The Massive Multiple Realization of Psychology Properties
In my talk today, I want to cover some excerpts from a series of papers I have coauthored with Carl Gillett. The central thesis of our work is what we call the Massive Multiple
Realization of Psychological Properties.
(Massive Multiple Realization) Many human psychological properties are multiply
realized at many neurobiological levels.
Something like this view is, of course, the orthodox view in the philosophy of mind, but it is an
orthodoxy that has recently been challenged by Shagrir, (1998), Bechtel & Mundale, (1999),
Bickle, (2003), Polger, (2004), and Shapiro, (2004). What we think unifies these criticisms of
orthodoxy is a drive to raise the standards for scientific accuracy and metaphysical clarity.
Shagrir, Polger, and Shapiro, all press for greater clarity on the metaphysics. Bickle and Bechtel
and Mundale propose to look much more closely at the results of actual neuroscientific research.
The general tenor of our reply to these criticisms is to re-raise the standards. We think that if one
really develops a precise metaphysical theory of realization and multiple realization and if one
really looks closely at actual neuroscientific practice, one finds good reason to believe in
(MMR).
Neuroscientists, like all biologists, hold two fundamental beliefs about nervous systems.
First, they believe that nervous systems can be studied at any number of distinct, but
interdependent, levels of organization in which entities at one level are explained by the
qualitatively different entities at one, or more, lower levels that are taken to compose them.
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Neuroscientists study structures as large as communities of interacting organisms and as small as
individual proteins. There are thus a number of neurobiological levels.
The second fundamental belief shared by neuroscientists is that nervous systems display
individual variation. Subsequent research has shown that Darwin surely understated the case,
especially in the case of the subject-matter of the neurosciences, when he observed that not all
individuals of a species are cast from the same mold. Organisms obviously vary in their genetic
make up, but given distinct histories of interaction with their environments even genetically
identical individuals will diverge in their phenotypic details. In truth, no two organisms are
exactly alike, molecule for molecule, or cell for cell, or organ for organ – especially when the
molecules, cells and organs in question are those studied by the neurosciences.
Combining these two fundamental beliefs, we may say that, as far as can currently be
determined, individual variation appears at every level of neurobiological organization. As a
result, since component entities such as realizer properties vary at particular levels, we contend
that we have overwhelming scientific evidence for what we will term the ‘Massive Multiple
Realization’ (MMR) hypothesis about psychological properties:
(Massive Multiple Realization) Many human psychological properties are multiply
realized at many neurobiological levels.
Putting the thesis in other words, Massive Multiple Realization is the claim that for many human
psychological properties the instances of these properties are realized by different lower level
properties at levels of entities studied in neuroscience. There are, of course, a number of
stronger, but related versions of this hypothesis. For example, one can change the two “many”
quantifiers to “most” or “all” and not limit the hypothesis merely to human psychological
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processes. For the space of the present discussion, however, I shall examine only this weaker
hypothesis.
My aim in this paper will not be to provide so much in the way of a defense of this
hypothesis. Instead I will work on two preliminaries. The first of these preliminaries is to
articulate the kind of realization and multiple realization we think is in play in this thesis. This is
where Carl and I propose to take the metaphysics of realization and multiple realization really
seriously. The second of these preliminary goals is to show how this apparatus applies in an
extended case study. Although Carl and I believe that we can offer additional justification in
support of (MMR), providing such justification will not be possible in the space of this talk. A
single case study will have to suffice. Maybe another way of introducing my talk is to say that it
is not so much a defense of a thesis as it is an introduction to one of my philosophical projects,
namely, a more metaphysically and scientifically sophisticated theory of realization and multiple
realization in the sciences. So, with that brief roadmap, let me begin explaining our take on
realization.
1. Realization
The term “realization” is used in many ways in different philosophical projects in
different areas of philosophy. The sense that we have in mind appears to be what John Searle
was describing in one of the “oldies, but goodies,” Minds, Brains, and Science:
it seems to me that there is no difficulty in accounting for the relations of the mind to the
brain in terms of the brain’s functioning to cause mental states. Just as the liquidity of the
water is caused by the behavior of elements at the micro-level, and yet at the same time it
is a feature realized in the system of microelements, so in exactly that sense of “caused
by” and “realized in” mental phenomena are caused by processes going on in the brain at
the neuronal or modular level, and at the same time they are realized in the very system
that consists of neurons. And just as we need the micro/macro distinction for any physical
system, so for the same reasons we need the micro/ macro distinction for the brain. And
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though we can say of a system of particles that it is 10°C or it is solid or it is liquid, we
cannot say of any given particle that this particle is solid, this particle is liquid, this
particle is 10°C. I can't for example reach into this glass of water, pull out a molecule and
say: “This one's wet.” (Searle, 1984, p. 22).
So, take Searle’s example of the temperature of a sample of gas being 10°C. For simplicity of
exposition, let us suppose that our sample consists of pure hydrogen, that is, molecules of
hydrogen made up of exactly two atoms of hydrogen. In this case there is a “macro-level”
individual (the sample) bearing a “macro-level” property, namely, having a temperature of 10°C.
