Against Selective Realism(s)

D. Tulodziecki∗†

Draft/October 2016

Abstract: It has recently been suggested (for example, Lyons 2006) that
realist responses to historical cases featured in pessimistic meta-inductions
are not as successful as previously thought. In response, selective realists have
updated the basic divide et impera strategy specifically to take such cases
into account, and to argue, on this basis, that more modern realist accounts
are immune to the historical challenge (cf. Vickers 2013). Using a case-study
– that of the 19th century zymotic theory of disease – I argue that these
updated proposals fail, and that even the most sophisticated recent realist
accounts are just as vulnerable as their predecessors.

1 Introduction

The pessimistic meta-induction (PMI) targets the realist’s claim that a the-
ory’s (approximate) truth is the best explanation for its success. It attempts
to do so by undercutting the alleged connection between truth and success
by arguing that highly successful, yet wildly false, theories are typical of the
history of science.1 There have been a number of prominent realist responses
to the PMI, most notably those of Worrall (1989), Kitcher (1993), and Psil-
los (1999). All of these responses try to rehabilitate the connection between
a theory’s (approximate) truth and its success by attempting to show that
there is some kind of continuity between earlier and later theories. One of the
most widely discussed proposals has been Psillos’s divide et impera strategy.

∗tulodziecki@purdue.edu
†Department of Philosophy, Purdue University, West Lafayette, IN
1For a recent and new take on the PMI, see Frost-Arnold (2014).

1

mailto: tulodziecki@purdue.edu


Psillos argues (1999, Chapter 5), first, that realists ought to make the notion
of a theory’s success more stringent so as to include use-novel predictions,
and, second, that realists face trouble only if it can be shown that those
elements of a theory that “really fuel” that theory’s genuine success are re-
jected and turn out to be completely false. It has recently been suggested –
for example, by Lyons (2006) –, however, that Psillos’s strategy is not as suc-
cessful as previously thought. Lyons tests Psillos’s move against a number of
historical cases, and concludes that this “form of realism remains threatened
by the historical argument that prompted it” (537). In response to Lyons,
recent realists such as Vickers (2013) have argued that, once the selective
realist strategy is updated appropriately, more modern realist accounts can,
in fact, meet the challenge that Lyons has set.

In this paper, I argue that even recent, sophisticated realist accounts such
as that of Vickers fail to be immune to the historical challenge and are just as
vulnerable as their predecessors. I make my point by providing an example of
such a case – that of the 19th century zymotic theory of disease, predecessor
to the germ theory – and by carefully showing that this theory was highly
successful in the realist’s sense of ‘genuine success’. I explain in detail what
elements of the theory were responsible for its successes, by providing deriva-
tions of its predictions and the theoretical posits involved in making these
predictions, and then show that the elements responsible for its success and
that “really fueled” the relevant derivations were discarded in later theories
and turned out to be completely false.

I will proceed as follows: In Section 2, I provide an overview of the zymotic
theory of disease; in Section 3, I discuss its successes. Section 4 deals with
the updated realist challenge and Section 5 is concerned with a derivation of
the zymotic predictions and those posits of the zymotic theory that brought
them about. In Section 6, I show how the updated realist challenge can be
met, before concluding, in Section 7, that even the most sophisticated recent
realist accounts are in trouble just as much as their predecessors.

2 The Zymotic Theory

The zymotic theory was the most sophisticated version of the miasma theory
and dominated British disease theory from the 1840s to the 1870s (although
it is frequently referenced well into the early 1900s). It sought to explain
diseases in terms of complex interactions between miasmas and so-called

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zymotic materials. Miasma was the result of rotting organic matter produced
by decomposition processes. It would be dispensed in the air which, in turn,
would act, via zymotic principles, on individual constitutions, causing one of
several diseases (cholera, yellow fever, typhus, etc.), depending on a number
of more specific factors. Some of these were thought to be extraneous, such as
weather, climate, and humidity, and would affect the nature of the miasmas
themselves; others were related directly to the potential victims and thought
to render them more or less susceptible to disease. Lastly, there were a
variety of local conditions that could exacerbate the course and severity of
the disease, such as overcrowding and bad ventilation.

