The (Multiple) Realization of Psychological and other Properties in the Sciences.
By Kenneth Aizawa and Carl Gillett.
…we should… not focus on weird cases while we are trying to get things straight about
multiple realizability as a matter of methodological concern in science;… Let’s free
discussions of realization, reduction, emergence, supervenience and the like from reliance
on very far out examples and counterexamples. (Boyd (1999), p.91).
…there is nothing intrinsically mentalistic (or social or cultural) about multiplerealizability... Though… [this trait has] been taken by some philosophers to be
characteristic of the mental, I would argue that [it is] characteristic of any move from a
lower compositional level to a higher one. That goes for the theory of chemical bonding
relative to fundamental quantum-mechanical theories of the atom no less than for the
relation between the neurophysiological… and the cognitive… (Wimsatt (1994), p.224)
Over recent decades, work on ‘realization’ and ‘multiple realization’ has been central in
both the philosophy of science and the philosophy of mind.1 However, the views presented in
these two areas have been very different, often embodying different notions of ‘realization’
offered in the course of distinct projects.2 The failure to heed such differences has arguably
produced a range of problems and in particular some of the most prominent accounts of
‘realization’ and ‘multiple realization’ offered in the philosophy of science have arguably been
badly underappreciated and misinterpreted.3 Our primary purpose in this paper is therefore to
more clearly articulate the standard view of ‘realization’ and ‘multiple realization’ offered in the
philosophy of science, focusing upon a number of concrete examples but examining scientific
cases of psychological properties, and their various kinds of realizer, in most detail.
What we will term the ‘received’ view of special sciences was defended by writers such
as Jerry Fodor, William Wimsatt, and Philip Kitcher.4 The proponents of the received view
overturned the Positivist’s jaundiced view of special sciences by looking at real examples from
the biological sciences, geology, linguistics, neurophysiology, psychology, and economics,
amongst other areas. And central to the received view’s critique was the ontological claim that
examples in the special sciences very often involve the ‘multiple realization’ of properties where
2
this phenomenon was then used to establish a range of important results. As our opening passage
from William Wimsatt highlights, the claims about multiple realization made by proponents of
the received view of special sciences were never intended to apply simply to psychology or
psychological properties, for the received view used cases of multiple realization drawn from a
wide range of special sciences and offers a general picture of all the disciplines above physics.
Our first goal will therefore be, in Part 1, to offer a general account of the realization of
properties in the special sciences and then to provide an equally general account of multiple
realization. Following the counsel of Richard Boyd in our other opening passage, we focus upon
real scientific examples to guide our account. To further sharpen our account of multiple
realization in the sciences, in Part 2, we consider an alternative theory recently offered by
Laurence Shapiro (2004) and the associated objections that Shapiro levels at our position. We
show that Shapiro’s criticisms are misplaced and argue that Shapiro’s own account of multiple
realization in the sciences faces grave problems: namely, that it fails to acknowledge all the kinds
of scientific explanations that reveal “causally relevant properties” and that it is unable to
accommodate the use scientists make of theories from a range of levels to guide or constrain
their research.
Although presented as part of a fully general view of special sciences, one of the most
prominent applications of the received view was to the psychological sciences, with writers, such
as Fodor (1968) and Block and Fodor (1972), presenting empirical evidence that they claimed
showed psychological properties are multiply realized. Unsurprisingly, recent philosophical
work on the neurosciences has examined whether these earlier claims actually hold-up in the face
of a more careful examination of empirical results and many of these assessments have argued
that current neuroscience challenges the multiple realization of psychological properties. 5 In
earlier work, we have replied to the most important negative arguments that purport to show
3
there is no multiple realization of psychological properties in the sciences.6 In the second half of
this paper, we therefore focus upon using our framework for scientific multiple realization to
assess whether there is positive evidence for the existence of multiple realization of
psychological properties in the sciences, focusing first upon evidence from biochemistry, in Part
3, and then neurophysiology in Part 4.
Our final conclusion is that it is important to provide more precise frameworks for the
nature of compositional relations generally, and for realization and multiple realization in
particular, using concrete and mundane scientific cases as Boyd and Wimsatt counsel. For such
examples allow us to clearly appreciate when, and why, there is multiple realization across the
sciences. Furthermore, following such a strategy, we show that given our present empirical
evidence we have strong reason to believe that psychological properties are indeed multiply
realized both at the biochemical and neuronal levels.
Part 1 - Realization and Multiple Realization in the Sciences.
Very little scientific work makes explicit use of the term ‘realization’ in anything like the
way in which it has been used by defenders of the received view. Thus, like so much philosophy
of science, investigation of the realization of higher level properties by lower level properties
constitutes a kind of examination and reconstruction of scientific practice, theorizing etc. To
guide our work we will begin by examining some familiar cases from the sciences and to make
our discussion of these examples manageable we follow recent debates in using a version of the
“causal theory of properties”. This is a variant of Shoemaker (1980)’s account under which a
property is individuated by the causal powers it potentially contributes to the individuals in
which it is instantiated. On this view, two properties are different when they contribute different
powers under the same conditions.
4
The basic features of realization relations can be illuminated by all manner of scientific
examples, but for brevity we will begin by considering an example whose details are compact,
and widely known. Later in the paper we will then examine a range of less familiar cases. But, to
start, consider the extreme hardness of a diamond, in its property of having a Knoop hardness
value 10,400 kg/mm2along a cubic face, and the bonding and alignment of carbon atoms.7 Our
illustration in Figure 1 highlights some of the mechanisms involved with this case. When the
edge of a cut diamond is pressed with force across a pane of glass, in a certain sequence of
directions, the diamond’s hardness causes a Z-shaped scratch in the glass. Here we have a causal
process, i.e. mechanism, which is grounded by the triggering of one of the diamond’s powers,
contributed to the diamond by its property of being very hard, which results in a certain effect – a
Z-shaped scratch on the glass.
As Figure 1 also roughly captures, the sciences have provided an elegant, and familiar,
explanation of this higher level process by illuminating its implementation. For the sciences have
shown how the lower-level mechanisms of particular carbon atoms breaking the bonds between,
and displacing, specific glass molecules compose, or more specifically ‘implement’, the process
of the diamond scratching the glass. Under the relevant background and triggering conditions,
the powers of the carbon atoms, to hold each other in close relative spatial relations and to break
the bonds between glass molecules when under pressure, are triggered in a host of carbon atoms.
The result is a large number of causal processes involving particular carbon atoms each of which
cause specific glass molecules to break their bonds with other glass molecules and change their
spatial locations. It is worth marking that none of these lower-level mechanisms involving
carbon atoms results in an effect that has a macroscopic breadth and depth, or has a Z-shape, or
any of the other features of the process of the diamond scratching the glass. Instead, when
triggered, the powers of particular carbon atoms each ground a mechanism that causes certain
5
glass molecules to break their bonds and change position. Nonetheless, the many mechanisms
involving carbon atoms together implement, i.e. non-causally result in, the qualitatively different
process involving the diamond.