The sample of gas has its property in virtue of a collection of “micro-level” individuals (the
hydrogen molecules) and their “micro-level” properties (their masses and velocities). So, it is
because the molecules of the gas have the masses and velocities they do that the sample of the
gas is 10°C. And (almost), as Searle says, even though the sample has a temperature of 10°C, no
molecule has a temperature of 10°C. This skips some important technical details and the
example ultimately does not work, but I want to stay with it because it has a simple structure that
will make it easier to convey the formal apparatus of our theory of realization.
We can state the foregoing a bit more formally by saying that the gas sample’s property,
G, of being 10°C is realized by the masses m1, m2, …, mm, and velocities v1, v2, …, vm of the
gas’s constituent molecules s1, s2, …, sm. Letting F1-Fn = {m1, m2, …, mm, v1, v2, …, vm} we can
say that G is realized by F1-Fn. Notice that, in focusing on the masses and velocities of the
molecules of hydrogen, we are picking out the properties of the molecules that are relevant to the
temperature of the gas. We are ignoring other properties of the molecules, such as the fact that
they are diatomic, that they contain two protons and two electrons each, that they are electrically
neutral, that their covalent bond is due to the filling of a 1s orbital, and so forth. All that matters
for the temperature of a gas are the masses and velocities of the constituent particles.
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The foregoing suggests the following sketch of an account of realization, what we call the
“Dimensioned view”:
(Dimensioned Realization) Property/relation instance(s) F1-Fn realize an instance of a
property G, in an individual s under condition $, if and only if, under conditions $, s has
G in virtue of s’s constituents having F1-Fn, but not vice versa.
This schema is again a simplification of the Dimensioned view, but I think it will do for present
purposes, since as it stands, it captures a number of important features of realization that may
well be only implicit in Searle’s examples. First, there are many cases in which the realization of
G by F1-Fn will be relative to, or will presuppose, certain background conditions, such as
standard temperature and pressure, a particular range of gravitational fields, and so forth. We
capture this in our schema by reference to the background conditions $.
Second, for present purposes, I will treat the “in virtue of” relation as the bottom of the
expository heap. One may go deeper in more metaphysical studies, but this seems to be deep
enough to capture the surface metaphysics of science. That said, I do wish to note that by “in
virtue of,” I will, however, note that I mean a logical or mathematical determination relation. So,
for example, the realization relation we are concerned with is not like the relation between the
determinable red and determinate shade of red, scarlet. In the sense that concerns us,
determinables are not realized by determinates. Although I will not go into it, this is the point
were Searle’s example will break down for us.
Third, the “in virtue of relation” is also a kind of non-causal determination relation. The
determination relation we are describing is unlike causation in a number of respects. A)
Causation is a kind of “horizontal” determination relation that is temporally extended, relates
wholly distinct entities and often involves the transfer of energy and/or the mediation of force.
So, when one billiard ball collides with another, the first moves toward the second, then collides
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with it, and both move apart. This process is extended in time. By contrast, it takes no time for
the masses and velocities of the hydrogen gas to realize the temperature of the gas. The
instantaneous masses and velocities determine the temperature. B) When two billiard balls
collide, the first ball is distinct from the second. By contrast, the molecules of the sample of gas
are literally parts of the whole sample. So, causation, but not realization, involves wholly
distinct entities. C) As we can see in the collision of the billiard balls, causation involves
transfer of energy from one billiard ball to another. But, there is no transfer of energy when the
molecules of the gas realize the temperature of the gas.
Third, there is Searle’s observation that it can happen that a higher level property, such as
having a particular temperature or solidity, is qualitatively different than are the properties that
realize it. Searle noted that, while a sample of gas may be 10°C, no molecule of the gas is. (This
is correct in a rough and ready way, since after all it is possible for a single hydrogen molecule to
have the right mass and velocity to bear the temperature of 10°C.) Where the gas has a
temperature, the individual molecules have qualitatively different properties, such as mass and
velocity.
Fourth, it is often the joint action of the lower level properties that brings about the higher
level property. So, it is all the masses and all the velocities that together bring it about that the
sample of gas has the temperature it does.
These are features of the realization relation that we will return to in our case study.
2. Multiple Realization
A temperature of 10°C can be realized in many ways. The same ensemble of molecules
we mentioned earlier can have their velocities changed while preserving their average velocity.
Or, a different set of molecules with different masses and different velocities could also realize
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the same mean kinetic energy. Let me formalize this a bit more as preparation for our definition
of multiple realization. So, again, G is again the “macro-level” property of being 10°C. As we
noted above, one way to realize G is to have molecules s1, s2, …, sm bearing properties F1-Fn =
{m1, m2, …, mm, v1, v2, …, vm} under conditions $. Another way involves a different set of
molecules s*1, s*2, …, s*p bearing properties F*1-F*m = {m*1, m*2, …, m*p, v*1, v*2, …, v*p}
under conditions $*. We, thus, get the gist of multiple realization if F1-Fn ≠ F*1-F*m.
