, whereas c 

belongs to orange. Now suppose, however, that CAT perceives b as more similar to a than to 

c, whereas NONCAT perceives b as just as similar to a as to c. CAT therefore meets the 

assumed standard for CP with respect to red, whereas NONCAT does not: CAT exhibits the 

requisite differential similarity effects at the boundary of red, whereas NONCAT does not.29 

 Now consider any arbitrary colour category C, and any suitably placed triad of 

discriminable shades a’, b’, and c’, for which CAT exhibits the differential similarity effects 

required for CP with respect to C. By stipulation, the shades a’, b’, and c’, will be equally 

discriminable to CAT and NONCAT. But now suppose that NONCAT fails to exhibit the 

differential similarity effects required for CP with respect to C. NONCAT would thereby fail 

to exhibit CP for any of the categories for which CAT exhibits CP. Moreover, I stipulate that 

there are no other colour categories for which NONCAT exhibits CP, but for which CAT 

doesn’t. NONCAT is therefore exactly similar to CAT in respect of her visual discriminatory 

abilities, and yet has total NCP for colour. If genuinely possible, this scenario establishes that 

a subject exactly like a normal human in respect of the constitutive, discriminatory, standard 

for colour vision could altogether lack CP for colour. 

Does NONCAT present a genuine possibility? Is it possible that two subjects exactly 

alike in respect of their visual discriminatory abilities nonetheless could differ in respect of 

their CP for colour? It might be thought not, for CP is often, if not typically, characterised as 

a discrimination effect. In the introduction to a seminal collection of papers on this topic, for 

example, Harnad ([1987], p. 3) states that CP requires that ‘the subject can discriminate 

smaller physical differences between pairs of stimuli that straddle that boundary than between 

28 See Palmer ([1999], pp. 671-72) for discussion of just noticeable differences. 
29 I assume that NONCAT also fails on all other implicit or explicit tests for CP noted above. 

27 
 

                                                        



pairs that are within one category or the other’. In another influential study, Bornstein and 

Korda ([1984], p. 208) say that CP is characterised by the ‘nonmonotonic character of 

discriminability along [the] correlate physical dimensions’ of a given perceptible feature. 

Taken at face value, this characterisation implies that two subjects differing in respect of CP 

for colour ipso facto would differ in their discriminatory abilities. This would cause trouble 

for our imagined scenario involving CAT and NONCAT, for CAT and NONCAT were 

stipulated to be exactly alike in respect of their discriminatory abilities and yet utterly 

different in respect of CP for colour. 

Despite the quoted assertions to the contrary, the overwhelming evidence seems to be 

that CP for colour is not in fact a function of discrimination performance. Hardin ([1997], p. 

293) comments as follows, 

 

It is important to understand that the resemblance that connects two instances of the 
same colour category is not necessarily a function of the perceptual [i.e. 
discriminable] distance between them. It is not hard to find three colour samples A, B, 
and C, [where] B is separated from A on the one side and C on the other by the same 
number of just-noticeable differences, and yet A and B are seen to belong to the same 
colour category whereas C is seen to belong to a different colour category. 

 

Hardin is clearly not claiming that ordinary subjects do not possess CP for colour, but simply 

pointing out difficulties in reducing the similarity effects that are characteristic of CP to 

discrimination performance. Franklin and Davies ([2004], p. 352, note 2) make the same 

point, 

 

[Inequality in discriminable distance] would not really be equivalent to adult 
categorical perception. Equivalent non-uniformities also occur within categories. 
Thus, quasi-categorical effects should also be found within categories. It might be 
thought that it is this variation in discriminability that drives category formation. 

28 
 



Category boundaries occur at local discrimination maxima. This fits the blue-green 
boundary; there is a discrimination peak at about the right place. But, it is much less 
clear for green-yellow and yellow-orange-red.30 

 

Franklin and Davies argue that differential similarity effects are not in fact a function of 

discrimination, because discrimination performance does not peak at the category boundaries 

between green and yellow, yellow and orange, and orange and red. If differences in 

discrimination underlay CP, moreover, then we should expect to experience more colour 

categories than we in fact do, given the existence of local discrimination maxima within 

categories.  

