Dissociable Realization and Kind Splitting


Dissociable Realization and Kind Splitting
Author(s): Carl F. Craver
Source: Philosophy of Science, Vol. 71, No. 5, Proceedings of the 2002 Biennial Meeting of The
Philosophy of Science Association<break></break>Part II: Symposia
Papers<break></break>Edited by Sandra D. Mitchell (December 2004), pp. 960-971
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Philosophy of Science, 71 (December 2004) pp. 960–971. 0031-8248/2004/7105-0028$10.00
Copyright 2004 by the Philosophy of Science Association. All rights reserved.

960

Dissociable Realization and Kind
Splitting

Carl F. Craver†‡

It is a common assumption in contemporary cognitive neuroscience that discovering
a putative realized kind to be dissociably realized (i.e., to be realized in each instance
by two or more distinct realizers) mandates splitting that kind. Here I explore some
limits on this inference using two deceptively similar examples: the dissociation of
declarative and procedural memory and Ramachandran’s argument that the self is an
illusion.

1. Introduction. The main point of clarifying the concept of realization
is to better understand how our taxonomy of kinds ought to track or
accommodate what we know about their realizers. Discussion of this
accommodating relation has focused almost exclusively on cases of mul-
tiple realization—cases in which the same kind could be realized by dif-
ferent realizers in different instances. Yet there has been almost no dis-
cussion of dissociable realization—cases in which instances of what was
previously thought to be a single kind are discovered to be realized by
more than one distinct and independent realizer. It is a common (but
rarely explicit) methodological assumption in neuroscience and elsewhere
that discovering a kind to be dissociably realized mandates splitting the
kind into as many as there are dissociable realizers.

Here I explore some limits on this assumption using two cases: one
that is accepted as an exemplar of successful application of this principle,
and one that is a bit more suspect. The first is the dissociation of pro-
cedural and declarative memory and the corresponding inference that
memory is not a single kind. The second is V. S. Ramachandran’s (1998)
argument that “the self is an illusion”: because the self is dissociably

†To contact the author, please write to: Department of Philosophy, Philosophy Neu-
roscience and Psychology, Washington University, One Brookings Dr., Busch Hall,
Rm. 225, St. Louis, MO 63130; e-mail: ccraver@artsci.wustl.edu.

‡Thanks to Lindley Darden, Stuart Glennan, Jeff Poland, Rob Wilson, Peter Ma-
chamer, and Barbara Von Eckardt for comments on earlier drafts.



DISSOCIABLE REALIZATION AND KIND SPLITTING 961

realized, what we previously thought of as “the self” should be split into
as many selves as there are realizers—which is a bit like having no self
at all. The arguments are deceptively similar and the empirical evidence
in each case is equally compelling. Yet there is a way to avoid the con-
clusion in the second without jeopardizing the inference in the first. The
difference between the two turns on the fact that each exemplifies a dif-
ferent variety of realization, one of which sustains the argument for split-
ting and one of which does not.

2. Splitting Memory. Memory, as understood from the perspective of con-
temporary cognitive neuroscience, is not a single kind of thing. This con-
viction is typically justified by noting that different kinds of memory can
be “dissociated” from one another through brain damage or experimental
manipulation. One example is the splitting of declarative and procedural
memory.

The argument for splitting typically opens with the story of H. M.
(Scoville and Milner 1957). In an effort to relieve life-threatening epileptic
seizures, H. M. consented to experimental surgery removing bilaterally a
structure known as the hippocampus. H. M. can still read and write, he
can learn new skills, he can remember much of his childhood, and his IQ
if anything increased after the surgery. The tragedy of H. M.’s life is that
he permanently lost the ability to retain new memories for facts and events.
He has lost declarative memory but has maintained procedural memory
(see Corkin 2002). From the case of H. M. and from studies in nonhuman
animals, researchers have concluded that the mechanisms for procedural
and declarative memory are distinct. Since the two are realized by distinct
mechanisms, the kind “memory” is confused, lumping together what we
now know to be two distinct kinds of memory.