The foregoing, we think, captures most of the familiar ideas about the kind of multiple
realization we are looking at. There is, however, one more absolutely essential refinement. For
multiple realization of G, F1-Fn and F*1-F*m must be on the same level. Elaborating on our
temperature example will show why. Suppose, as before, that we take one realization of G to be
the molecules s1, s2, …, sm bearing properties F1-Fn. Now, think about the two atoms of
hydrogen that make up each molecule of hydrogen. Instead of molecules s1, s2, …, sm we might
enumerate the atoms s’1, s’’1, s’2, s’’2, …, s’m, s’’m. Similarly, instead of the molecular properties
{m1, m2, …, mm, v1, v2, …, vm} we can think of the atomic properties, {m’1, m’’1, m’2, m’’2, …,
m’m, m’’m, v’1, v’1, v’2, v’2,…, v’m, v’’m}. Similarly, we can denote this last set by F*1-F*m.
Now, suppose, merely for the sake of expository simplicity, that the atoms also realize the
temperature of the gas. This apparently allows one to have the temperature of a gas realized both
by the molecules of the hydrogen and by the atoms of hydrogen, so that we get two realizations
of the temperature, hence multiple realization of the temperature. But, this is not the kind of
example that is typically counted as multiple realization. So, we need to be sure to exclude this
kind of case.
We can now extrapolate from the foregoing example to generate a schema for the kind of
multiple realization we have in mind:
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(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 instances of
properties/relations F1-Fn to 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 instances of
properties/relations F*1-F*m of s*’s constituents, but not vice versa; (iii) F1-Fn ≠ F*1F*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.
Where I think that conditions (i)-(iii) are pretty familiar, I think it is important to emphasize the
importance of the less widely appreciated condition (iv). Because of condition (iv), Carl and I
often describe our theory as the theory of indexed realization and multiple realization. Even
though it is common practice to speak merely of “multiple realization,” we think that it is
important to treat this only as a kind of typically harmless shorthand. In all strictness and
explicitness, we think that realization and multiple realization make an implicit reference to one
or another level. One motivation for this strictness and explicitness noted above is simply doing
justice to what is commonly meant by multiple realization. There are, however, two other
reasons for noting this relativization. First, it forestalls certain objections to particular examples
of multiple realization, and, second, it helps us understand at least part of the philosophical
dialectic in debates about multiple or unique realization. Both of these I want to explain by some
allusions to biochemistry.
In my 2007 paper in Synthese, I claimed that all human psychological properties are
multiply realized by distinct sequences of amino acids. Here is the quick and dirty version of an
argument for this claim. Every human psychological property are realized, in part, neurons,
neurons are realized, in part, by proteins, and proteins are realized, in part, by amino acids. In
addition, our best scientific evidence suggests that there are different combinations of amino
acids that realize the same psychological property. There is plenty of room here for arguments
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about the scientific details and the state of empirical knowledge in this area, but there is also a
simple, armchair philosophical objection as well. I want to address that briefly here.
One might say that any notion of multiple realization that says that psychological
properties are multiple realized by biochemical properties is ipso facto wrong. Any such theory
simply trivializes the notion of multiple realization. If that’s what you, Aizawa, mean by the
multiple realization of human psychological properties, then of course there is that kind of
multiple realization. That’s just trivial. Is there anyone who would ever have doubted this?
The answer here appeals to our condition (iv). The sense that the multiple realization of
psychological properties by biochemical properties is obvious and trivial is captured by the
indexed theory of multiple realization, since it allows us to say that in fact psychological
properties are multiply realized by biochemical properties, but it also allows for the existence of
other interesting questions, such as whether or not psychological properties are multiply or
univocally realized by some other, higher level of neuroscientific structure. The long-standing,
philosophically interesting question is not whether psychological properties are multiply realized
at some level. They obviously are. The long-standing philosophically interesting question is
whether psychological properties are multiply realized at all levels. If there were some level at
which psychological properties were uniquely realized, then there would be some level at which
psychological properties might be type-type reduce. If psychological properties were uniquely
realized in, say, some neuronal level properties, then there would be a point at which an
argument for type-type reduction might begin to get some traction.
So, the indexing of multiple realization provides us with a way of meeting the charge of
trivialization. The short of it is that the indexing allows us to recognize the truth of some (trivial)
multiple realization claims, while allowing us to explore another long-standing philosophical
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question. The second reason for recognizing indexing actually grades into the first. Recognizing
indexing provides us with a way of more clearly articulating certain replies to multiple
realization arguments. Part of what is going on in certain replies is to concede multiple
realization at one level, but to hold out hope for univocal realization at another. Let me run
through just one illustration. This appears to be at least part of what is going on in this wellknown observation by Jaegwon Kim,
[T]he fact that two brains are physico-chemically different does not entail that the two
brains cannot be in the “same physico-chemical state.” ... To argue that the human brain
and the canine brain cannot be in the same brain state because of their different physicochemical structure is like arguing that there can be no microphysical state underlying
temperature because all kinds of objects with extremely diverse microphysical
compositions can have the same temperature; or that water-solubility cannot have a
microstructural “correlate” because both salt and sugar which differ a great deal from
each other in atomic composition are soluble in water. If the human brain and the
reptilian brain can be in the same “temperature state,” why can they not be in the same
“brain state,” where this state is characterized in physico-chemical terms? (1972, pp.