Further evidence for the dissociation between CP for colour and discrimination 

performance is presented by Roberson and colleagues ([2009]). Their study focuses on 

speakers of Korean, who display characteristic CP effects for three categories in the blue-

green region, parang (approximately: blue), chorok (approximately: green), and cheongnok 

(approximately: greenish-blue), whereas English speakers display these effects for only two 

categories: blue and green. Despite these robust differences in CP for colour, however, 

discrimination thresholds across the spectrum were not significantly different for speakers of 

English and Korean. For most actually encountered differential similarity effects with respect 

to colour, then, researchers have failed to account for these effects in terms of differences in 

discrimination. Such evidence provides good reason to think that CP for colour is not 

reducible to discrimination performance. If this is correct, then we should not dismiss the 

possibility involving CAT and NONCAT on the grounds that their differences in CP for 

colour must imply differences in their discriminatory capacities. 

30 Roberson et al. ([2009]) argue that discrimination performance also cannot explain the blue-green category 
boundary. See also Hanley & Roberson ([2011]). 

29 
 

                                                        



Given the evidence cited above, how should we explain the persistence of the 

discriminatory characterisation of CP in the empirical literature?31 I believe that this 

characterisation persists due to a failure clearly to distinguish between notions of 

discriminability and similarity. In experimental contexts, the term ‘discrimination’ is 

typically used to describe any situation in which two sets of stimuli receive different types of 

response. This usage obscures the fact that two sets of stimuli might be equally visually 

discriminable for a subject and yet still be treated differently due to differences in their 

apparent similarity. To make this point concrete, consider the recent study of CP by Winawer 

and colleagues ([2007]). Their experiments involve an ‘ABX test’ paradigm in which 

subjects are presented with a target square together with two test squares, one of which is 

(type-)identical to the target while the other ‘distracter’ square is drawn either from the same 

linguistic category F as the target (the within-category condition) or a different linguistic 

category G (the across-category condition). Subjects are asked to report which of the two test 

squares is the same as the target square. It is assumed that if the subject possesses CP for F, 

then she will be significantly faster at identifying the identical square in the across-category 

condition than the within-category condition. The logic is that if the subject possesses CP for 

F, then the test squares in the across-category condition should appear in some sense more 

different than the test squares in the within-category condition, which should make it easier 

for the subject to identify which of the test squares matches the target square in the across-

category condition. The authors gloss this by saying that subjects with CP demonstrate ‘faster 

discrimination’ between the two fields in the across-category condition. It is crucial to note, 

however, that these experiments do not establish that subjects with CP for F show better 

discrimination of colours, in the sense of being able to make finer visual discriminations 

between colours that lie across category boundaries than between colours that lie within the 

31 What’s more puzzling is that even those who claim to dissociate discrimination and CP continue to 
characterise CP in terms of discrimination performance. See Franklin and Davies ([2004], p. 350). 

30 
 

                                                        



same category. All that is needed to explain the difference in response is to suppose that the 

test fields in the across-category condition appear more dissimilar to subjects with CP for F 

than the test fields in the within-category condition. 

Perhaps the strongest objection to premise five derives from the widespread practice 

among vision scientists of using colour category terms to describe very low level processes in 

the visual system. An early example is the trichromatic theory of Young and Helmholtz, 

which proposed that all colour experiences were the product of the combined outputs of three 

retinal photoreceptor types, which were labelled ‘red’, ‘green’, and ‘blue’ receptors. It is now 

widely acknowledged that these labels are inappropriate, however, and that these receptors 

are better termed ‘long’, ‘medium’, and ‘short’ (L, M, S) wavelength cones.32 The practice 

persists, however, in attempted physiological reductions of opponent process theory (OPT). 

OPT is founded on a set of observations about the phenomenal structure of our ordinary 

visual experience of colour. OPT posits four unique hues, which are hues that do not 

phenomenally appear to be a mixture of any other hue. These hues are typically labelled as 

unique red, unique green, unique yellow, and unique blue. All other colours phenomenally 

appear to be binary, in the sense that they appear to be mixtures of two hues. No colour ever 

appears to be a mixture of red and green, however, and likewise for yellow and blue. These 

pairs are therefore referred to as phenomenally opponent colours.33 On discovery of 

physiologically opponent processes in animal colour vision systems in the 1950s, researchers 

were predictably quick to apply the categorical terminology of phenomenal OPT to these 

processes.34 This practice remains widespread today in both scholarly research articles and 

textbook introductions to colour vision. 

32 See De Valois and De Valois ([1975], p. 107) and Gordon & Abramov ([2001], p. 103). 
33 See Hardin ([1988], pp. 26, 30, 37-39, 52). 
34 Opponent processes were first discovered in the retinal ganglion cells of fish by MacNichol & Svaetichin 
([1958]), and in the lateral geniculate nuclei of macaques by De Valois et al. ([1967]). For a summary, see 
Gordon & Abramov ([2001]). 