Against this backdrop, since properties are our primary focus, let us more carefully
consider the compositional relations posited in this type of case between property instances and
their powers, starting with the latter. A property instance is an entity that makes a difference to
the causal powers of individuals, usually by contributing powers to individuals. Most scientific
properties are thus individuated by the causal powers they potentially contribute to individuals
under certain background conditions. We can consequently see that the sciences again illuminate
how, and why, the properties and relations of the carbon atoms compose, or ‘realize’, the
properties of the diamond. For we have now seen that the powers of the carbon atoms that, as we
will put it, comprise the powers individuative of the properties of the diamond. As a result, the
sciences explain why if one has the carbon atoms with their properties and relations, then the
latter properties will together realize the diamond’s property of hardness. For the carbon atoms’
relations of bonding and alignment together contribute powers that comprise the powers
individuative of the property of being very hard and hence non-causally result in, and realize, an
instance of the property of hardness in the diamond.
Using the causal theory of properties, we can formalize these observations in a
‘thumbnail’ account for the kind of realization we have found in our case in this theory schema:8
(Realization) Property/relation instance(s) of F1-Fn realize an instance of a property G, in
an individual s under conditions $, if and only if, under $, s has powers that are
individuative of an instance of G in virtue of the powers together contributed by the
instances of F1-Fn to s or s’s constituent(s), but not vice versa.
Given its abstract nature, we can better illuminate our schema if we now draw out of the
diamond case a number of features that all compositional relations share.
6
First, we should mark that relations like realization are obviously a species of
determination relation, but are rather different from causal relations. The ‘horizontal’
determination involved with causation is temporally extended, relates wholly distinct entities and
often involves the transfer of energy and/or the mediation of force. In contrast, compositional
relations are not temporal in nature, since their ‘vertical’ determination is instantaneous, does not
relate wholly distinct entities, and does not involve the transfer of energy and/or the mediation of
force. Composition is thus a variety of what has been termed ‘non-causal’ determination.
Second, scientific composition usually relates qualitatively different kinds of entity. For
example, diamonds have properties like hardness, and hardness contributes the power to scratch
glass. But no carbon atom has the property of hardness, nor any property that contributes the
power to scratch glass. In our case, we thus have individuals that constitute other individuals
with which they share no properties, as well as properties that realize other properties with which
they share no common powers, and similar points hold for the relevant powers and mechanisms.
A quick examination of the compositional relations illuminated by the full spectrum of special
sciences shows that this feature is common to all of their findings.
Third, and perhaps most importantly, as we noted with realization relations above, we
should mark a common feature of scientific composition. For as in the realization of properties,
given their interconnections, the entities studied by lower level sciences together compose the
qualitatively different entities studied by higher level sciences. The simple secret of
compositional relations, and the mechanistic explanations that posit them, is therefore that,
although individually the component entities are qualitatively different from the composed
entity, nonetheless the components together non-causally result in the composed entity.
Compositional relations in the sciences are thus often ‘many-one’ with regard to their relata, with
many component entities and one composed entity. This distinctive feature of the compositional
7
relations posited in the sciences consequently allows one to mechanistically explain entities of
one kind in terms of entities of very different kinds. For example, what makes for the hardness of
a diamond is not the property of any single carbon atom, but the way in which the properties and
relations of individual carbon atoms work together.
Noting these general features of scientific composition helps illuminate the character, and
strength, of our theory schema for realization. Since we can now see that our schema has the
merit of accommodating all of these characteristics of scientific relations of composition. The
Dimensioned account allows that realization is a non-causal relation that is synchronous,
between entities which are not wholly distinct and which does not involve either the transfer of
energy and/or mediation of force. Furthermore, our schema accommodates realizer and realized
properties that are qualitatively distinct and instantiated in different individuals, like the carbon
atoms and the diamond. Finally, the schema embodies the idea that realizer properties together
play the causal role of the realized property without directly contributing any of the realized
property’s individuating powers.
The second and third of these general features of scientific composition also serve to
illuminate the underlying nature of multiple realization in the sciences. If we reflect on the
implications of combining the characteristic that scientific realization often relates qualitatively
different kinds of property with the feature that properties together realize other properties, then
we can see why we will have very different realizer properties that, on different occasions, all
realize instance of the same property. Putting things crudely, it is because realizer properties, that
are qualitatively distinct from other realizers and the realized property, together realize some
special science property that we get multiple realization. Since distinct combinations of lowerlevel properties, that are different from each other in the powers they contribute and hence the
causal mechanisms that they ground, may nonetheless still together non-causally result in
8
instances of the same special science property. We can use these points to frame a more precise
abstract account of such multiple realization:
(Multiple Realization) Instances of a property G are multiply realized if and only if, (i)
under condition $, an individual s has an instance of property G in virtue of the powers
contributed by instances of properties/relations F1-Fn to s, or s’s constituents, but not
vice versa; (ii) under condition $* (which may or may not be identical to $), an individual
s* (which may or may not be identical to s) has an instance of a property G in virtue of
the powers contributed by instances of properties/relations F*1-F*m of s* or s*’s
constituents, but not vice versa; (iii) F1-Fn ≠ F*1-F*m and (iv), under conditions $ and
$*, F1-Fn of s and F*1-F*m of s* are at the same scientific level of properties.
Overall, the theory schema is fairly mundane. Conditions (i)-(iii) simply frame the demand for
distinct sets of realizer properties for instances of the same realized property. However, the final
condition deserves more comment.
Implicitly, philosophers of science have always had something like condition (iv) in
mind, but we show later that making this condition explicit has a variety of advantages. To see
why one needs (iv), either implicitly or explicitly, consider the following common situation.
Properties and relations of carbon atoms realize hardness; but obviously properties and relations
of certain fundamental microphysical properties realize the properties and relations of carbon
atoms and realize the property of the diamond. But since (the properties and relations of the
carbon atoms) ≠ (the properties and relations of fundamental microphysical individuals), then it
appears that in such cases if we only use conditions (i)-(iii) then this entails we have a case of
multiple realization. Obviously, what is awry is that the two sets of properties are not at the same
level and hence are implicitly excluded as even candidates to ground a case of multiple
realization. Addition of condition (iv) resolves this problem, though we suggest that all sides
implicitly endorse (iv) as a shared background condition. 9
An added advantage of using (iv) is that it combats a common and problematic
philosophical practice. The practice in question is that of talking simply about the ‘multiple
realization’ of some property, whether psychological, biological or whatever, and saying nothing
9
further. Often, given the context, this may be a harmless way of talking, but we should note how
it may be damaging. Suppose that some higher level psychological property G is multiply
realized by microphysical properties of fundamental particles and hence multiply realized at the
microphysical level. This does not, of course, mean that G is multiply realized in, say, distinct
physiological properties. After all, it is logically possible to have G be univocally realized in the
physiological properties of two organisms where these physiological properties are themselves
multiply realized by the microphysical properties of the fundamental particles that constitute
these two organisms. So, our two instances of G might be univocally realized at level X (the
physiological level), but multiply realized at, say, level X – 1 (the microphysical level). In such
cases simply discussing ‘multiple realization’ of G potentially leads to confusion which is easily
avoided by indexing such claims to particular realizers and levels. We will see later that failing
to appreciate, or respect, such points has lead to substantive problems.