Arguments from dissociable realization are central to the contemporary
neuroscience of memory. Daniel Schachter writes:

we have now come to believe that memory is not a single or unitary
faculty of the mind, as was long assumed. Instead, it is composed of
a variety of distinct and dissociable processes and systems. Each
system depends on a particular constellation of networks in the brain
that involve different neural structures, each of which plays a highly
specialized role within the system. (1996, 5)

Similar arguments can be found in Weiskrantz 1990, Squire 1992, Schac-
ter and Tulving 1994, and Squire and Knowlton 1994. Philosophers of
neuroscience have used this case as an illustration of the ability of neu-
roscientific research to drive conceptual revision in psychology (e.g., Bickle
1998, Churchland 1986).

In each case, the authors argue for kind splitting on the basis of a



962 CARL F. CRAVER

premise about dissociable realization. The argument might be represented
as follows:

M1. Declarative memory and procedural memory can be indepen-
dently disrupted.

M2. If phenomena X and Y can be independently disrupted, then
they have distinct realizers.

M3. Procedural and declarative memory have distinct realizers.

M4. No Dissociable Realization (NDR). Instances of a natural kind
have one and only one realizer. If there are two distinct realizers for
a putative instance of a kind, there are really two kinds, one for each
realizer.

M5. So procedural and declarative memory are distinct kinds of
memory.

Here I will be concerned primarily with NDR. NDR is stated as a
sufficient condition for kind splitting; of course there are other kinds of
evidence as well. Researchers studying memory (and cognition generally)
have a variety of accepted techniques for distinguishing cognitive phe-
nomena, thereby leaping from M1 to M5 directly (an example opens the
next section). The distinction between procedural and declarative memory
predates H. M. But the case of H. M. (and subsequent research) seemingly
removed any doubt: the declarative/procedural distinction is reflected in
the architecture of the brain. NDR is phrased to reflect the force of such
evidence for kind splitting.

NDR stops short of requiring one-to-one correspondence between re-
alized kinds and their realizers. It asserts only that dissociating realizers
mandates splitting kinds. It is consistent with NDR that there might be
two realized kinds and one realizer. (Plaut 1995 argues that one can in-
dependently disrupt memory categories that share a common neural net-
work realizer.) NDR also leaves it an open question whether the unifi-
cation of two or more realizers that were previously presumed to be
disparate would mandate unifying the realized kinds (as in, perhaps, the
case of rusting, burning, and breathing). And finally, NDR takes no stance
as to whether discoveries about realized kinds should influence views
about lower-level realizers. These are all important (and underdiscussed)
questions; but here I confine my attention primarily to NDR.

3. Splitting the Self. In Phantoms in the Brain, V. S. Ramachandran (1998)
provides a structurally similar argument that our sense of self is an illusion.
He develops his argument with the aid of “Titchener’s circles” (Figure 1).
A circle looks larger when surrounded by small circles than when it is



DISSOCIABLE REALIZATION AND KIND SPLITTING 963

Figure 1. Titchener’s circles.

surrounded by large circles. The effect, which can be measured by allowing
subjects to change the size of one center circle until it visually matches
the other center circle, can increase the apparent size of the center circle
by as much as 30%. Recent experiments by Salvatore Aglioti et al. (1995)
show that if the experimental subject reaches for a poker chip the size of
the center circle, the fingers are right every time: the forefinger and thumb
open the right amount to pick up the chip, and they open as much when
the circle is surrounded as when it is alone. It appears that the system
reporting perceptual experience and the system doing the reaching are
distinct: one is fooled; the other is not.

This behavioral argument for dissociation is reinforced by evidence
from people who have brain damage. Consider David Milner’s case-study,
DF, who suffered anoxic brain damage when carbon monoxide from her
water heater overwhelmed her (Milner and Goodale 1995). When she
came to, she was blind. Although she gradually regained the ability to
recognize colors and textures, she could not visually recognize the shapes
of objects or faces. She says that she can’t see, and she fails tests of object
recognition and naming. Nonetheless, she can perform complex reaching
tasks—such as putting a letter through a mail slot or reaching for a
pencil—without error. And if you ask DF to reach for the center chip,
her hands aren’t fooled. DF’s lesion uniquely damaged her visual system’s