The term ‘zymotic’ (from the Greek for fermentation) goes back to William
Farr (1807–1883), Statistical Superintendent of the General Register Office
from 1842 to 1879. Farr coined this term to indicate that disease processes
“are of a chemical nature, and analogous to fermentation; by which they
are moreover to a certain extent explained” (1842, 201), yet not identical to
vinous fermentation. Since decomposition figured heavily in the various ac-
counts of disease causation, disease theorists drew heavily on contemporary
chemical theories, such as those of Liebig, who had both a comprehensive
system for explaining the various morbid processes of decomposition, pu-
trefaction, and fermentation, but also his own specific zymotic pathology.
Chemical theories like those of Liebig were well suited to explaining diseases,
because they explained the interaction between living and non-living things,
such as human bodies and the environment, and they did this on a molecular
basis. Moreover, Liebig’s chemical theories were popular, highly respectable,
and they had already had great successes, and so the zymotic theory may be
seen as drawing on some of the most successful science at the time.

Liebig and Farr held a so-called contact theory of decomposition. Accord-
ing to this, diseases occur as a result of introducing into the body (through
inhalation or direct contact) various zymotic principles. These were thought
to be “organic matter in a state of pathological transformation” (Farr 1842,
202). This would be absorbed by the blood, and, through the transfor-
mation, zymotic diseases had “the property of communicating their action
[i.e. decomposition], and effecting analogous transformations in other bod-
ies” (ibid.). The zymotic principles were the ‘exciters’ of the various diseases
and “in the blood corresponding bodies exist, which are destroyed, and by
the transformation of which the exciters are generated or reproduced” (ibid.,
199). In short, pre-existing stuff in a victim’s blood catches the process of de-
composition and communicates this state to other particles of blood (which,

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in turn, would transmit it to various body parts), until it ran out of sus-
ceptible particles to contaminate. The underlying idea of the contact theory
was that zymotic matter was like ferment, a volatile chemical substance that
could transfer its volatility to other materials. So, just as ferment produced
fermentation, zymotic material produced disease (zymosis).

Two things about the zymotic theory are worth stressing: first, zymotic
material was not a specific substance; according to the zymotic theory, the
disease was not the (presence of) zymotic materials, but the zymotic pro-
cesses of transformation. Second, the zymotic account was purely chemical,
and Liebig (and others) explicitly rejected the view that zymotic materials
were living organisms.

3 Successes of the Zymotic Theory

The zymotic theory was highly successful with respect to a number of phe-
nomena. Specifically, it was successful both in terms of explanation and pre-
diction. Among its explanatory successes were explanations of well-known
disease phenomena, such as the fact that diseases were known to be seasonal,
and often tied to particular regions (such as marshy ones) or particular lo-
cations (barracks, prisons, etc.). Similarly, it was well known that sickness
and mortality rates in poor, crowded urban centres were worse than in their
less poor and crowded counterparts, and that, in turn, those parts were af-
fected worse than rural areas. It could also explain a number of facts tied to
epidemics, such as why epidemics began, took the course they did, and then
subsided, yet often came back several years later. It could account for the
fact that epidemic diseases often moved around when there was no known
contact with previous victims, why quarantines were ineffective for some dis-
eases (such as cholera), why only some, but not all people were affected by
a given disease, and, lastly, why certain diseases were endemic.

The zymotic theory could explain all of these phenomena through its claims
that decomposing material produced miasmas. Diseases peaked when condi-
tions for putrefaction were particularly favourable: this was the reason why
certain diseases were particularly bad during periods of high temperature and
in certain geographical regions (for example, the many fevers in Africa), why
urban centres were much more affected than rural areas, and why even spe-
cific locations in otherwise more or less healthy areas could be struck (sewage,
refuse, and general ‘filth’ would sit around in badly ventilated areas). More-

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over, since zymotic material interacted with the blood, the zymotic theory
could provide an account for individual disease susceptibility, and explain
why certain diseases were contracted only once (once the relevant material
in a victim’s blood had been ‘converted’, the person became immune).

While this degree of explanatory success is quite impressive, as we have
seen, however, realists tend to think that, in addition, genuine success also
requires a theory to make use-novel predictions, i.e. predictions that did not
play a role in the theory’s original formulation. Here, the zymotic theory
also delivers. It predicted, for example,

(i) that the air in areas with higher disease incidence ought to be worse
than the air in healthier areas; more specifically, that it should contain
more decomposing organic material,

(ii) a number of different disease incidence patters, based on the prevalence
of decomposing and putrefying materials (in conjunction with facts
about ventilation), including

(a) relationships between disease prevalence and season, temperature,
rainfall, wind, and so on,

(b) a relationship between disease incidence and population density,

(c) a relationship between disease incidence and elevation,

(iii) facts about the course and duration of various epidemics,

(iv) facts about the relation between mortality rates and different occupa-
tions, and, lastly,

(v) facts about relationships between mortality from various diseases and
age.