In order to articulate how we intend our account of multiple realization to be understood,
and to further illuminate its nature, we now want to work through some simple scientific
examples. Our goal will be to explain both when our account says we do not have multiple
realization, as well as when we do.
Case I. Take some group of carbon atoms s1-sp. At one time, bonded together in one
way, i.e. with one set of bonding and spatial relations, these carbon atoms have
properties/relations that realize an instance of the property of having a Knoop hardness value
10,400 kg/mm2along a cubic face –that is, the hardness of a diamond. However, at another time,
the very same carbon atoms when bonded together in another way, i.e. with distinct bonding and
spatial relations, can realize an instance of the far lower Knoop hardness found in graphite. Our
example thus illuminates how differing properties/relations of the same individuals, either at a
10
time or over time, may realize instances of distinct higher level properties. We return to this
point below when we consider recent discussions of multiple realization.
Case II. Consider two individual diamonds s and s* that contain exactly the same number
of carbon atoms in exactly the same relations of bonding and spatial alignment at standard
temperature and pressure. Here we have what one might naturally call two numerically distinct
realizations of the property having a Knoop hardness value of 10,400 kg/mm2 along a cubic face.
This, however, is not an example of multiple realization for the received view, since the
instances of the properties involved in this case clearly fail condition (iii) because we do not have
instances of distinct realizer properties/relations in the relevant constituents of the two diamonds.
Merely having distinct instances of the very same properties does not suffice for multiple
realization under the received view or our theory schema.
Case III. Consider an example where we have distinct materials each of which have the
same Knoop hardness. Diamonds do not provide a clean example, but other cases of materials
with the same hardness are easy to find. Thus, to take an example from modern dentistry, we
have a variety of Cobalt-Chromium alloys and Palladium alloys that are used as fillings in
humans and which instantiate instances of the property of having a Knoop hardness in the range
350-400 kg/mm2. In addition, of course, much sound, unetched human and bovine enamel (i.e.
human and bovine teeth) also has the property of having a Knoop hardness in the range 350-400
kg/mm2.10 In such cases, under our schemata we will have multiple realization. For example,
assume that we have distinct instances of the property of having a certain Knoop hardness in
both a Palladium alloy filling and also in a human tooth. The properties and relations of the
constituent atoms and/or molecules that are the realizers in these individuals are obviously
distinct, since the bonding relations between the metal atoms/molecules, and the molecules in the
enamel, are different. Furthermore, these atoms/molecules, and their bonding relations, are
11
plausibly at the same level of properties. As a result, our theory schemata say that the relevant
instances of the property of having this Knoop hardness are multiply realized.
To conclude, we now have a better grip on the general features of the compositional
relations posited in mechanistic explanations that defenders of the received view focused upon in
their work. In particular, we have more precise theory schemata for both realization and multiple
realization. Furthermore, following on from Case III, it should also be obvious to the reader that
multiple realization will be common in materials science, engineering sciences, and a host of
other disciplines. For these areas are replete with distinct substances known and valued, like the
Palladium alloy fillings, because they realize common higher level properties, whether having a
certain Knoop hardness, density, refractive index, conductance, etc., which are realized by the
properties and relations of some other, distinct, material with rather different accompanying
features. Whether we also have multiple realization in a still wider range of disciplines, and in
particular in the neurosciences and psychology, is an issue which we examine further in Parts 3
and 4. However, in the next section, we turn to objections, and a rival, to our account of multiple
realization.
Part 2 – An Opposing View and Objections: Shapiro on Scientific Multiple Realization.
At this point, it useful to briefly assess our theory schemata and we can do this if we now
turn to the important work on multiple realization of Larry Shapiro ((2000), (2004),
(Unpublished)). For Shapiro not only offers a contrasting account of multiple realization in the
sciences, but also gives voice to some common concerns about our position. We will begin by
outlining two opposing objections Shapiro presents to our view (that it is too permissive and too
restrictive), briefly respond to these criticisms, and then detail Shapiro’s alternative account of
12
multiple realization, since his challenges to our theory schemata appear most plausible against
the backdrop of his own positive view.
One common worry about our precise account, and the received view generally, is that it
is too permissive and leads to there being too much multiple realization in the sciences and the
natural world. This is often argued by critics to be counterintuitive and hence objectionable. In
recent work, for example, Shapiro (Unpublished) has argued that our account of multiple
realization would mean that a difference in “just one quark” would lead to multiple realization,
where the underlying intuition is apparently that this would consequently make the existence of
multiple realization far too widespread. Let us call this the ‘Permissiveness Objection’.
In response, it is worthwhile to start by emphasizing that we have offered our accounts of
realization and multiple realization as parts of the wider project of understanding the
compositional concepts used in the sciences. We suggest that the success of such accounts
should be judged by how well they do in capturing the features of the concepts actually used in
scientific explanations. But what if the resulting account does not accord with some
philosopher’s prior expectations about compositional concepts? For example, what if our view
entails that realization, and multiple realization, are more widely found than analytic
philosophers of mind of an earlier generation assumed? As in so many other cases where the
sciences have surprised us, we fail to see why such a conflict with the expectations of
philosophers poses a problem for the success of our accounts of realization and multiple
realization. Instead, we contend that the success of our theory schemata is, at least to a large
measure, a thoroughly empirical matter to be judged, for example, by whether they ascribe, or
fail to ascribe, realization and/or multiple realization in accord with our scientific evidence.
We have thus seen that the Permissiveness Objection in the bare form Shapiro presents it
fails to pose a problem for our account, but we invite those concerned about our view to explore
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whether the Permissiveness Objection can be made more successfully using a richer range of
empirical evidence. In contrast to the latter concern, Shapiro (2004) gives voice to a complaint of
exactly opposite kind – namely, that our accounts of realization and multiple realization are far
too restrictive in their ascriptions. Let us call this the ‘Restriction Worry’. For example, Shapiro
objects that in virtue of our accounts of realization and multiple realization we are:
…committed to the claim that, ultimately, every property is realized in the same way.