189-190).
There are at least two things going on in this passage. When Kim says that, “all kinds of objects
with extremely diverse microphysical compositions can have the same temperature,” part of
what he may mean is a point I articulated earlier. In spelling out what realizes the temperature of
a sample of hydrogen gas, only certain properties matter. The masses and the velocities of the
hydrogen molecules matter, but the fact that they are diatomic, that they contain two protons and
two electrons each, and so forth does not matter. Another thing that could be going on is an
issue of levels. Kim might mean that two samples of a gas can have the same temperature and
the same mean kinetic energy, but then all sorts of different combinations of masses and
velocities for the atoms can have the same mean kinetic energy. So, the picture could be that
there is one high level property, temperature, then one middle level property, mean kinetic
energy, and, finally, several lower level ensembles of masses and velocities. The level theory of
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realization accommodates this idea. Stated quite generally, in point of logic, one can have a
property at level n that is univocally realized at level n-1, then multiple realized at level n-2, then
uniquely realized at level n-3, and so forth.
So, leveled realization can help articulate part of what Kim might have been up to. But, it
assuredly articulates some of the dialectic I’ve encountered in claiming that psychological
properties are multiply realized by distinct amino acid sequences. In replying to my argument in
Synthese, Bickle has not denied that there are multiple chains of amino acids underlying
psychological properties. Instead, he has sought a higher level at which there might be univocal
realizations. Another way to frame my argument is to say that psychological properties are
multiply realized by distinct primary structures of amino acids, i.e., the sequences of amino acids
in chains. Bickle has claimed (roughly) that these amino acids fold up into more complicated
tertiary structures and these higher level tertiary structures provide a unique realization of
psychological properties. I do not think that this is correct, but my present point is merely that
one can make sense of Bickle’s move in terms of higher level structures that might provide for a
unique realization.
So, to summarize what I have said about multiple realization, the idea is basically that
you get multiple realization when you can make a higher level property out of different
combinations of relevant lower level properties at the same level. The indexing of multiple
realizations to levels is valuable for three reasons. First, it allows us to express what
philosophers commonly mean by multiple realization. Second, it allows us to express what is
correct, but not the entire story behind, claiming that psychological properties are multiply
realized by distinct sequences of amino acids. Third, it allows us to articulate more clearly some
of the dialectic of arguments about multiple realization.
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3. A Scientific Case Study
To this point, I have been presenting the theories of realization and multiple realization
with an eye toward a clear exposition of the view. With an eye on clarity, I have cut a few
corners, but I think the general picture comes through. The idea has been to be true to the
metaphysics of the target conceptions. Now, however, I want to discuss the second dimension of
the Gillett-Aizawa project, namely, how these theories of realization and multiple realization
make sense of actual scientific practice. To do this, I will be working part way through a case
study, the realization and multiple realization of color vision in humans. I will show how
realization works at a variety of levels and how it is at least plausible to conclude that there is
multiple realization at these levels. At the outset, I should say that 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 in the philosophy of mind. 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.
The case study to be presented here involves human color vision and in particular the
contribution made by the eye. If there is multiple realization of the color-relevant properties in
the eye at many levels, then there will be multiple realization of color vision at many levels. So,
consider, first, the relevant individuals involved in the hierarchy shown in Figure 1. 1A shows
the entire eye containing the retina. 1B shows the retina and the primary types of nerve cells to
be found within it. 1C shows the outer segment of a cone cell. Cones are the photoreceptors that
are involved in color vision; the outer segment is the portion of the cone in which light is
converted into a biochemical signal. 1D shows two photopigment molecules embedded in the
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membrane of the outer segment. 1E is a schematic drawing showing the individual amino acids
that constitute the opsin portion of the photopigment molecule. The seven cylinders represent
the portion of the opsin that spans the outer segment of the membrane. To see how our
realization and multiple realization schemata work, we will begin with the realization of
properties of the green cone photopigments.
Human color vision is classically taken to involve three distinct cone photopigments,
sometimes called the red, green, and blue photopigments. The green photopigment is made up of
a protein component called an opsin and a chromophore (11-cis-retinal, a derivative of vitamin
A). The green photopigment has peak light sensitivity of about 530 nm (λmax = 530 nm) and the
sensitivity curve has roughly a bell shape. (See Figure 2.) This sensitivity curve determines the
probability of capturing a photon of a given wavelength. The green cone opsin is a chain of 364
amino acids. So what looks to be going on here is that the “micro-level” or lower level
properties and relations of the 364 individual amino acids in the chain, in conjunction with the
properties of the 11-cis-retinal, realize the “macro-level” or higher level properties of the green
cone pigment. Our theory of realization allows us to describe this.