964 CARL F. CRAVER

ventral pathway (featuring inferotemporal cortex), which houses crucial
mechanisms involved in consciously recognizing “what” objects are. Yet
the event spared her dorsal pathway (featuring the posterior parietal cor-
tex), which houses crucial mechanisms involved in guiding visual inter-
actions with objects (and so is sometimes called the “how” pathway).
Ramachandran concludes:

The most obvious fact about existence is your sense of being a single,
unified self “in charge” of your destiny; so obvious, in fact, that you
rarely pause to think about it. And yet Dr. Aglioti’s experiment and
observations of patients like DF suggest that there is in fact another
being inside you that goes about his or her business without your
knowledge and awareness. And as it turns out, there is not just one
such zombie but a multitude of them inhabiting your brain. If so,
your concept of a single “I” or ‘Self” inhabiting your brain may be
simply an illusion—albeit one that allows you to organize your life
more efficiently, gives you a sense of purpose and helps you interact
with others. (83–84)

Although the self may be a “practical kind” (helping to organize our lives
and our commerce with others), it is not a natural kind. Ramachandran
clarifies his conclusion: “Here and elsewhere, when I say that the self is
an ‘illusion’ I simply mean that there is no single entity corresponding to
it in the brain” (272). Ramachandran does not notice an inferential gap
between the claim that there is no single entity in the brain corresponding
to the self and the claim that there is no such thing as the self (i.e., that
our sense of self is illusory). He does not notice it because the inference
is so imbedded in the methodological medium of contemporary biology
and cognitive neuroscience.

Ramachandran’s argument is superficially similar to the argument for
splitting memory:

S1. Conscious visual object recognition and visually guided action
can be independently disrupted.

S2. If phenomena X and Y can be independently disrupted, then X
and Y have distinct realizers.

S3. So conscious visual object recognition and visually guided action
have distinct realizers.

S4. (NDR). Instances of a natural kind have one and only one re-
alizer. If there are two distinct realizers for a putative instance of a
kind, there are really two kinds, one for each realizer.

S5. There is no single thing, the self, that is responsible for both



DISSOCIABLE REALIZATION AND KIND SPLITTING 965

conscious visual object recognition and visually guided action; the
notion of a single unified self is an illusion.

If anything, a stronger case can be made for premise S3 in the case of
the self than can be made in the case of memory. After all, the central
dogma of contemporary neuroscience holds that functions localize in the
brain, and if that is right, then the brain is composed of distinct and
independent mechanisms realizing more or less distinct and independent
kinds of phenomena. If NDR is right, the conclusion follows.

But the conclusion is suspect. There are many areas of neuroscience
and psychology where the concept of the “self” is not merely respectable
but fashionable: research on alien thoughts and actions, dissociative iden-
tity disorder, development of the self-concept, egocentric maps of space,
episodic or autonoetic memory, kinesthetic and proprioceptive self-aware-
ness, mirror self-recognition, and the recent panoply of work on con-
sciousness and the biological basis of self-reference—these are all areas
of cognitive science in which the notion of the self plays a central role in
the causal and inferential structures described by their theories. Perhaps
Ramachandran would take this diversity of topics as reinforcing his point:
each of these fields focuses on a different capacity with different realizers
and labels it, exclusively, the self. But one resisting Ramachandran’s eli-
minativist conclusion (as I am inclined) has available the option of re-
jecting NDR. Why can’t the self simply be the bundle of these dissociable
capacities?

There are other clear violations of NDR. We did not give up on mice
or frogs as kinds (or split them into numerous distinct and independent
kinds of things) when we learned that instances of them were made up
of innumerable distinct mechanisms. Nor did we give up on hearts, or
kidneys, or solar systems. In fact, in cases such as these, the robustness
underwriting their status as kinds depends precisely on the fact that they
are associated with a cluster of regularly co-occurring properties and ac-
tivities. This association between clusters of properties and kinds has been
persuasively urged by, for example, Boyd (1999), Griffiths (1997), and
Wilson (2001). Further, some of our best evidence for belief in different
kinds of entities comes from the fact that they can be detected and ma-
nipulated with multiple causally and theoretically independent techniques.
These considerations, as I will argue in Section 5, suggest a principled
way to restrict NDR so as to block the conclusion for mice, frogs, and
selves that follows so naturally for memories. But first, we must consider
a necessary condition for splitting realized kinds.