Talk of multiple realizability, on the dimensioned account [of realization], vanishes, for,
it turns out, that the same properties that realize, say, a digital watch also realize an
analog watch (as well as everything else under – and over – the sun). (Shapiro (2004),
p.44)
This version of the Restriction Worry appears to be that the same properties and relations of
physics, what Shapiro terms a “handful of basic properties” (Shapiro (2004), p.44) will be the
realizers of all properties. Again disambiguating the claims of multiple realization at issue here
by indexing to particular properties, the Worry is that our account entails that all higher level
properties will be univocally realized by the properties and relations of physics which appears
highly implausible given our empirical evidence.
In response to this concern, it is useful to consider how, for example, someone might
object in a similar way that our view of realization and multiple realization is mistaken using the
case of hardness. For, such an objector might argue, our account takes the same kind of
individual, in carbon atoms, and the same kinds of relation, in bonding and spatial alignment, to
be involved in the realization of both the high Knoop hardness of a diamond and the low Knoop
hardness of graphite by the properties and relations of materials science. Thus, in an analogous
fashion, the objector might conclude that our account is too restrictive by putatively entailing
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that all instances of hardness, whether of a high or low Knoop value, are all univocally, rather
than multiply, realized by the properties and relations of carbon atoms.
The obvious point to make in response, highlighted by our discussion of Case I above, is
that although the same very general kinds of property and relation of carbon atoms may be
involved in realizing the hardness of diamonds and graphite, nonetheless the specific properties
and relations that are realizers in these cases are different in ways outlined by the sciences
concerned. Similarly, on a grander scale, though the same very general kinds of property and
relation illuminated by fundamental physics have plausibly been shown to be realizers of very
many special science properties/relations at the microphysical level, nonetheless the sciences
have also shown that the specific microphysical properties and relations involved in realizing
various special science properties are usually different – though where, and what form, multiple
realization takes at any level is obviously an empirical matter. We can therefore see that
Shapiro’s bare version of the Restriction Worry also fails, though again we invite anyone
concerned with our position to explore whether a richer base of empirical evidence can bolster
such a concern.
Shapiro’s objections are offered against the background of his positive account of a
certain kind of explanation in the sciences and corresponding views of both realization and
multiple realization. The latter account is at odds with our own view and is developed most fully
in Shapiro (2004), so let us now turn to this competing position. Shapiro develops his theory of
realization, and multiple realization, primarily through the use of his understanding of Cumminsstyle functional analysis. According to Shapiro, a property G, of an individual s, is multiply
realizable if there exist distinct functional analyses of G. For example, Shapiro considers whether
the property of being a watch is realized by the properties of being, respectively, an analog watch
and a digital watch.11 Shapiro then asks what properties of analog and digital watches make them
15
distinct realizations of watch (if they are in fact distinct). His overall way of thinking about the
issue of multiple realization emerges when he writes:
…I propose to use the name R-property as a label for those properties of realizations
whose differences suffice to explain why the realizations of which they are properties
[sic] count as different in kind. In short, two types of realization of some functional kind,
like corkscrew or watch, count as different kinds of realization if (by definition) they
differ in their R-properties… “Which properties of a realization are R-properties?”… Rproperties are those that are identified in the course of functionally analyzing some
capacity. It is to functional analysis, I claim, that we should look for the identification of
R-properties because it is functional analysis that uncovers those properties that causally
contribute to the production of the capacity of interest. (Shapiro, 2004, pp. 52-53).
Given these commitments, when Shapiro turns to the question of whether the properties and
relations of lower level constituent individuals, such as quarks, electrons, or atoms, can be
realizers, Shapiro’s understanding of functional analysis, and his key notion of an R-property,
lead him to conclude that such properties and relations are not realizers, and hence not the basis
for multiple realization. With two corkscrews made of aluminum and steel, or to vary the
example, two objects each with a Knoop hardness of 370 kg/mm2 where one is made of
Palladium alloy and the other of unetched human enamel, our account will say we have multiple
realizations of the property of being a corkscrew or the property of having a certain Knoop
hardness. In contrast, given his understanding of functional analysis and an R-property, Shapiro
denies that properties/relations of the relevant atoms/molecules are R-properties at all, hence
denies that these are examples of multiple realization (Shapiro (2004), pp. 56-7). This marks one
difference between our account and Shapiro’s.
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A further divergence is that on our view realization is a transitive relation, where under
Shapiro’s account realization is not transitive.12 In order to better assess the impact of these
differences, and to evaluate the competing accounts of realization and multiple realization, we
need to see how they accord with our evidence from the sciences taken in the broadest sense to
include not just our scientific findings about particular phenomena, but also the various features
of scientific explanations, methodology and more. And although Shapiro’s rejection of the
transitivity of scientific realization might appear innocuous, we will now argue that this and
other features of his account lead to a couple of crucial difficulties for his view.
As Shapiro (2004) outlines in a range of concrete cases, theories and findings in the
higher and lower level sciences that respectively investigate realizer and realized properties are
often used to guide, constrain or evaluate each other. In fact, it is well documented (Darden and
Maull (1977); Bechtel and Richardson (1993)) that working scientists readily look downwards,
or upwards, across levels of realizing and realized properties in order to find the explanatory
resources to help overcome difficulties. We suggest that one obvious reason for such intertheoretic constraint is that the objects of these theories bear compositional relations, in the shape
of realization relations, which are ontological determination relations and that this ontological
constraint leads to inter-theoretic constraint. Furthermore, the ready ability to reach downwards,
or upwards, across a number of levels of realizing and realized properties also strongly suggests
that the relevant ontological determination relations are transitive – for otherwise there is no
explanation of why working scientists may move so easily between theories at one or more
lower, or higher, levels.
As we have argued at length elsewhere, the nature of these scientific practices and the
inter-theoretic constraint that underpins them apparently supports the view of realization and
multiple realization we have offered.13 For under our account, realization is a transitive
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ontological determination relation and thus under our view of the nature of scientific realization
one may, for example, track realization relations down one, or more, levels of realizer properties
in order to inter-theoretically constrain one’s higher level theory of some higher level realized
property. Given Shapiro’s rejection of transitivity his account of realization cannot account for
this common methodological phenomenon in the sciences which poses a real problem for his
theory of realization and multiple realization. Furthermore, we should also note that Shapiro’s
limitation of R-properties to properties of the same individual, through his sole focus upon
functional analysis, also independently leads to a similar problem, leaving Shapiro’s account of
realization unable to explain why scientists may so readily make use of the relations of
determination between the properties of distinct individuals at different levels.
So far we have focused on difficulties that primarily arise from Shapiro’s rejection of
transitivity, though we have seen that Shapiro’s narrow conception of R-properties produces
related problems. We can now see that the latter feature of Shapiro’s account leads to another set
of difficulties. As we have seen, Shapiro’s exclusive focus is upon just one type of explanation
found in the special sciences, namely, explanation by functional analysis as he understands it.
