Reduction and Elimination in Philosophy and the Sciences

The Functional Unity of Special Science Kinds
Daniel A. Weiskopf, Tampa, Florida, USA
1. Reduction vs. elimination redux
Realization is a relation between a property Φ at one level
of organization and a property Ψ or family of properties
Ψ1-Ψn at a lower level of organization. According to the
Multiple Realizability (MR) thesis, psychological properties,
as well as the properties in the domain of many other special
sciences, are multiply realizable. MR is true of properties
that are defined by some purpose, capacity, or contribution
they make to some end—generally, by their functional
role. Where there are interestingly different ways of
playing the role that defines the property Φ, then Φ has
different realizations. For Φ to be multiply realizable, the
Ψs must belong to distinct kinds, as defined by some independent
taxonomy.
Against this consensus, Shapiro (2000) argues that
the MR thesis is not even coherent. Consider the Ψs that
realize Φ. Either:
(1) these realizers differ in their causally relevant
properties, or
(2) they do not.
‘Causally relevant properties’ are those that enable
something having Ψ to perform the function of a Φ. If (1) is
the case, then the Ψs are different kinds. “But if they are
different kinds then they are not the same kind and so we
do not have a case in which a single kind has multiple
realizations” (Shapiro, 2000, p. 647). That is, if the Ψs
possess different causally relevant properties, then Φ itself
does not constitute a kind, and hence one higher-level kind
isn’t being multiply realized. But if (2) is the case, then they
are not different realizations and the thesis is false in this
instance.
I agree that if Ψ1 and Ψ2 are different independently
certified kinds then we have genuine MR. But I deny that
their being different kinds entails that Φ is not a kind.
Whether Φ is a kind or not depends on whether there is a
sufficiently large and interesting body of empirical
regularities in which Φ itself is implicated. Kindhood
depends on there being a rich cluster of properties that
reliably co-occur with something’s being Φ, where these
properties do not cluster together by chance but by the
operation of some governing principle or mechanism.
It might seem that if Ψ1 and Ψ2 are causally different
ways of bringing about Φ that this would automatically
show that they did not participate in any (nonanalytic)
common regularities. But this isn’t obviously true, since
distinct mechanisms can still give rise to shared properties
and generalizations. We can see this by looking at an
example that Shapiro himself discusses: the case of
compound vs. camera eyes.
2. The eyes of others
Arthropod compound eyes and vertebrate camera eyes
are all eyes in virtue of falling under the functional description
‘organs for seeing’. But different mechanisms are involved
in the production of sight in each kind of eye;
hence, by the anti-MR argument, eyes should not be a
single kind. However, both kinds of eyes can display simi-
lar psychophysical phenomena despite having different
optical properties.
The main phenomenon of interest is the perception
of Mach bands: regions of especially high or low
brightness that occur at the high or low ends of a
brightness gradient. While perception of Mach bands
occurs in many organisms, including primates, cats, and
horseshoe crabs (Limulus polyphemus), the neural circuits
that underlie it differ radically across species. This can be
illustrated with respect to the Limulus eye and the
mammalian eye.
The lateral eyes of Limulus are composed of ~1000
cones that terminate in ommatidia (Battelle, 2006).
Ommatidia contain photoreceptive cells that depolarize a
central eccentric cell, the axons of which form the optic
tract. Eccentric cell axons also distribute collaterals to their
neighbors in adjacent ommatidia. These interwoven
branching collaterals form the lateral plexus of the eye,
which enables one ommatidium to be inhibited by activity
in adjacent ones (Hartline and Ratliff, 1957). Lateral
inhibition enhances contrast and sharpens perception of
edges, and also explains the perception of Mach bands.
Mammalian eyes, while physically and optically
different from compound eyes, also contain inhibitory
mechanisms that produce Mach bands. In contrast to the
loose organization of the lateral plexus, mammalian retinas
are tightly organized into distinct layers. They also use a
vastly greater range of cell types than does the Limulus
eye. Photoreceptive cells feed into a network containing
horizontal, amacrine, and bipolar cells, finally terminating
at ganglion cells that project to higher regions. While there
are many loci for lateral inhibition in the retina, it occurs
initially in the horizontal cells linking adjacent rods and
cones. These cells have highly specific connectivity
patterns, as opposed to the near-random wiring of Limulus
(Field and Chichilnisky, 2007; Sterling, 1998).
So we have two distinct mechanisms for producing
lateral inhibition, and therefore two ways of constructing
visual systems that can perceive Mach bands. Yet both
eyes share more extensive causal properties than just
enabling sight. Implementing lateral inhibition and
producing Mach bands are two such interconnected
properties. While these are not exhaustive of the possible
kinds of eyes, they illustrate the point that distinct kinds at
one level can have numerous other common causal
properties, and therefore constitute a higher-level kind.
Thus Shapiro’s inference from different causal properties
to the absence of a higher-level kind is not generally
sound. Whether a functionally defined property also
constitutes a kind is something to be decided on a caseby-case
basis.
3. Unity through constraint?
Interestingly, Shapiro considers the case of lateral inhibition,
but draws the opposite conclusion from it (Shapiro,
2004, pp. 117-120). Rather than concluding that the use of
this common strategy of visual processing in different species
shows its multiple realizability, he suggests that it
shows that the evolution of visual systems occurs under
tight constraints. These constraints mean that there will
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