31 
 

                                                        



As Mollon ([1997], pp. 868-872) explains, however, the application of category terms 

here is once again precipitous, 

 

We still believe today that there are chromatically antagonistic channels in the early 
visual system—that is, channels that draw inputs of opposite sign from different 
classes of cone, but these channels simply do not correspond to the 
phenomenologically defined channels of Hering […] In fact, no one has found a site 
in the visual system where colour appears to be represented according to Opponent 
Colour theory—that is, a site where the cells might be held to secrete redness and 
greenness or yellowness and blueness... Thirty years ago we thought we understood 
the existence of four unique hues, hues that are phenomenally unmixed. Today this is 
perhaps the major unsolved problem of colour vision. 

 

While colour category terms doubtless provide a helpful façon de parler in describing these 

opponent mechanisms, then, such attributions in no way establish that CP for colour is an 

inherent or fundamental aspect of colour vision. 

A possible rejoinder is that although the application of colour category terms to 

physiological opponent mechanisms is inappropriate, the use of such terms in the original 

phenomenal form of OPT is entirely permissible. Given that phenomenal OPT captures the 

universal structural features of ordinary colour experience, moreover, it follows that colour 

vision is essentially categorical in nature. Let’s flesh this rejoinder out in more detail. As 

noted above, phenomenal OPT is founded on the observation that all colours appear either as 

unique shades of red, green, yellow, and blue, or as binary complexes of these categories, 

such as reddish-blue and greenish-yellow, subject to the constraints of the opponent hue 

pairings.35 Some people seemingly take these phenomenal structural features to impose 

unassailable constraints on the nature of colour vision. For example, Thompson ([1995], p. 9) 

claims that ‘colour vision, whatever else it might be, is the ability to see visual qualities 

35 The ‘hue scaling’ techniques of Jameson & Hurvich ([1959]) and Sternheim & Boynton ([1966]) indicate that 
the terms ‘red’, ‘green’, ‘yellow’, and ‘blue’, and combinations of these terms, are necessary and sufficient to 
describe all of the spectral hues. 

32 
 

                                                        



belonging to the phenomenal hue categories, red, green, yellow, blue’. Now presumably 

Thompson would agree that it is implausible that all animal colour vision involves the ability 

to see qualities belonging to the anthropocentric categories of red, green, yellow, and blue. 

Viewing these comments in light of Thompson’s categorical view of colour constancy 

discussed above, however, it seems reasonable to conclude that he takes phenomenal OPT to 

imply that colour vision constitutively involves the ability visually to represent some colour 

categories or other. 

I offer two points in response. First, as made clear at the outset, a central aim of this 

work is to develop an alternative to the mainstream experiential approach to theorising about 

colour vision. On the present constitutive approach, the fact that colour experience 

canonically displays some feature F is not taken to entail that F is a constitutive feature of 

colour vision. It seems, however, that the argument from the apparently categorical nature of 

phenomenal OPT, to the categorical nature of colour vision, employs just this type of 

inference. Phenomenal OPT is a partial account of the structure of colour appearance, of the 

topology of our ordinary conscious experience of colour. Whether or not this structure 

implies CP for colour, the entire point of this paper has been to challenge the assumption that 

any such structural feature of experience provides an incontrovertible constraint on a 

constitutive account of colour vision. 

It might be objected that by resisting such experiential constraints, I have simply 

changed the changed the subject from that commonly agreed upon by philosophers of 

perception. I reject this charge. My subject is colour vision, which as noted in Section 2 is 

agreed by all parties to be a foundational explanatory notion in this subfield. What has been 

changed is the method or emphasis in studying this phenomenon. My approach seeks to 

understand the distinctive nature of colour vision qua psychological kind, rather than to 

describe the features of our ordinary visual experience of colour. One is of course free to 

33 
 



object to any or all aspects of my account. What is not acceptable, however, is to impose an 

experiential conception of colour vision as a precondition on the debate. 