In our hydrogen gas example, we had only one type of micro-individual, a hydrogen
molecule. In this case, however, we have one or more instances of 20 different amino acids to
contend with along with the 11-cis-retinal. In the hydrogen gas example, only two properties of
each molecule were relevant to the realization of the temperature of the gas, namely, the
molecule’s mass and velocity. In the green photopigment case, however, we have many more
properties and relations that are relevant to the peak and the shape of the photopigment’s light
sensitivity curve. The 11-cis-retinal, for example, has the property of capturing photons in a
portion of the visible spectrum of electromagnetic radiation. The individual amino acids,
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however, have properties such as polarity, size, and charge, along with peptide bonds between
them. This makes it a bit messy to come up with a nice notation such as we had for the masses
m1, m2… mm, and velocities v1, v2, …, vm in our hydrogen gas example, but so I will provide
only a rough account. So, it might be something like P’1, P’’1, P’’’1, P’2, P’’2, P’’’2, P’’’’2 , …,
P’365, where the subscripts indicate which of the 365 different individuals bears the
properties/relations and where the superscripts indicate one or another of the properties/relation.
In our formal schema, however, we just wash out all this additional notational apparatus and
enumerate all the properties and relations as F1-Fn. So, plugging this into our schema, we would
say that property/relation instances F1-Fn realize an instance of the property G of having a λmax =
530 nm in an individual green photopigment molecule s under normal physiological conditions
of the retina $, if and only if the photopigment molecule has G in virtue of the molecule’s having
F1-Fn under conditions $, but not vice versa. And, of course, we could tell a comparable story
about the shape of the molecule’s spectral sensitivity curve.
What makes the biochemical level so apt for multiple realization is that one expects, on
general biological principles, that there should be various genetic mutations that give rise to
differences in amino acid sequences, but which give rise to proteins with the same property.
That is, one of the cornerstones of evolutionary biology is that there is a pool of variation upon
which natural selection can act and that many of these variations are selectively neutral. So, in
Aizawa, (2007), this was the empirical basis upon which I argued for the multiple realization of
psychological properties. What makes this example of the green photopigment especially
interesting is that in the last ten years, neuroscientists have actually measured the absorption
spectra of molecularly distinct photopigments and found them to have the same λmax. That is,
there are genetic mutations that have given rise to chemically distinct green photopigments that
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have the same λmax. These are the gray and the green curves in the upper left panel of Figure 3.
And, as one might expect, the same holds for the red photopigment. So, we now have two
chemically distinct green photopigment molecules that have different sets of lower level
properties, but the same λmax. Put in our formalism we can say that two instances of the property
G of having λmax = 530 nm are multiply realized since (i) under normal physiological conditions
$, one molecule has λmax = 530 nm in virtue of the instances of properties/relations F1-Fn
belonging to that molecule’s constituents, but not vice versa and (ii) under those same
physiological conditions $ another molecule has λmax = 530 nm in virtue of the instances of
properties/relations F*1-F*m of the other molecules constituents, but not vice versa; (iii) F1-Fn ≠
F*1-F*m and (iv), under conditions $ and $*, F1-Fn and F*1-F*m are at the same scientific level
of properties.
With our first step in our case study, I want to remind everyone of the features of
realization to which we have drawn attention. First, there is a non-causal, non-logical
determination relation between the properties/relations of the amino acids and 11-cis-retinal and
the properties of having λmax = 530 nm. Second, the lower level properties are qualitatively
different than the higher level properties. No individual amino acid or the chromophore (11-cisretinal) has a λmax = 530 nm, a property that the entire protein does. Instead, the relevant
properties of the amino acids are their size, charge, polarity, bonding arrangement, and so forth.
Third, it is the joint action of the properties of the amino acids and the chromophore that together
realize higher level property λmax = 530 nm. This concludes our application of our schemata for
realization and multiple realization to the biochemical level. Now I will move up to a slightly
higher level to the realization of an entire nerve cell, green cone. At this level, I will offer a bit
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less about the realization and multiple realization schemata and a bit more about the scientific
details.
To explain this case, let me begin with the individuals involved. At the lower level, the
relevant individuals include water molecules, ions (such as Ka+, Na+, and Ca++), phospholipid
molecules, proteins, etc., and, at the higher level, the individual under consideration is a human
green cone. (See Figures 4 and 5.) Phospholipid molecules have both a hydrophilic and a
hydrophobic region. Given this configuration, they spontaneously form a bilayer structure in
which the hydrophilic regions face outward to an external aqueous environment in either the
extracellular space or the cytoplasm, while the hydrophobic tails of the molecules cluster
together inside the bilayer. This phospholipid bilayer constitutes the cell membrane, illustrated
in the right half of Figure 5. Proteins, for their part, also have hydrophobic and hydrophilic
portions which help embed them in the cell membrane. (See again the right half of Figure 5.)