However, although functional analysis may be a sub-species of mechanistic explanation it is very
plausible that there are very many mechanistic explanations which are not functional analyses.
For example, in our earlier case we saw that the hardness of the diamond is mechanistically
explained using the properties and relations of the diamond’s constituents. Although it is far
from clear that this is an example of functional analysis, such an explanation is clearly a
mechanistic explanation. More importantly for our purposes, whether or not one classes such
inter-individual mechanistic explanations as “functional analyses”, such an inter-individual
mechanistic explanation clearly “uncovers those properties that causally contribute to the
production of the capacity of interest” (Shapiro (2004), p.53). Consequently, a thorny dilemma
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faces Shapiro and appears to establish that, however one resolves questions over the relation of
functional analyses and inter-individual mechanistic explanations, his accounts of realization and
multiple realization must be mistaken since they conflict with scientific findings and practice.
The dilemma proceeds as follows. Either inter-individual mechanistic explanations are
functional analyses or they are not. On one hand, if such inter-individual mechanistic
explanations are functional analyses, then R-properties can include properties of constituents,
like the carbon or Palladium atoms, and we will have realization, and multiple realization, in
cases like the aluminum and steel corkscrews or enamel and Palladium alloy fillings. So, in such
cases, Shapiro’s stated account of realization and multiple realization is mistaken. On the other
hand, if such explanations are not functional analyses, then it is not only functional analysis that,
in Shapiro’s words, “uncovers those properties that causally contribute to the production of the
capacity of interest”, since inter-individual mechanistic explanations of this type clearly do so.
So, again, Shapiro’s account of realization and multiple realization is mistaken. Consequently, it
appears that closer attention to all species of mechanistic explanation in the special sciences
suggests that Shapiro’s account of realization and multiple realization is, in fact, mistaken.
To conclude our discussion of Shapiro’s recent work, we have seen that the theory
schemata for scientific realization and multiple realization that we have presented have passed
their test against Shapiro’s objections about their scope. Furthermore, we have also found reason
to believe that our understanding of realization and multiple realization fits better with scientific
methodology and practice than one of its significant rivals in Shapiro’s own account. Given the
success of our theory schemata, in the final two sections of the paper we therefore propose to
apply our framework to the key question of whether psychological properties are multiply
realized at any neuroscientific level of properties.
19
Part 3 - The Scientific Realization of Psychological Properties (I): Amino Acid Sequences
and the Multiple Realization of Psychological Properties by Biochemical Properties.
We will begin our discussion by using our framework to determine whether our present
scientific evidence supports psychological properties being multiply realized by biochemical
properties, since this question has recently come to the fore with the pioneering work of John
Bickle (2003) on molecular neuroscience. Bickle has presented an empirically informed and
sophisticated defense of the claim that psychological properties are univocally realized by
biochemical properties, arguing that there exists a psychological process of memory
consolidation that is univocally realized across numerous terrestrial species at a single level of
neuroscientific structure, namely, the biochemical level. Thus, we have a concrete case of an
indexed claim of univocal realization that is putatively grounded in neuroscientific evidence.
Aizawa (2007), however, argued that the empirical evidence did not support Bickle’s
view, but this argument proceeded without appeal to a well-formalized theory of realization or
multiple realization. All that was said in this earlier paper was that realization was supposed to
be a non-logical, non-causal determination relation. Here, we can elaborate upon this earlier
work by appeal to the framework we have offered in this paper.14 We shall not challenge the
scientific evidence or theory to which Bickle appeals. Instead, in combination with our
framework for realization and multiple realization, we will dispute Bickle’s analysis by drawing
attention to some additional scientific evidence that we believe Bickle fails to take into proper
account.
For the sake of argument we shall make a number of assumptions. First, we shall follow
Bickle in supposing that there is a single cognitive process of memory consolidation that may be
found in Aplysia, Drosophila, and mice. Second, by ‘memory consolidation’ we shall understand
any cognitive process that enables memories to persist for hours, days, and weeks, rather than
20
merely seconds or minutes. Roughly speaking, memory consolidation is the process of
transforming short term memories into long term memories. And, third, we assume, along with
Bickle, that this process is implemented by numerous biochemical reactions of the sort we will
shortly describe, and that the higher level psychological properties associated with memory
consolidation are together realized by the properties of many such biochemical reactions.
Turn now to a simplified account of the biochemical level, one boiled down to just the
elements that are essential for current philosophical purposes. During electrical activity of
neurons, cells come to have increased levels of a secondary messenger, cyclic adenosine
monophosphate (cAMP). cAMP molecules bind to molecules of protein kinase A (PKA). PKA
is a tetrameric protein containing two regulatory and two catalytic subunits. When cAMP binds
to the PKA regulatory subunits, the entire molecule dissociates leaving two free PKA catalytic
subunits in the cytosol. As the concentration of PKA catalytic subunits increases in the cytosol,
the concentration also increases in the neuronal cell nuclei, where they add phosphate groups to
various other molecules farther along in the biochemical cascade. Eventually, this biochemical
cascade of reactions leads to the synthesis of more proteins that effect structural changes in the
neuronal synapses. These newly synthesized proteins bring about synaptic modifications that are
familiar to philosophers of mind and psychology as one of the basic postulates of connectionist
theories of cognition. Bickle’s claim is that these biochemical reactions provide the univocal
implementation, at this level, of the process of memory consolidation across species – and thus
that the higher level psychological properties associated with memory consolidation are
univocally realized by the properties of these biochemical reactions. Thus in this case, which has
the significant feature of being one of the few where our neuroscience has moved beyond the
fledgling stage, Bickle challenges earlier claims that psychological properties are multiply
realized across species.
21
The science that Bickle fails to consider, however, is the work that shows that PKA
corresponds to distinct amino acid sequences in the different species of organisms that Bickle
considers. Bergold, et al. (1992) for example, report differences in the amino acid sequences of
the regulatory subunits in Aplysia, Drosophila, and mice. Beushausen, et al. (1988) report
differences in the catalytic subunits. Let the regulatory subunit of PKA have the property G of
pairing and binding to PKA catalytic subunits, and dissociating upon binding cAMP.15 There are
many combinations of amino acids that can be bound into chains that have this property. So, one
instance of G might be realized, under normal physiological background conditions $, by a set of
properties (such as charge, size, and polarity) and relations (such as being covalently bonded and
hydrogen bonded), F1-Fn, of one set of amino acids, s1-sp. Additionally, another instance of G
might be realized, under normal physiological conditions $*, by a different set of properties and
relations, F*1-F*m, of a distinct set of amino acids, s1-sq. Given the different characters of the
relevant amino acids, it is plausible that F1-Fn ≠ F*1-F*m, but that F1-Fn and F*1-F*m are all at
the same level. Thus, contrary to Bickle’s claims, once one attends to the evidence about the
differing amino acid sequences in distinct species, then it appears we actually have a paradigm
case of multiple realization of the properties of the biochemical that Bickle focuses upon and
hence of the psychological properties, associated with the process of memory consolidation, that
are realized by the properties of such biochemical reactions.