Thus, the zymotic theory had successes on both explanatory and predictive
levels. Before showing (in Section 6) that these successes were, in fact, due
to a number of essential working posits that did not get retained in successor
theories and that turned out to be completely false, however, let us be clear
about what exactly the new selective realist challenge amounts to.

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4 The New Selective Realist Challenge

In response to the pessimistic meta-induction, besides making the notion
of success more stringent, selective realists have suggested that we ought
to focus only on those parts of past theories that were, in fact, responsible
for their genuine success. Prominent approaches include Worrall’s structural
realism, Kitcher’s distinction between working and presuppositional posits,
and Psillos’s divide et impera strategy. And, while this line of response
has also been popular among more recent realists, such as Harker (2013),
Peters (2014), Saatsi (2005), and Vickers (2013), they also acknowledge the
shortcomings of the traditional responses. In this vein, Vickers, for example,
argues that the basic divide et impera strategy needs to be updated, since (i)
first, there now are cases of successful theories that did make novel predictions
but that, nevertheless, turned out to be completely false (cf. Lyons 2006),
and (ii) second, “the divide et impera position needs significant refinement,
especially concerning the crucial concepts scientific success and responsible
for on which so much weight is placed” (2013, 190).

Thus, Vickers agrees with his predecessors that a given historical case poses
a problem only “if posits that ‘really fuel the derivation’ [of a novel predic-
tion] turn out to be definitely not approximately true” (194), but he thinks
more light needs to be shed on what it means for a posit to “really fuel a
derivation”. To this effect, Vickers proposes a distinction between derivation-
external and derivation-internal posits. The former are those that “merely
influenced the thinking of scientists” (198), but, since they only guide sci-
entists and are not part of the relevant derivation, they are not eligible for
realist commitment. The latter are those “posits [that] were actually in-
volved in the derivation of that prediction” (198). However, Vickers argues,
derivation-internal posits are not “necessarily the ‘working posits’ since any
individual posit might ‘contain within it’ some other posit that is the real
working part” (198). In other words, a posit might contain a ‘weaker’ posit
that is sufficient for the prediction, such as the posit “the passengers are
50kg too heavy” containing within it the weaker posit “the passengers are
too heavy” (198.; originally due to Saatsi 2005, 532).

Thus, contrary to previous realists – especially Psillos, according to whom
scientists’ judgements play an important role in determining what fuelled a
derivation – Vickers thinks that realists ought to commit themselves only to
posits that do logical work, and moreover, to their weakest possible versions
(i.e. if one weakened the posits any more, one could no longer derive the

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prediction). Thus, Vickers makes a distinction between actual, historical
derivations, and (possibly much weaker) logical, epistemic derivations (cf.
Peters 2014, 386). And, while Vickers believes there are now cases showing
that there are theories that contain working posits that turned out to be
false, he does not think that there are cases showing that there are essential
parts of derivation-internal posits that turned out to be false. Coming up
with such cases, then, is the new and updated selective realist challenge.

5 Zymotic Predictions

In order to show that the zymotic theory can rise to it, let’s look at some
of its predictions in more detail, starting with its predictions concerning air
quality. Here, the zymotic theory predicts, (i) first, that air quality ought to
be proportional to disease incidence, so that the ‘right’ locations should have
good and bad air, respectively, and (ii) second that differences in air quality
ought to be related to decomposition and ventilation. A number of mid-19th
century chemists tried to test these predictions; however, for brevity’s sake,
I will restrict my focus to a small subset of the experiments of Robert Angus
Smith (1817–1884), a contemporary of Farr’s, and often cited by the latter.2

Smith began by collecting indoor condensation liquid from crowded rooms
and compared it to fresh rainwater, finding that the indoor liquid, but not
the rainwater, had a strong perspiration smell, and, “on standing it formed
a glutinous mass in which the microscope revealed “Confervae”, “greenish
globules”, “various species of Volvox” [a type of algae], and “monades many
times smaller”” (ibid., 219–220; Smith, 1848, 18). While this was not a strong
result by itself, Smith believed that it at least showed that the the indoor
liquid contained organic material on which the Volvox and the monades were
feeding (ibid., 220). Further, upon burning the liquid’s residue, he obtained
the smell of ammonia, which was significant, since ammonia was tied to the
last stage in decomposition. At the larger scale of towns, these results were
thought to be exacerbated, not just because of the various exhalations of
living bodies, but, in addition, those of animal refuse and fuel combustion
(Smith, 1848).