4. Constitutive, Contextual, and Etiological Realizers. Realization is typ-
ically understood as a constitutive relationship. The realized phenomenon



966 CARL F. CRAVER

is some activity W of an object or system S in context C, and W’s con-
stitutive realizer is located within S’s spatial boundaries. Wilson (2001)
rightly points out that a “constituency” thesis is part of nearly every
explicit analysis of realization in contemporary philosophy. But, as Wilson
notes, many of the activities in which objects engage cannot be exhaus-
tively (sufficiently) realized by the parts of those objects. Sometimes the
activities are realized (at least in large measure) by causal (or other) re-
lations between S and its context. And sometimes activities are realized
(at least in part) by the historical processes leading up to them. To ac-
commodate these possibilities, we may wish to recognize cases of contex-
tual realization and cases of etiological realization (cf. Craver 2001, section
4) as distinct from the more familiar constitutive variety. Each of these
approaches has figured prominently in discussions of multiple realizability.
Fully articulating either would require considerable fine tuning. Yet the
effort is unlikely to pay off in the present cases. The contextual and
etiological realizers in each of our cases are as dissociable as the consti-
tutive realizers.

Consider memory first. Declarative memories are triggered by the pre-
sentation of facts or the occurrence of events in the life of the person,
and they play important roles in, for example, conversation, autobiog-
raphy, or the simple act of reminiscing. Nondeclarative forms of memory
(like procedural memory, iconic memory, priming, etc.) have their own
unique triggering conditions (procedural memories are acquired by doing
things, iconic memories by visual impressions, etc.) and play different
roles in the life of the organism. These differences are reflected in the
different kinds of stimuli used to produce and evoke memories of the
different types. Similarly, the etiological realizers of declarative and pro-
cedural memory are likely to be distinct and independent as well. Phy-
logenetically, procedural forms of memory appear earlier than declarative
memory (they can be found, for example, in sea slugs). Ontogenetically,
skills, habits, and priming predate the ability to lay down lasting memories
for facts and events. Appealing to contextual or etiological realizers in
the memory case reinforces the splitter inference.

Similar arguments can be made for the two systems dissociated in Ra-
machandran’s argument about the self. Contextually, the inferotemporal
“what” system damaged in DF is privy to different information than the
dorsal “how” system. And it is clear from Aglioti’s experiments that the
two systems are able to support different sorts of behavior in the world.
The “how” system is contextually linked to reaching and the “what”
system is contextually linked to object recognition. Etiologically, the
“how” system is phylogenetically much older than the “what” system,
and the two develop ontogenetically at different time scales. So, as with



DISSOCIABLE REALIZATION AND KIND SPLITTING 967

the memory case, appeal to contextual or etiological realizers only rein-
forces the dissociation of realized kinds.

Differences in contextual or etiological realization do seem necessary
for kind splitting. Maximally redundant dissociable realizers illustrate this
point. The mechanisms of each kidney can be disrupted independently of
those in the other, they are the same kinds of mechanism, and they both
do the same thing. It seems reasonable to say that there are two mech-
anisms of the same kind jointly filtering the blood rather than that there
are two kinds of filtering. After all, discussions of multiple realizability
center on the fact that even different kinds of realizers might realize the
same realized kind. A more complicated case involves two realizers, each
of which is necessary for a realized kind, neither of which is sufficient for
the kind, and neither of which produces any intelligible behavior operating
in isolation (each lurches and hiccups incomprehensibly in the absence of
its partner). In this case, failure to find an intelligible way to distinguish
two realized phenomena by attention to their contextual or etiological
realizers may trump the discovery that the constitutive realizers are dis-
sociable. Each example illustrates that our willingness to split kinds ex-
tends only to those cases in which it is possible to distinguish them con-
textually and etiologically as well.

5. Varieties of Constitutive Realization. Focusing attention back on con-
stitutive realizers, one could perhaps restrict NDR by recognizing different
varieties of constitutive realization, depending upon what is getting re-
alized and what is realizing (cf. Poland 1994). In particular, kinds of
entities stand in a different relationship to their realizers than do kinds
of properties or activities. The distinction between the two rests on the
importance of explanatory relevance in articulating the latter, but not the
former, sort of realization. It is open to those inclined against Rama-
chandran’s argument to hold that the self, conceived as an entity like
mice or frogs, need not track realizing material constituents, properties,
or mechanisms.