Human cone opsins, for example, have an evolutionarily well-conserved set of seven
transmembrance amino acid sequences. (See Figure 6.) Ion channels, also, have amino acid
sequences that enable them to span the cell membrane and provide bindings sites on one or
another side to regulate the flow of ions through the channel. Cytoskeletal proteins, also
partially embedded in the cell membrane, shape a cell into exotic configurations such as those of
the rods or cones.
Now consider how this apparatus transforms the capture of a photon into a neuronal
signal. Photopigment molecules are embedded in the cell membrane in the outer segment of the
cone. (Recall Figure 5.) Upon absorption of a photon, a single photopigment molecule will
change conformation and release into the cytoplasm a molecule of all-trans-retinal leaving an
activated opsin molecule in the membrane. (See Figure 7.) One activated an opsin molecule
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binds to a single G protein molecule located on the inner surface of the cell membrane. This G
protein molecule, in turn, activates a molecule of an enzyme, cGMP phosphodiesterase, which
breaks down cGMP. When intracellular cGMP concentrations subsequently decrease, cGMP is
removed from a cGMP-gated Na+ channel, leading to the closure of the channel. Closing of the
channel, blocks the influx of Na+ into the cell. In concert, vast numbers of photopigment
molecules, G protein molecules, ion channels, and Na+ ions go through this process leading to
the hyperpolarization of the cell. This hyperpolarization propagates from the outer segment to
the synaptic contact of the cone, where it reduces the rate of release of the neurotransmitter
glutamate. This reduction in neurotransmitter release is the cone’s signal that the cell has been
illuminated. The foregoing lower level processes may be summarized schematically as follows:
Photon capture → all-trans-retinal release → G protein activation →
cGMP phosphodiesterase activation → cGMP decrease →
cGMP released from ion channels  ion channel closure →
cone hyperpolarization → decreased glutamate release.
Obviously a large number of these molecular processes occur together and these lower level
processes implement the cellular process of signaling the presence of light by release of
glutamate. Consequently, we can thus also see that the cone’s property of releasing a
neurotransmitter in the presence of light is evidently realized by the properties and relations of
the molecular individuals within the cell.
So, that is a fair bit of the science of the biochemistry of phototransduction. Let me now
plug this into our realization and multiple realization schemata. Our higher level property G will
be the property of reducing the release of a neurotransmitter in response to the absorption of light
of a particular band of frequencies. Or, oversimplifying somewhat, we might just say that G is
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the property of transduction. The lower level properties F1-Fn will be the multitude of properties
of the many individual green photopigment molecules, the many G protein molecules, the many
cGMP phosphodiesterase molecules, the many cGMP molecules, the many Na+ ion channels, the
many Na+ ions, the many synaptic vesicles, etc. One property of a green photopigment molecule
that is relevant to vision is its λmax, another relevant property of the green cone opsin is its ability
to bind and activate a G protein molecule, a relevant property of the G protein molecule is its
capacity to activate a molecule of cGMP phosphodiesterase, the relevant property of cGMP is to
bind and unbind from the Na+ channel, and so forth. These properties together can be placed
into our schema for realization. This is a bit quick and imprecise, but I hope the basic picture of
how this counts as realization is clear.
Now turn to multiple realization. What we would need for this would be to have two
cones that have the higher level property G of changing its release of neurotransmitter in
response to the absorption of light, but which differ in their lower level realizer properties. To
this end, we cannot rely on cGMP or Na+ ions. cGMP is cGMP and Na+ ions are Na+ ions. They
do not vary in the properties they contribute to the realization of G. Also, since one might think
that the identifying properties of a medium wavelength cone come from the properties of its
constituent green photopigment, I want to argue for the multiple realization of the cone based the
properties of other proteins in the biochemical cascade. So, think for a minute of just, say, the G
protein molecule. Among its relevant properties is the fact that it is activated by the green cone
opsin. But, as with the green cone opsin, there will likely be different amino acid sequences for
the G protein molecule, hence different G proteins that differ slightly in their activation by the
green cone opsin. Note that sometimes different amino acid sequences give rise to the same
higher level property and at other times different amino acid sequences give rise to different
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higher level properties. When different amino acid sequences give rise to the same higher level
property, we can point to such cases as instances of multiple realization at the protein level.
When different amino acid sequences give rise to different higher level properties at the protein
level, we can use those higher level properties at the protein level as a basis for a case for
multiple realization at the cellular level. So, the idea in short is this. We get one set of realizing
properties and relations F1-Fn using the properties of one G protein and another set of realizing
properties and relations F*1-F*n using the properties of another G protein.
As a caveat on this, I should say that I do not know of experimental work specifically
verifying my claim that G proteins vary in their amino acid sequences, hence in their higher level
protein properties, but such studies may exist. In this case, I base my estimate of the likelihood
of different amino acid sequences in G proteins, hence different properties of G proteins, on
general evolutionary considerations.