Essentially the same kind of story can be told about the catalytic subunit of PKA that has
the property of binding to free pairs of PKA regulatory subunits, dissociating from PKA
regulatory subunits binding cAMP, and phosphorylating certain additional components of the
biochemical pathway. Indeed, essentially the same kind of account could be given for any
protein in the relevant biochemical reactions. Thus, for any element of these biochemical
reactions a plausible case can be made that the properties of this element will be multiply
22
realized across species. This finding thus squares with the intuitively formulated contention in
Aizawa (2007) that the properties of distinct amino acid sequences multiply realize the properties
of the biochemical reactions underlying memory consolidation and hence multiply realize the
psychological properties associated with memory consolidation. For realization is plausibly a
transitive relation. Thus since the properties of Bickle’s favored biochemical reactions are
multiply realized at the biochemical level, and given that we are assuming for the sake of
argument that the properties of these reactions together realize the psychological properties
associated with memory consolidation across species, then the transitivity of realization implies
that the psychological properties associated with memory consolidation are multiply realized at
the biochemical level across a variety of species.
The realization and multiple realization schemata offered here articulate informal notions
apparently in play in Bickle, (2003), and Aizawa, (2007). In addition, however, this exchange
illustrates some of the features built into the schemata. The example and the schemata are
mutually supporting. So, in the biochemistry of memory consolidation, we find that biochemical
properties (such as charge, shape, and polarity) and relations (such as covalent and hydrogen
bonding) non-causally determine a psychological property. We also find that the realizing
properties and relations found in biochemistry are wholly unlike, and hence qualitatively
different from, the realized property from psychology. Charge, shape, and polarity are
qualitatively different from such properties as dissociating upon binding a cAMP molecule. A
third feature is that many biochemical properties and relations together to realize the
psychological property. It is not the properties or relations of a single macromolecule that brings
about memory consolidation. Finally, there is an implicitly understood level at which the
psychological properties are supposed to be univocally, or multiply, realized.
23
As well as supporting our earlier findings about realization and multiple realization
generally, the foregoing considerations potentially also support a much stronger conclusion about
multiple realization at this level in particular. Proteins are major components of all neurons, and
indeed all cells in all known cognitive agents. Certain cellular components, such as water,
sodium ions, potassium ions, and calcium ions, will be the same across many distinct types of
organisms, but the proteins will vary. Even homologous proteins will differ to a greater or lesser
degree in their amino acid sequences, so that they will differ to a greater or lesser degree in their
physico-chemical properties. These different amino acid sequences will contribute different
properties and relations toward the realization of higher level properties. Thus, insofar as any
given psychological property is realized, in part, by the properties and relations of such proteins
then that property will evidently be multiply realized at the biochemical level. This appears to
hold for both cognitive and qualitative properties, suggesting that all psychological properties
may be multiply realized across a range of species at the biochemical level.
Obviously a full evaluation of these claims requires one to consider a range of interesting
suggestions about why the latter reasoning is too quick and does not suffice for multiple
realization at the biochemical level. Since one of us has carefully examined many of these
responses in detail elsewhere the natural thing to do is to look at a somewhat higher level of
neuroscience to see whether there is a better chance for univocal realization at that level.16 In the
next section we will therefore consider this question in more detail by focusing on empirical
work at the neuronal level.
Part 4 - The Scientific Realization of Psychological Properties (II): Neuronal Plasticity and
the Multiple Realization of Psychological Properties by Neural Properties.
24
We contend the existence of multiple realization of psychological properties by neuronal
properties can be supported using recent neuroscientific research on a variety of different
cognitive processes, whether neuroanatomical studies of primates and other mammals or
neuroimaging work with human subjects. Elsewhere we have reviewed some of this evidence,
but for our illustrative purposes here we wish to focus on one particular example drawn from
more recent scientific research.17 In this case, in contrast to what we contend the biochemical
evidence shows, we defend a less sweeping conclusion on the basis of some very recent in vivo
microscopic work on neurons. What we think this work strongly suggests is that any
psychological property that depends on the electrical activity of excitatory neurons, across
distinct regions of the cortex, will turn out to be multiply realized by neuronal
properties/relations. What the work actually shows is that certain neurons in the adult cortex
change the structure of their dendritic spines. This, however, suggests that there will be regular
fluctuations in the excitatory electrical signaling properties of these neurons. And this, in turn,
strongly suggests that these neurons and their dendritic spines have properties that multiply
realize many higher-level psychological properties. The current position in recent research, and
our claims about multiple realization at the neuronal level built upon it, should be much clearer
following a review of some of the principal empirical findings.
The orthodox view in contemporary neuroscience is that neurons are the principal
information processing elements in the brain and that they are responsible for many dimensions
of psychological processing.18 Neurons, of course, have dendrites, axons, and cell bodies. The
dendrites are tree-like structures that primarily receive information from other neurons by way of
synaptic connections to them. The cell body contains much of the cellular apparatus for
metabolic maintenance of the cell. The axon connects the neuron to other neurons by way of
25
further synapses. The topic of our neuroscience discussion here focuses on a sub-structure of the
dendrites, namely, dendritic spines.
Dendritic spines appear in many distinct forms as roughly finger-shaped or mushroomshaped extensions on the shafts of neuronal dendrites (see Figure 2). These forms vary in the
size of the neck, spine length, spine volume, head volume, and size of a region called the
“postsynaptic density area”. (See Figure 3 for data on dendritic spines in the hippocampus.)
Each of these properties of a dendritic spine has a plausible role to play in shaping the electrical
signaling properties of neurons and will thus plausibly have a role to play when the properties of
neurons together realize psychological properties. For example, individual dendritic spines can
serve as extensions that can add additional connections between more distantly placed neurons
(see Figure 4A) and they provide for structures that facilitate modulation of the effects of one
presynaptic terminal on the dendrite of a cell (see Figure 4B).
Consider now some recent findings regarding the plasticity of dendritic spines.
Techniques have recently been developed that enable scientists to insert genes that code for a
green fluorescent protein (gfp) into the part of the genome coding for neurons.19 Individual
neurons containing gfp can be reliably reidentified day after day through microscopy. In
addition, scientists can track changes in individual dendritic spines over the course of hours,
days, and weeks. What they find is that, although much of the structure of the dendritic tree is
constant over many days, there are nonetheless individual variations over time.