Some realized kinds are entities or objects, e.g., gems, corkscrews, and
computers. The realizers of entities are typically described as material
realizers. Material realizers are constituents: the material parts (however
those are to be individuated) within the entity’s spatial boundaries. Neph-
rite is said to be realized by its molecular constituents, corkscrews by steel,
and computers by silicon. Such simplified examples typically abstract from
the messy details scientists confront; they turn heterogeneous material
realizers (in which S is realized by many different kinds of materials) into
homogeneous material realizers (in which the stuff realizing S is the same
throughout). Either of the jades is a good example of the latter. But steel
corkscrews can have silver engravings, and computers contain a great deal



968 CARL F. CRAVER

besides silicon. Most entity kinds in our world have heterogeneous ma-
terial realizers, and it is rare that the discovery of heterogeneous material
realization leads us to split realized entity kinds. Precious metals and gems
are items for which homogeneity frequently counts, but there is little
temptation to split Aplysia, pyramidal cells, or even water into more than
one distinct kind simply because we learn that they are composed of
heterogeneous substances. The idea that these kinds would split does not
arise naturally from consideration of theory building in science.

Learning how entities are materially realized is often an important
opening stage of science. Cell fractionation and centrifugation were es-
sential to the development of biochemistry and molecular biology, for
example, precisely because they allowed investigators to discover cellular
constituents. But when attention shifts from describing to explaining, en-
tities and their material realizers are not the primary focus. Instead, at-
tention shifts to the properties or activities as realized kinds and to com-
ponent parts, their properties, and their activities as realizers.

Realizers for properties and activities lie along a continuum between
aggregative and more mechanistic cases. Aggregate realization is reali-
zation by constituents where the property of the whole is a simple sum
of the properties of the parts (see Wimsatt, 1997). The mass of a pile of
sand is realized aggregatively by the masses of the individual grains (other
units are equally useful). In aggregate cases, we do not typically split kinds
when we split lower-level realizers. If we found that our pile of sand were
made up of sands of ten different grains (coarse to fine), we would not
be tempted to say that there are ten masses (any more than we would be
tempted to say that there are right and left masses or bottom and top
masses); and given that there are arbitrarily many ways to divide up the
total mass into constituent units, there would be arbitrarily many ways
of dividing kinds of mass. Note, though, that even in cases of aggregate
realization, realizers include only some of the properties of the constit-
uents: those that are explanatorily relevant to the realized property or
activity. It is the mass of the individual grains (and not their color, melting
point, or texture) that explains the mass of the whole pile.

In cases of mechanistic realization, the burden of realization is borne
by some constituents (working parts or components) more than others,
and organization among the components figures increasingly in our de-
scription of the realizer. The realized kind might be some activity, such
as sensitization in Aplysia, a pyramidal cell’s firing, or a computer program
running. Realizing mechanisms are composed of the working parts of the
mechanism (such as amino acids, neurons, and filters) and their activities,
the things that these entities do (their binding, firing, filtering). These
components are organized such that they produce the behavior of the
mechanism as a whole (Machamer et al. 2000). This organization may be



DISSOCIABLE REALIZATION AND KIND SPLITTING 969

spatial (locations, orientations, shapes) or temporal (order, rate, duration)
or active (who acts and interacts with whom when) (cf. Bechtel and Rich-
ardson 1993; Glennan 1996; Thagard 2000; Cummins 1975). There are
no mechanisms simpliciter; all mechanisms are mechanisms of something,
and it is by reference to that something that the relevance of components
is established. Relevance thus has a central place for mechanistic realizers
as well as for property realizers.