The last case I want to consider is a cell-to-tissue case which concerns how a certain
property of the retina is realized and multiply realized by cellular level properties. In this
example, the lower level individuals are the particular photoreceptor cells, the amacrine cells,
bipolar cells, horizontal cells, and retinal ganglion cells. (See Figure 8.) The higher level
individual is a retina. The lower level realizing properties and relations are properties of the
cones, amacrine, bipolar, horizontal, and retinal ganglion cells. These lower level properties and
relations include releasing glutamate in the presence of light within a given band of frequencies,
binding certain neurotransmitters, having certain electrochemical synapses, and certain patterns
of connectivity. The higher level property of the retina is its signaling a pattern of color in the
visual field.
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At this level, for the sake of simplicity, I will only focus on the contribution of the
photoreceptors to the realization and multiple realization base of human color processing in the
retina. As you know, human color vision is generally treated as trichromatic. The three types of
cones in the normal human eye are sometimes referred to as red, green, and blue. That is, each
type of cone changes its glutamate release in response to a different band of frequencies of
incident light. (See Figure 9.) Each type of cone releases glutamate as it does in virtue of
containing a chemically distinct photopigment. That is, each photopigment consists of a protein
component, an opsin, covalently bonded to an 11-cis-retinal component and it is the opsin
components that vary from cone to cone. The amacrine, bipolar, horizontal, and ganglion cells,
of course, contribute to the retina as well, but again for simplicity we will set their role to one
side.
This cell-to-tissue example again supports the existence of the features of the realization
relation we described in our previous cases. First, the lower level properties and relations of the
cells stand in a synchronous, non-causal, non-logical determination relation to the higher level
property of signaling patterns of color in the visual field. And there is no transmission of energy
or mediation of force between the lower level properties and relations of the cells to the higher
level property of the retina, where the relevant properties, and the individuals that have them, are
not wholly distinct. Third, the relata in this case of realization are again of qualitatively distinct
kinds. The relevant determining properties and relations of the cells include their capacity to
release glutamate in response to illumination, releasing certain neurotransmitters, binding certain
neurotransmitters, having certain electrochemical synapses, having certain patterns of
connectivity, etc., whereas, in contrast, the determined property of the retina is its signaling
patterns of color in the visual field. Fourth, in the case of the retina, once again many properties
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and relations of individual cells together non-causally result in the retina’s property of signaling
a pattern of color in the visual field.
What reason is there to think that we have multiple realization of the color processing
capacity of the retina? There are many. The simplest, however, seems to come from the recent
discovery of large individual variations in the cone mosaic, the layout of the cones in the fovea
of the retina. One of the things that matters to color vision is the presence of red, green, and blue
cones. Yet, the red-green cone ratio varies substantially. Figures 10 and 11 are illustrations of
the differences between two subjects, JW and AN, reported by Williams and Roorda, (1999).
The following provides quantitative data on these differences.
Subject
Red cone (%)
Green cone (%)
Blue cone (%)
Red: Green cone ratio
JW
75.8
20.0
2.1
3.79
AN
50.6
44.2
5.6
1.15
Although all three types of cones (R, G, & B) matter for normal color vision and although the
ratios of these types of cones matters for color vision, the ratios can still vary by a sizeable
amount. So, it looks as though there are various combinations of instances of the properties and
relations of the red, green, and blue cones that can enable normal color vision.
Another way to get multiple realization of a color processing retina is through
dichromacy. Dichromacy is a genetic disorder in which one type of cone is not present, typically
either the red or green cones. These are the most familiar types of color blindness. Yet, color
blind individuals are not completely unable to see colors. Instead, there are certain color
discriminations they cannot make. They make fewer color discriminations than do color normal
individuals. So, their retinas are still color processors. There are, however, three ways of being
a dichromatic retina, each corresponding to the loss of a distinct photopigment. The retina of a
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‘protanope’ lacks red cones, the retina of a ‘deuteranope’ lacks green cones, and the retina of a
‘tritanope’ lacks blue cones. Each form of dichromacy corresponds to a distinct realization of an
instance of the property of signaling patterns of color in the visual field. So, we have a second
type of multiple realization in the human retina.
A third type of multiple realization involves the less dramatic, and perhaps somewhat less
familiar, cases of what are termed ‘anomalous trichromats’. These individuals possess three
distinct types of cones, but the sensitivities of the cones are not those of normal cones. So, for
example, in one of the most common forms of red-green colorblindness, the red cones and the
green cones release glutamate in response to relatively similar bands of electromagnetic
radiation, hence there is not enough difference in the properties of the red and green cones to
implement a higher level process that can signal differences between certain patterns of color.