Given the plausible role of the properties of dendritic spines in the realization of
psychological properties, the implication of these findings about the neuroplasticity at the
dendritic spine level is that we will likely have multiple realization of psychological properties at
this level. Although this is not the final word on this form of neuroplasticity, it does provide
some evidence for the contention that the central nervous system is highly labile and that a given
26
type of psychological process is in fact often associated with a variety of distinct neurological
structures.20 Our accounts of realization, and multiple realization, nicely capture what appears to
be emerging from this research in neuroscience. Let G be any psychological property, such as
remembering the location of a food source, that scientific and philosophical investigation
discovers is realized by the properties/relations F1-Fn of the individual neurons n1-np in a neural
network s under standard physiological conditions $. On our view of realization, this means that
scientific and philosophical investigation have found that, under standard physiological
conditions $, the neural network s has the psychological property G in virtue of the powers
together contributed by properties (such as releasing certain neurotransmitters, binding certain
neurotransmitters) and relations (such as synapsing upon), F1-Fn, of the constituent neurons n1np. What makes for the multiple realization of G is the plasticity of the dendritic spines, the
changes in the patterns and strengths of synaptic connections. Although the individual neurons
realizing G may, or may not vary, it appears that the manner in which they are interconnected
and the strengths of these interconnections vary over periods of hours, days, and weeks. That is,
the initial properties F1-Fn of the relevant neurons will likely change over time, to be replaced
by other properties F*1-F*m, where clearly F1-Fn ≠ F*1-F*m and where these properties are at
the same level. But in such cases it appears that the subject often continues to instantiate the
same psychology and thus continues to have the same psychological property G. Thus, we
apparently have a textbook case of multiple realization of the psychological property at the
neuronal level within the same individual over time.
As with the biochemical example, this neuronal case and our theory schemata are
mutually supporting. Our earlier, more philosophical, discussion of the general nature of
scientific composition makes the present scientific case look mundane, for recall that the
compositional relations posited in mechanistic explanations in the sciences often relate
27
qualitatively different entities. The key to such relations is that although components, such as
realizer properties and relations, are qualitatively distinct from the entity they compose, such as a
realized property, the component entities together non-causally result in the qualitatively distinct
composed entity. But this feature of composition underpins, as we noted earlier, the prevalence
of multiple realization, for it means that differing realizers may together non-causally result in
instances of the same realized property. And this is apparently what we find with neurons, for
although their relevant properties and relations change over time it is often none the less the case
that preceding and succeeding properties and relations of the same neurons, although diverse,
each contribute powers that together non-causally result in, and hence realize, instances of the
same psychological property.
At the biochemical level, we argued for the multiple realization of psychological
properties across individuals of different species. However, we should mark that the evidence we
have just offered for multiple realization at the neuronal level supports such multiple realization
within the very same individual over time. Such evidence the claims of earlier writers, such as
Horgan (1993) or Endicott (1993), who contended that psychological properties are multiply
realized by neuronal properties even within a single individual.
Although the recent work on the plasticity of dendritic spines offers a promising defense
of the multiple realization of psychological properties by neuronal properties, we should close
this section with a note on some limitations of this argument. There is, of course, the empirical
possibility that changes in dendritic spines will simply turn out not to be relevant to any
psychological properties at all. Less dramatically, there could be some properties of some
psychological processes or states, such as moods, that do not depend on the structure of dendritic
spines. Perhaps moods and their properties are entirely a matter of the amounts of particular
neurotransmitters perfusing the neurons and perhaps dendritic spines will have nothing to do
28
with this. If so, then the properties of such psychological processes or states will not be shown
by the foregoing considerations to be multiply realized at this level. This shows that, over and
above an account of realization and multiple realization, it is always primarily a matter of
scientific inquiry to determine what realizes what, whether multiply or univocally. However, we
should also again emphasize that a diverse range of other evidence in addition to that we have
briefly outlined here, for example from both neuroanatomical and neuroimaging studies, can also
be used to support such multiple realization of psychological properties at the neuronal level.
Conclusion
Our opening passage from Boyd encouraged an approach to scientific realization and
multiple realization, amongst other issues, that focuses squarely on real, well-confirmed cases
from the sciences themselves, rather than the weird and wonderful thought experiments that
guided much of the last generation of discussions of multiple realization. Our work in this paper
confirms the utility of Boyd’s approach. For by starting with well-confirmed scientific examples
we have provided a better understanding of compositional relations in the sciences generally, as
well as precise theory schemata for realization and multiple realization.
Furthermore, when taken together our case-studies focused on biochemistry and dendritic
spines suggest that psychological properties are multiply realized at the biochemical and
neuronal levels.21 For we have argued that the amino acid sequences of the relevant proteins
vary across species and the properties of these sequences plausibly contribute the powers
relevant to the realization of higher level properties. Thus psychological properties are plausibly
multiply realized at the biochemical level. We have also given some reason to believe that many
psychological properties (those that depend on the electrical signaling properties of neurons) are
29
also multiply realized at the neuronal level given our evidence about changes in the properties of
dendritic spines over time.
Obviously, the evidence we have offered for these specific multiple realization
hypotheses at particular levels is empirical and these hypotheses are thus defeasible. However,
we have used scientific evidence, from a range of disciplines, to confirm the suggestion of our
other opening passage, from Wimsatt, that multiple realization is not special to the case of
psychology. We have shown that, as the defenders of the received view of special sciences
argued, multiply realized properties are to be found in a range of scientific disciplines, from
materials science and biochemistry to the neurosciences and psychology. Given the success that
apparently results from following Boyd’s advice, we suggest that more writers on multiple
realization, as well as scientific realization and other topics, would do well to focus on concrete,
well-confirmed accounts from the sciences in crafting their views.22
30
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34
Figure 1. The higher and lower level processes involved in the implementation of the process of
a diamond making a Z-shape scratch in glass.
35
Figure 2. A 3D Reconstruction of Dendritic Spines (from Sorra & Harris, 2000).
Ranges in Dimensions of Hippocampal Dendritic Spines and Their Synapses
Dentate Gyrus
Area CA3
Area CA1
Neck Diameter (μm)
0.09-0.54
0.20-1.00
0.038-0.46
Spine length (μm)
0.20-1.78
0.60-6.50
0.160-2.13
3
Spine volume (μm )
0.003-0.23
0.13-1.83
0.004-0.56
Head Volume (μm3)
0.003-0.55
3
Postsynaptic density area (μm )
0.003-0.23
0.01-0.60
0.008-0.54
Figure 3. Variation in Functionally Significant Properties of Dendritic Spines (from Sorra & Harris, 2000)
36
Figure 4. Possible Functions of Dendritic Spines (from Sorra & Harris, 2000). A: Spines increase
connectivity between more distantly placed neurons. The gray dots represent synaptic vesicles containing
neurotransmitters. The black bars represent so-called postsynaptic densities which appear to have a
functional role in the uptake of neurotransmitters. B: Dendritic spines allow structures in which synaptic
connections can be modulated by other neurons synapsing on their necks. C: Sizes and shapes of dendritic
spines allow for the modulation of the effects of a given synaptic connection and for associative connections
between adjacent spines. D: Dendritic spine heads localize the effects of synapses making them more effective
at inducing postsynaptic potentials. E: Dendritic spines facilitate the localization of proteins synthesized for
modifications of the postsynaptic structures.