In general, realized entity kinds track neither kinds of realizing prop-
erties nor kinds of realizing mechanisms. As noted above, entity kinds
are typically characterized in terms of clusters of co-occurring properties
and our best evidence for the existence of an entity comes from the multiple
independent lines of evidence for detecting and manipulating it via dif-
ferent of those properties. And entities don’t track realizing mechanisms
for exactly the same reason: Entities (especially complex entities) tend to
be associated with myriad lower-level mechanisms. There is no mechanism
of the Aplysia, nor a mechanism of the cell; rather, they are composed of
innumerable mechanisms for each of the many things that Aplysia and
cells do. Explanatory relevance is central to our notion of realization for
properties and activities (Lepore and Loewer 1989 and Kim 1993 include
explanatory relevance in their account of realization), but not to reali-
zation for entities. We do not seek explanations for Aplysia, the cell, or
computers. We do not ask “How the Aplysia?” or “Why the pyramidal
cell?” but rather, “How do Aplysia sensitize to repeated tail shocks?” and
“Why do pyramidal cells have their characteristic dendritic arboriza-
tion?”1 In the realization of entities, spatial containment defines what is
included in the realizer and what is not; what is in the realizer are the
constituent parts. In the realization of properties and activities, “in” and
“out” mean something different: in this case “in” means that the item is
explanatorily relevant to the realized kind.

This distinction provides some basis for distinguishing cases where
NDR holds from those where it does not, one that we might apply to
avoid Ramachandran’s conclusion without jeopardizing the memory case.
The different kinds of memory (declarative, procedural, sensitization,
priming, etc.) are different kinds of activities. The self (as typically con-
ceived) is an entity. If we take complex composite objects as our kinds,
we are almost always going to find them to be realized by several inde-
pendent kinds of constituents, their properties are almost always going
to be realized by several independent sets of properties, and their activities

1. In some cases, a constitutive realizer explains why the realized is the kind of thing
that it is. This thing’s being water is explained by its being composed of H2O molecules,
this rock’s being jade is explained by its being composed of a silicate of magnesium
and calcium. Being water and being jade are properties and should be handled as such.



970 CARL F. CRAVER

are going to be explained by different and in many cases nonoverlapping
mechanisms. Entity kinds do not typically track kinds of constituent sub-
stances, kinds of constituent properties, or kinds of constituent mech-
anisms.

In clear cases of dissociation, we are able to identify two or more distinct
sets of relevant entities, properties, and/or activities, one set for each of
two or more independently detectable realized kinds. The parts, properties,
and activities that we appeal to in explaining procedural memory are not
all of them relevant to explaining declarative memory and vice versa. But
the realizers of mice and Aplysia are localized sums of materials, prop-
erties, and activities, and such sums can be formulated without special
reference to the relevance of those constituents to the realized kind.

The self is no exception. Like most complex entities, selves are com-
plexes of diverse capacities that are only loosely associated with one an-
other. Selves are conceived as centers of conscious awareness, as agents
of actions, as authors of narratives, as seats of autobiographical memory,
as things that can refer to themselves, and as things that can recognize
themselves in mirrors. They could not be all of these things without a
variety of different capacities, many of which can be explicated by mech-
anisms that are independent. Think of the impact on the self suffered by
people with brain damage. When H. M. lost his declarative memories, he
lost one of these capacities, but by no means lost his self. (This is so even
on a relatively simple formulation of Locke’s memory criterion, given
that H. M. maintains memory ties to himself prior to the surgery.) Memory
is but one of the psychological capacities contributing to a self. Rama-
chandran’s patient DF has lost conscious object recognition and main-
tained visually guided behaviors. The corpus of contemporary neurosci-
ence is replete with tragic cases of people who, by virtue of having suffered
damage to parts of their brains, have lost the ability to engage in activities
that lie very close to the heart of what we call “the self.” But in truth,
normal selves, prototypically, are (or, better, do) all of these things.

6. Conclusion. The problem of dissociable realization is to come up with
a principled way to restrict NDR to only those cases in which it is truly
appropriate. Here I have identified a range of cases in which NDR ar-
guably fails. These different cases correspond to different varieties of
realization at work in contemporary cognitive neuroscience. The meta-
physics of realization ought to help us sort out the problems that arise
in the process of refining our taxonomies of kinds. To play such a role,
the metaphysics ought to be sensitive to different varieties of realization
encountered in the process of building theories and ought to recognize
the possibility that those different varieties of realization may carry vastly
different implications for our taxonomy of kinds.



DISSOCIABLE REALIZATION AND KIND SPLITTING 971

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