Smith also tried to measure air quality more directly. He washed air sam-
ples by changing the air in a bottle containing distilled water, shaking the

2Smith was not a particularly distinguished scientist (Eyler 1980, 216), but is, for this
reason, quite representative of a number of people and their work.

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bottle after each change of air (with up to 200 changes), and then performed
a chemical analysis on the water, determining its amount of organic mate-
rial. He found these results to confirm those previously obtained from the
experiments above (Smith 1859, 218–225; cf. also Eyler, 220).

Most telling, however, were experiments in which he tried to show that
there was organic material in vapours given off by putrefying meat, blood
(due to the strong smell, he only performed a limited number of these), and
cesspools. He proceeded to compare these samples to the air in a number of
different locations – everything from different areas of Manchester, to “closely
packed railway carriages[s]” (1859, 221), to the air in bedrooms before and
after someone had slept in them, to the occasion on which a “strong smell of
a sewer entered my laboratory” (221), the fronts and backs of various houses,
alleys, and so on. Smith obtained rather a large variation in air quality, and,
specifically, concluded that by inhaling putrid air from decomposing animal
matter (such as decomposing mutton), “we might be inhaling 9000 times
more of some organic substance or other than we should be doing by inhaling
the purest air” (1859, 222). These figures include the vapours given off by the
putrefying meat, but, without these, he concluded that the difference among
different areas of town is about 22 times, and, within industrial areas, ranges
from about 9 to 22 times, while over the Atlantic Ocean and the Highlands,
the air was clean (223; cf. also Eyler 220–221). Most importantly, however,
as Eyler points out, the ranges Smith found were in proportion “to the range
of the crude death rates for the districts of Manchester” (221): in the districts
with the highest death rates, the air had most organic material in it, and the
ones with the lowest death rate had the cleanest air. Smith concludes that
“[t]hese differences . . . enable us to account for the number which represent
the deaths of the various districts” (1859, 223).

He also performed an investigation for the mining commission, during
which he observed people in air-tight lead chambers, recording their pulse,
respiration, and so on. He systematically measured the carbonic acid con-
centration inside the chamber, and concluded that carbonic acid was quite
harmful, that it “almost always comes in bad company” (Eyler, 222), and
then proceeded to use carbonic acid concentrations to test how well or badly
ventilated a given place was.

However, Angus Smith’s experiments are not the only ones speaking in
favour of the zymotic theory. As we have already seen, the zymotic theory
also predicted a number of other relationships, such as those between disease
incidence and population density and between disease incidence and distance

8



from sources of decomposition. Both of the latter were confirmed by Farr.
First, Farr showed that “the mortality of town districts has a certain rela-
tionship to their density” (207). Based on his analyses of the data sets of
a number of Annual Reports of the General Register Office, Farr came up
with a law on whose basis he made a number of precise predictions. When
he then proceeded to compare the calculated, expected mortality to that ac-
tually observed in the 30 London statistical districts, he found the results
agreeing “very directly with the results of direct observation” (ibid.; due to
space constraints, I won’t discuss this result in any detail, but, for a quick
flavour of the kind of law Farr put forth, see figure 1 (ibid.)).

Figure 1: Farr’s Density Equation

� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � �� � � � � � � � � �� � � � � � � � � � � � � � � � � � � � �� � � � � �� � � � � � � � � � � � �� � � � � �� � � � � �� � � � � � � � � �� � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 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� � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � �� �� �� � � � � � � � � � � � �� � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �His crowning achievement, however, was his elevation law, relating choleramortality to soil elevation. This relation relation followed straightforwardlyfrom those parts of the zymotic theory having to do with the dilution ofmiasma in the atmosphere. Here, what Farr did was to capture the exact re-lation between the decline of cholera and increased soil elevation in the form
of the following equation: c = c′ × (e′ + a)/(e + a)(e is the elevation above
the Thames high water mark, c the average cholera mortality rate at that
elevation, and e′ and c′ the elevation and mortality at a higher elevation, a
is a constant; 1852, lxiii). Farr then calculated the expected series according
to the formula, compared it to the actual series recorded in London, and
found remarkable agreement (1852, lxiii). Seeking further confirmation, he

9



then proceeded to “submit the principle to another test, by comparing the
elevation and the mortality from cholera of each sub-district ”, and found
that this “entirely confirms the announced law” (xv-xvi). Lastly, Farr’s pre-
dictions were also confirmed by others in different regions. For example,
“William Duncan, Medical Officer of Health for Liverpool, wrote that when
he grouped the districts of his city by elevation as Farr had done, that cholera
mortality in the last epidemic obeyed Farr’s elevation law for Liverpool as
well” (Eyler, 1979, 228).