My second response is that even if one remains unconvinced by the last two 

paragraphs, it is in any event mistaken to think that phenomenal OPT structurally implies the 

presence of CP for colour. The unique hues postulated by OPT are typically labelled ‘unique 

red’, ‘unique green’, ‘unique blue’, and ‘unique yellow’. This might suggest a straightforward 

relationship between visually perceiving the unique hues and seeing the colour categories red, 

green, blue, and yellow. This thought is bolstered by empirical evidence that subjects who 

possess CP for these categories typically locate their foci or paradigmatic examples at or very 

close to the corresponding unique hues.36 Despite the undoubted correlations, however, there 

is no constitutive connection between the traditional phenomenally opponent colour space 

and CP for colour. Actual empirical cases suffice to make the point. The aforementioned 

study by Winawer and colleagues ([2007]) indicates that Russian subjects possess CP for two 

categories in the bluish region, roughly corresponding to light blue and dark blue. In 

accordance with OPT, we can assume that Russian subjects perceive a single unique hue in 

the bluish region. This unique hue could not possibly form the focal point for the category 

blue for these subjects, for the simple reason that these subjects do not possess CP for blue, 

but rather for light blue and dark blue. Similarly, Roberson and colleagues ([2009]) found 

that Korean subjects possess CP for three categories, chorok, cheongnok and parang, in the 

blue-green region. In accordance with OPT, we can assume that Korean subjects perceive 

two unique hues in this region: so-called ‘unique blue’ and ‘unique green’. Once again, 

however, these hues could not possibly form the focal points for the colour categories blue or 

green for these subjects, for the simple reason that Korean subjects do not possess CP for 

36 McDaniel ([1972]) first proposed the link between colour category foci and the unique hues. See Wooten & 
Miller ([1997], p. 83). MacLaurey ([1997], p. 263) discusses some problems with this proposal. Philipona & 
O’Regan ([2006]) provide an ecological argument for the origin of the unique hues, focusing on certain 
illumination-invariant regularities in reflectance properties. 

34 
 

                                                        



blue or green. There is no straightforward implication, then, from having experiences of 

colour that can be characterised by phenomenal OPT, to possessing CP for the colour 

categories blue, green, and so on. 

Extending this point, we can see that phenomenal OPT does not imply the possession 

of CP for any colour categories whatsoever. The above cases all involve subjects who possess 

CP for more colour categories than ordinary English speakers. We can easily imagine cases 

in which subjects have CP for fewer colour categories than English speakers, but who still 

perceive the four unique hues posited by OPT. A moderate case would be a subject who 

possessed CP for a category that spanned both blue and green, and hence included both the 

‘unique green’ and ‘unique blue’ hues. A more extreme case would be a subject like 

NONCAT who lacked CP for any colour categories whatsoever, and yet still had experiences 

as of the four unique hues. These points undermine any constitutive connection between 

phenomenal OPT and CP for colour. By the same token, these cases demonstrate how 

misleading it is to label the unique hues postulated by OPT with the category terms ‘red’, 

‘green’, ‘yellow’, and ‘blue’. This response nullifies the biggest threat to premise five, and 

thus clears the way for my conclusion that colour vision does not constitutively involve the 

ability to see colours in the natural, categorically committed, sense. 

The reader at this point might remain unconvinced. Surely, one might object, if a 

normal perceiver perceives unique blue and unique red and also perceives non-unique blues 

and non-unique reds, then she ipso facto perceives colour categories. My response is simply 

to reiterate the adopted characterisation of CP for colour, which requires the presence of 

differential similarity effects between shades of colour at category boundaries. According to 

this standard, even if NONCAT perceives shades that English speakers would call ‘unique 

red’ and ‘non-unique red’, then so long as she does not exhibit the canonical differential 

similarity effects between these shades, she does not possess CP for red. 

35 
 



 

4. Whorf and the View from Cognitive Neuropsychology 

While colour CP has not featured prominently in philosophical discussions about colour, the 

psychological origin of this ability has been a matter of considerable debate in cognitive 

psychology. The argument of the previous Section introduced evidence for cross-linguistic 

variations in CP, as in the examples of the categorisations made by Russian and Korean 

speakers. These results are part of a recent revival of relativist or Whorfian views of 

perceptual categorisation.37 This marks a significant departure from the Universalist 

orthodoxy of the last forty years, on which the colour categories apparent in our ordinary 

visual experience are deemed universal, presumably very low-level, features of colour vision 

systems. Universalism has undoubtedly permeated philosophy to some degree, perhaps partly 

explaining the widespread assumption that the ability to see colour categories such as red and 

green is somehow inherent to, or part of the nature of, colour vision.38 In its strongest form, 

the Whorfian view holds that all of our perceptual categories are determined (in some sense) 

without constraint by the language that we speak. In Whorf’s words ([1940], p. 213), 

 

The categories and types that we isolate from the world of phenomena we do not find 
there because they stare every observer in the face. On the contrary the world is 
presented in a kaleidoscopic flux of impressions which have to be organised in our 
minds. This means, largely, by the linguistic system in our minds. 