This gives rise to color-sensitivity “blindspots”. Anomalous trichromacy is, of course, a kind of
color processing, and its subjects do realize instances of the property of signaling patterns of
color in the visual field, but in addition it grades off into normal color vision. There is no sharp
dividing line between anomalous trichromacy that is normal and anomalous trichromacy that is
abnormal. To take but one well-known example in the biochemistry of vision literature, the
human red cone appears to be ‘polymorphic’. That is, it comes in two forms. One form of the
red cone has the photopigment with an amino acid sequence with a serine amino acid at position
180, where the other photopigment has alanine at that position. Both forms are quite common in
the human population. They correspond to the two red curves in all the panels of Figure 3. So,
one has the multiple realization of normal human color vision by having some individuals with
cones containing one red photopigment and other individuals having cones containing the other. 1
1
See Sharpe, et al. (1998).
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As a fourth and final example of multiple realization of color processing in the retina, I
want to draw on some recent work on the biochemistry of opsins. In a classic paper, Nathans,
Thomas, and Hogness (1986) identified the gene sequences, hence the amino acid sequences, of
the three opsin components of the photopigments. In addition, they found that normal humans
vary in the number of genes coding for the green pigment. In other words, there are multiple loci
each coding for a green photopigment. Subsequent research has also found that normal humans
vary in the number of genes coding for the red pigment.2 This suggests the possibility that a
given individual can possess distinct versions of the gene for the green and red photopigments at
the different loci, hence can possess distinct green and red photopigments, and distinct green and
red cones. Further, it is hypothesized that part of the reason for individual variation in color
sensitivity within humans is due to differences in the number of different kinds of cones. Some
individuals might thus have, say, only one type of green cone and one type of red cone, where
other individuals might have, say, two different green cones and seven different types of red
cone.
So, let me summarize how I have applied our schemata for realization and multiple
realization to my scientific examples. First, I argued that the properties and relations of amino
acids and 11-cis-retinal realized the light absorbing property of the human green photopigment
and that this could be multiply realized by distinct amino acid chains that have the same
absorption spectrum. Second, I argued that the properties and relations of photopigment
molecules, phospholipid molecules, water molecules, potassium ions, sodium ions, G protein
molecules, cGMP molecules, etc. realize a cone’s property of transducing light. I argued that the
cone’s property of transducing light was multiply realized by G proteins with distinct activation
2
Neitz and Neitz, (1995).
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properties. Third, I argued that color processing in the human retina is realized by properties of
the cones, amacrine cells, bipolar cells, horizontal cells, and retinal ganglion cells. Color
processing is multiply realized by variations in the cone mosaic, dichromacy, anomalous
trichromacy, and the apparent variety in the number of distinct cone opsins normal humans carry.
4. Conclusion
The long term goal of my work with Carl is to provide a clear and correct theory of a
particular kind of realization relation and a corresponding notion of multiple realization, then to
use that in combination with recent neuroscientific work to justify the Massive Multiple
Realization Hypothesis that many human psychological properties are multiply realized at many
neurobiological levels. In this talk, I tried to provide an introduction to this project. I presented
a bare bones sketch of our views on realization and multiple realization, then showed how it
provides an account of the realization and multiple realization relations that neuroscientists have
found to underlie some psychological processes in human vision. This is far from a completed
project, so I would be very interested in comments and question on it.
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Figure 1. A) The Eye. B) A Cross-section of the Retina Showing the Principal Cell Types (Including the Rods
and Cones). C) The Outer Segment of a Cone. D) Photopigments Embedded in the Membrane of the Cone
Outer Segment. E) The Amino Acid Chain of a Cone Photopigment.
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Figure 2. Green Cone Photopigment Sensitivity Curve. (Modified from Blake and Sekuler 2002, Figure 2.23,
p. 74)
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Figure 3. Absorption Spectra of the Normal Green Human Photopigment (Green Curve), the Two Common
Red Photopigments (Red Curves), and Various Red-Green Hybrid Pigments (Gray Lines). The labels, such
as R2G3, name the hydrids. (Merbs and Nathans 1992, Figure 1)
Figure 4. Human rods and cones. (From Blake and Sekuler 2002, Figure 2.29, p. 69).
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Figure 5. Photopigment Molecule Embedded in the Cell Membrane and Phospholipid Molecules of the
Membrane Constituting a Cone. (Modified from Sharpe, Stockman, Jägle, and Nathans 1999, Figure 1.2, p.
6.)
Figure 6. Schematic of an opsin embedded in the cell membrane. The seven cylinders represent portions of
the opsin spanning the cell membrane. (Based on Sharpe, Stockman, Jagle, and Nathans 1999, Figure 1.17A,
p. 43
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Figure 7. Phototransduction Path (in a Rod).
Figure 8. Cells Constituting the Human Retina. 1) Rods, 2) Cones , 3) Horizontal Cells, 4) Bipolar Cells, 5)
Amacrine Cells, & 6) Retinal Ganglion Cells. (Modified from Wässle 2004, Figure 1, p. 2.)
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Figure 9. Absorption Spectra of the S-, M-, and L-cones. (From Blake and Sekuler 2002. Figure 2.23, p. 74).