6 Meeting the New Realist Challenge

Note that all of the above predictions were use-novel: they could not have
played a role in the construction of the zymotic theory, since they were not
even formulated by then, and, in Farr’s case, the data on which his laws were
based did not even exist. As such, the case of the zymotic theory meets the
realists’ criteria for ‘genuine success’.

However, as we have seen, Vickers does not think that this is enough; in
addition, he requires that it be shown that the derivation of these predictions
involved ineliminable parts of derivation-internal posits that cannot be weak-
ened. Here, I want to argue that the zymotic theory can meet this challenge,
too.

The first question is what the zymotic theory’s relevant posits are. Crucial
to the derivations of the above predictions are the following three:

1. Organic matter given off by putrefying material is the exciting cause
of diseases.

2. This organic matter is suspended in the air (i.e. there are miasmas).

3. This, in turn, is transmitted through the air.

Clearly, all of the above posits are derivation-internal, not derivation-
external: they did not just guide practitioners’ thinking, but they were all
crucially involved in making the various predictions. They were essential in
the air quality predictions confirmed by Angus Smith: if any of the above
posits are taken away, the entire prediction disappears. While the second
and third are directly about quality, the first is also necessary: without spe-
cific reference to (sources of) decomposition, we lose the geographical and

10



other location patterns, and, as a result, the entire prediction about air dif-
ferences. Thus, all of these posits are clearly doing work in the production of
the prediction. Similarly for Farr’s predictions about density and elevation.
Both putrefying material as a source of miasma and air as a medium of its
transmission are crucial, since, taking either one away makes the prediction
disappear. Thus, it is clear that the above posits are doing work.

However, as we have seen, according to Vickers, this is not enough – it also
needs to be the case that they don’t entail weaker versions of themselves,
on the basis of which the prediction still goes through. Examining our three
posits, we see that this, too, is the case. Ironically, the fact that the zymotists
knew relatively little about the chemical make-up of the alleged miasmas is
to their advantage here, since, absent any specific information about the
make-up of the allegedly responsible organic matter, these posits are already
as weak as they can get: they effectively state that whatever is given off
during decomposition is transmitted through the air and involved in causing
diseases. Weakening them any more would take away either the decomposing
sources, or air as a medium, and, as we have already seen, both of these are
necessary in order to make the predictions in the first place. Thus, since
these posits cannot be weakened, they ought to count as essential parts of
derivation-internal posits, according to Vickers. Moreover, they did not get
retained in any way, shape, or form, in the germ theory: decomposition is not
responsible for disease, neither is disease material dispensed in the air, neither
is the air a medium for disease transmission. But, if that is right, according
to Vickers, and by his own admission, we ought to have been realists about
miasmas.3

So much for miasmas, but what about zymes? Further zymotic posits we
might add to the miasmatic ones above are:

1. Diseases are a type of decomposition,

2. Disease processes are analogous to processes of fermentation (which is
a type of decomposition),

3. Zymotic material acts like a ferment on pre-existing stuff in a victim’s
blood.

3Note that this poses a problem not just for Vickers, but for selective realists more
generally.

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Now, clearly, none of these posits are necessary for the predictions we
have seen, since it was possible to provide derivations of these predictions
without appealing to any of the zymotic posits. Thus, obviously, they are
non-essential. However, I want to stress at this point that, even if the zymotic
posits fail to be essential, note that the miasmatic posits clearly are, and that
this is already enough of a problem for selective realists. After all, selective
realists face trouble if essential parts of derivation-internal posits turn out to
have been completely false, and the miasmatic posits fit that bill. But, of
course, it is not a requirement that every (rejected) posit be essential (which
is obviously not right). Thus, showing that the miasmatic posits are essential
and were rejected suffices to get the realist into trouble.