 

37 For a summary, see Kay & Regier ([2006]). 
38 The Universalist view originates in the seminal cross-cultural study of Berlin & Kay ([1969]). Arguably the 
most widely read and influential philosophical introduction to the science of colour vision, Hardin’s Colour for 
Philosophers, presents the standard Universalist line at ([1988], pp. 156-169).  

36 
 

                                                        



Nobody today would endorse such an extreme view.39 Nevertheless, cross-cultural studies of 

the sort carried out by Winawer and colleagues ([2007]) have been taken to support a weaker 

neo-Whorfian thesis, 

(WT) Some structural differences between colour language systems will be paralleled 

by differences in the colour categories apparent or represented in the visual 

experience of the native speakers of these languages. 

I now introduce some fascinating cognitive neuropsychological cases which have been cited 

as further support for the neo-Whorfian view. Such cases demonstrate that the possibility of a 

total dissociation between core colour vision abilities and CP for colour, on which my 

argument rests, is not only coherent, but also not nearly as distant as one might think. I should 

stress, however, that my central argument is not at all dependent on the truth of neo-

Whorfianism, which is just as well given the highly contentious nature of the view.40 

Reflecting on the neo-Whorfian view is simply helpful in opening our minds to certain 

possibilities as to how things might have been with respect to our colour categorisation 

abilities. 

Some studies in cognitive neuropsychology have addressed a rare condition known as 

visual colour anomia or optic aphasia for colours. Aphasias are linguistic impairments 

typically resulting from a head injury or stroke. Anomia is a type of aphasia involving a 

specific impairment in the production of names or verbal labels. Optic aphasia for colours is 

partly characterised by impairment in the naming of colours. Unlike colour aphasia, which is 

a purely verbal deficit in producing or comprehending colour terms, optic aphasia for colours 

is a visuo-verbal deficit which also results in impairments in the subject’s ability visually to 

39 Compare Quine ([1948], p. 11). 
40 Some of the results taken to motivate a neo-Whorfian view have been called into question. For example, 
Witzel & Gegenfurtner ([2011]) report that attempts to replicate Gilbert et al.’s ([2006]) finding that CP is 
lateralised to the right visual field have been inconsistent. Franklin & Davies ([2004]) also present evidence for 
colour categorisation in pre-linguistic infants, which is hard to explain on a thorough-going Whorfian view. 

37 
 

                                                        



categorise colours. The earliest reports of such cases appear in work by the German 

neurologists Sittig ([1921]), Goldstein and Gelb ([1918]), who described it as an ‘impairment 

of the categorical attitude’ towards colour, and Lange ([1936], p. 88), who says that, 

 

The patients do no behave conceptually but in terms of concrete reality[...] The 
patients have no principle of classification in colour sorting, but depend on the 
experience of coherence between patterns and the other colours. 

 

More recently, Beauvois and Saillant ([1985]) distinguished optic aphasia for colours from 

the more general linguistic deficits associated with colour aphasia, and also from colour-

specific visual agnosia. 

Beauvois and Saillant ([1985]) describe the case of MP, who is diagnosed as having 

optic aphasia for colours. MP’s visual and linguistic abilities with respect to colour were 

assessed via purely verbal, purely visual, and combined visuo-verbal tasks. MP performed 

within the normal range on tasks that required purely verbal processing of colour information. 

The purely visual tasks included colour matching tests, in which subjects are given two wool 

skeins and asked to decide if they are identical or not, and standard pseudo-isochromatic 

plates for colour discrimination. Once again MP’s performance on such tasks was within the 

normal range. In contrast, MP’s performance on visuo-verbal tasks was extremely impaired. 

In one task, MP was asked to produce a verbal colour label in response to a visually presented 

colour sample, or to point to a colour sample given a verbal request. MP’s errors here were 

gross and difficult to understand: she labelled a bright red sample as ‘light yellow’, pointed to 

navy blue when asked to point to orange, and pointed to pale blue when asked to point to the 

colour of parsley. MP was also assessed on the Gelb-Goldstein colour sorting test, which 

requires the subject to ‘select a skein of wool he likes and to pick out all the skeins which he 

38 
 



believes can be grouped together with the chosen one’ ([1985], p. 15). Once again, MP’s 

performance was highly unusual ([1985], p. 15), 

 

[S]he started with a pale grey and put it with the other grey, the white and the dark 
wools; then with them she put, in the following order: A pale pink, a pale yellow, a 
brown; she continued by adding all the pale pink and brown wools, and she finished 
by putting all the wools in the same category... On part 4 of the test, in which she was 
asked to select just the red wools, she picked up all the wools except for 10 (white, 
black, grey, one brown, four grey and two green); she seemed to select them without 
any apparent logic in order (e.g., she picked out a green wool just after a pink one, or 
a red just after a green). 