37
Notes.
1
Philosophers of mind have often discussed multiple realizability, rather than multiple
realization. Here we limit our attention to multiple realization, since it allows us to sidestep
issues about the proper modality, it simplifies discussion, and once multiple realization is
established then multiple realizability simply follows.
2
See Endicott (2005) for a survey of some of main varieties of the concept of realization.
Given how often writers fail to realize the varieties of such concepts, and hence confuse them, it
is worth briefly noting three kinds familiar to philosophers. First, there is a group of semantic
notions that we term ‘Linguistic’, or ‘L’, realization and which hold between entities in the world
and some set of sentences. Famously, for example, the work of David Lewis on topic-neutral
Ramseyfication and theoretical terms uses a notion of L-realization. Basically, on Lewis’ view of
‘realization’ it holds between entities in the world and the set of Ramseyfied sentences putatively
defining some theoretical term ‘F’ – crudely put, an entity X realizes ‘F’, in this sense, when the
entity X satisfies the relevant Ramsey sentences for ‘F’. Second, there is a kind of computational
or mathematical relation commonly referred to as ‘realization’, and used in both the sciences and
philosophy, we term ‘Abstract’, or ‘A’, realization. Very crudely, X is taken to A-realize Y if the
elements of X map onto, or are isomorphic with, the elements of Y. This notion of ‘realization’ is
commonly utilized with formal models and hence with work utilizing such models, for example
in computational accounts of cognitive processes. Note that here the relata of such ‘realization’
relations are largely unconstrained because A-realization simply holds in virtue of a
mathematical mapping, or isomorphism, which can obviously hold between all manner of
entities. Finally, the third class of ‘realization’ relations are what we may term ‘CausalMechanist’ or ‘M’ realization. The latter contrast with L- and A-realization by having as relata
causally individuated entities in the world, often (though not exclusively) property instances. M-
38
realization has been the focus of many writers, but in particular philosophers of science have
been especially interested in such relations which they take to be posited in so-called
‘mechanistic’ explanations in a range of the special sciences. In our discussion, for obvious
reasons, we focus exclusively on M-realization when we discuss ‘realization’.
3
For a detailed discussion of how these problems impact our understanding of the metaphysical
forms of ‘functionalism’, and their associated concepts of a ‘functional property’, ‘realization’,
‘causal role’ etc, see Gillett (2007) and (Forthcoming).
4
See, for example, Fodor (1968), (1974), and (1975); Kitcher (1984); and Wimsatt (1974),
(1976) and (1994), amongst many others.
5
See, for example, Bechtel & Mundale, (1999), Bickle, (2003), and, to a lesser degree, Shapiro,
(2004).
6
See Aizawa & Gillett, (under review).
7
The reader should note that due to limits of the test method, the Knoop hardness value of a
diamond is not so reliably known.
8
This ‘thumbnail’ account is defended in Gillett (2002) and (2003a). A full account of the
‘Dimensioned’ view of realization, as a part of integrated view of the compositional relations
posited in the sciences between ‘packages’ of powers, properties, individuals and mechanisms, is
offered in Gillett (Unpublished).
9
Some readers may be concerned that adding (iv), and the notion of a ‘level’, leaves us with
what many regard as the vague notion of a ‘level’. However, there are reasonably clear scientific
notions of a ‘level of entities’, under some condition, as entities that do, or can, participate in the
same causal mechanisms under those conditions (or which participate in processes that together
implement other processes). Furthermore, elsewhere one of us has outlined a precise definition
39
of this notion of a ‘level’. See Gillett (Unpublished) for a detailed formulation and defense of the
nature and coherence of this notion of ‘level’.
10
There are examples of both types of alloy and enamel that fall outside of these ranges, but all
we need is that some forms of these substances fall within the range to run our example.
11
To take another example, Shapiro considers whether the property of being a corkscrew is
multiply realized, respectively, by the properties of being a waiter’s corkscrew and a doublelever corkscrew.
12
Shapiro’s denial of transitivity for realization appears to be in conflict with his own
assumption that realization relations are determination relations (Shapiro, 2004, p. 39). At least
initially, it appears plausible that if A determines B, and B determines C, then A will determine
C. This need not be the case, but it appears reasonable to ask for some reason why this is not the
case, as Shapiro appears to claim.
13
See Aizawa and Gillett (Under review).
14
Aizawa, (2007), also argued that Shapiro’s, (2004), account of realization does not do justice
to the implicitly understood multiple realization involved in the biochemical case. Insofar,
however, as our theory schemata for realization and multiple realization do justice to the
intuitively understood features of this case, we have further support for the value of our theory
schemata.
15
Nothing of philosophical significance changes if we treat the regulatory subunit of PKA as
having three properties (pairing together, binding to pairs of PKA catalytic subunits, and
dissociating upon binding cAMP) or simply one property (i.e. the combination of those
properties).
16
See Aizawa (2007) for an extended treatment of various important objections to these claims.
17
See Aizawa and Gillett (Forthcoming) and Aizawa and Gillett (Under Review).
40
18
Some recent neuroscientific work has suggested a neuromodulatory role for glial cells (see,
for example, Cotrina, Lin, López-Garcia, Naus, & Nedergaard, (2000)), but nothing of
philosophical import will be lost by setting aside this complication.
19
Grutzendler, Kasthuri, & Gan, (2002), Holtmaat, Trachtenberg, Wilbrecht, Shepherd, Zhang,
Knott, and Svoboda, (2005), Trachtenberg, Chen, Knott, Feng, Sanes, Welker, & Svoboda, K.
(2002), and Zuo, Lin, Chang, & Gan, (2005), use this and related techniques.
20
Cf., Block & Fodor, (1972).
21
Here our focus has not primarily been to assess the extent of the multiple realization of
psychological properties by the properties studied by the neurosciences. However, as the reader
may have guessed from some of our comments we think that such multiple realization is very
extensive. In fact, elsewhere we have defended what we term the ‘Massive Multiple Realization’
hypothesis: the claim that, within a given species, instances of many psychological properties,
are multiply realized at many neuroscientific levels. See Aizawa and Gillett, (forthcoming), for
evidence that supports this hypothesis.
22
We would like to thank the audiences for their comments at the 2006 Wisconsin Workshop on
Special Sciences, in Madison, and at the 2007 conference of the Southern Society for Philosophy
and Psychology, where we presented versions of this paper. We owe special thanks to Fred
Adams, Tom Polger, Larry Shapiro, and Elliot Sober.