More interesting, however, is the fact that even though the zymotic posits
might not have been essential for making predictions, they play a different,
and perhaps equally important role in the zymotic theory: that of providing
unifying explanations.4 More specifically, among other things, the zymotic
posits explain why there is a link between decomposition and disease (they
are essentially variations of each other), why diseases vary with season and
temperature (it was well known that fermentation processes are temperature-
sensitive), why certain diseases only occurred once (victims’ blood would run
out of material to ferment), why certain diseases were childhood diseases
(children’s blood was different from adults’), and why epidemics began (exis-
tence of sufficiently virulent zymotic material) and ended (lack of new victims
with the appropriate blood). Lastly, it had been clear for some time that
disease material needed to replicate itself somehow, and fermentation pro-
cesses offered an explanation of how this was possible. The various zymotic
posits above can make sense of all of these at once. And, while there are no
concrete predictions based directly on these posits, it is clear that the above
explanations disappear without zymosis. Thus, while zymotic material might
not have been primarily responsible for the theory’s predictive success, it was
certainly crucially implicated in the theory’s explanatory success. Without
the fermentation aspect of zymosis, the entire disease mechanism would have
disappeared, and with it disease pathology. Moreover, without it, the link
between diseases and decomposition would have been lost, and, as we have
seen, without decomposition, the predictions of the zymotic theory disap-

4Peters (2014) stresses the importance of this in accounts of essentialness. For this
reason, the zymotic theory might also turn out to be problematic for Peters’ proposal.
However, due to space constraints, I will not pursue this further here.

12



pear, too.5

7 Conclusion

What, then, is the upshot of this? What is clear is that the zymotic the-
ory made use-novel predictions, such as those about air quality, and Farr’s
predictions about elevation and density. Miasmatic posits were essential
derivation-internal working posits that, further, could not be weakened, and,
so, according to Vickers, they deserved realist commitment. Yet, they were
rejected, and no miasmatic posits, as we have seen, were carried over to the
germ theory. Zymotic posits were not involved in the predictions in these
crucial ways, but were essential to the theory’s explanatory power, since, tak-
ing away zymosis left a disease theory without a disease mechanism, without
a pathology, and without any explanation of many of the well-known disease
phenomena that needed accounting for, including the all-important link to
decomposition. Thus, even modern, sophisticated selective realist accounts,
such as that of Vickers, cannot rise to the historical challenge that underlies
the pessimistic meta-induction. Realism remains in as much trouble as ever,
at least on this front.

8 References

Eyler, John M. (1979). Victorian social medicine: the ideas and methods of
William Farr. Johns Hopkins University Press.

Eyler, John M. (1980). “The Conversion of Angus Smith”. Bulletin of the
History of Medicine, 54(2), 216.

Farr, William (1842). “Letter”, 4th Annual Report to the Registrar General,
Appendix, London: W. Clowes

Farr, William. (1852). Report on the Mortality of Cholera in England, 1848-
49, London: W. Clowes.

5For brevity, I cannot discuss in detail why zymes cannot be regarded as proto-germs.
I want to stress, however, that even if such an argument could be made (although I am
confident that it couldn’t), this does not help the realist, since the miasmatic posits were
the ones responsible for the zymotic theory’s novel predictions, and miasma certainly did
not get retained in the germ theory.

13



Frost-Arnold, Greg (2014). “Can the Pessimistic Induction be Saved from
Semantic Anti-Realism about Scientific Theory?” British Journal for the
Philosophy of Science 65(3): 521-548.

Harker, David (2013). “How to Split a Theory: Defending Selective Realism
and Convergence without Proximity”. British Journal for the Philosophy of
Science, 64(1): 79-106.

Kitcher, Philip (1993). The Advancement of Science. Oxford University
Press

Lyons, Timothy D. (2006). “Scientific realism and the stratagema de divide
et impera”. British Journal for the Philosophy of Science 57(3):537-560.

Peters, Dean (2014). “What Elements of Successful Scientific Theories Are
the Correct Targets for “Selective” Scientific Realism?”. Philosophy of Sci-
ence 81(3): 377-397.

Psillos, Stathis (1999). Scientific Realism: How Science Tracks Truth. Rout-
ledge

Saatsi, Juha (2005). “Reconsidering the Fresnel-Maxwell Case Study”. Stud-
ies in History and Philosophy of Science, 36: 50–38.

Smith, Robert A. (1848). “On the air and water of towns”, Report B.A.A.S.
18: 16–31.

Smith, Robert A. (1859). “On the air of towns”. Q. J. Chem. Soc. Lond..
11: 196–235.

Vickers, Peter (2013). “A Confrontation of Convergent Realism”. Philosophy
of Science 80(2): 189–211.

Worrall, John (1989). “Structural realism: The best of both worlds?” Di-
alectica, 43(1-2): 99–124.

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	Introduction
	The Zymotic Theory
	Successes of the Zymotic Theory
	The New Selective Realist Challenge
	Zymotic Predictions
	Meeting the New Realist Challenge
	Conclusion
	References