 

It is significant to note that both of these visuo-verbal tasks are paradigmatic explicit tests for 

colour CP, which I mentioned in Section 3.2. MP’s behaviour thus presents an intriguing 

dissociation between purely visual discriminatory abilities with respect to colour, on the one 

hand, and performance on explicit tests for colour CP, on the other. Despite their extreme 

problems with explicit colour categorisation tasks, however, patients with optic aphasia for 

colour often perform within the normal range on implicit categorisation tasks, including the 

canonical test for differential similarity judgements across category boundaries.41 By the 

lights of the assumed standard for CP in this paper, then, these subjects class as possessing 

CP for colour. 

As Roberson and colleagues ([1999]) point out, a probable cause of this unusual 

categorising behaviour is the subject’s linguistic deficit. Roberson and colleagues thus take 

such cases as evidence for the neo-Whorfian view of colour categorisation. As noted above, 

the success or failure of such empirical arguments for neo-Whorfianism is not my concern. 

The relevance to the present argument is that these cases provide a significant validation of 

41 See the discussion of the patient LEW in Roberson et al. ([1999]). It is not unusual for patients with visual 
deficits as measured by performance on explicit tasks to perform relatively normally on implicit tasks. Nijboer 
([2007], pp. 144-145) notes related findings in blindsight, object agnosia, and prosopagnosia. 

39 
 

                                                        



the pivotal dissociative possibility discussed in the previous Section involving total NCP for 

colour. Although MP meets the official standard for CP assumed in this paper, her radically 

impaired performance on explicit categorisation tasks suggests that she lacks just about every 

other ability usually associated with CP for colour. Such subjects thus come extremely close 

to the sort of dissociation between constitutive colour vision abilities and CP envisaged in the 

case of NONCAT. It is certainly no stretch to imagine a subject just like MP also failing 

implicit tests for CP, which would suffice to establish the relevant possibility. Despite the 

apparent strangeness, then, perhaps the scenario involving NONCAT is not as distant as one 

initially might think. 

 

5. Conclusion 

I have motivated and implemented a constitutive approach to understanding colour vision. 

The constitutive approach seeks to understand colour vision qua psychological kind, as 

opposed to a primarily phenomenological or experiential phenomenon. I proposed colour 

constancy as a core constitutive condition on colour vision. Constancy in turn was explicated 

via DV as a discriminatory, yet fully representational, capacity. I then argued that the 

possession of CP for colour, a pervasive and vivid feature of ordinary visual experience, is 

not implied by the possession of these constitutive discriminatory abilities. In sum, colour 

vision does not by its nature confer the ability to see colours in a natural, categorically 

committed, sense. This claim was further motivated by evidence of closely related 

dissociations in cognitive neuropsychology. As is often the case, the unexpected actual 

extends our knowledge as to what is possible. Such cases confirm that our ordinary 

experience of colour possesses a latent complexity that is obscured by ordinary experiential, 

introspective, approaches to understanding colour vision. 

40 
 



Many issues deserving of extended discussion inevitably have been omitted or passed 

over too briefly. One issue of outstanding importance concerns the implications of the 

constitutive approach for colour ontology. If my argument is sound, then the ability visually 

to represent categorical colour properties such as red and green is not part of the constitutive 

nature of our colour vision abilities. Somewhat paradoxically, this suggests that colour 

ontology similarly will be free of the properties that we ordinarily describe simply as the 

colours.42 Further discussion of these consequences, however, will have to await another 

occasion. 

 

Acknowledgements 

For extremely helpful comments on this paper (and its predecessors) I would like to thank 

Alex Byrne, Mazviita Chirimuuta, John Hawthorne, Mark Kalderon, John Morrison, Scott 

Sturgeon, and Timothy Williamson; an audience at the Oxford Philosophy of Mind Work in 

Progress group; and two anonymous referees for this journal. 

 

Oxford Uehiro Centre for Practical Ethics 

Suite 8, Littlegate House 

St Ebbes Street, Oxford OX1 1PT 

will.davies@philosophy.ox.ac.uk 

 

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