The following explanation of functionalism is an excerpt from my essay on functionalism and consciousness or qualia.
Functionalist Theories of Mind:
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The first formulations of functionalism were the product of quasi-behavioristic principles and various arguments against type-identity theories of mental and physical states. From behaviorism, early functionalist theories took the concepts of input and output as crucial to understanding the workings of a cognitive system.
However, in a radical departure from behaviorism, functionalism defined input and output in terms of a system's internal states. In other words, functionalists were realists about the internal mental states of systems. Also, unlike behaviorism, functionalism considered the relation(s) between internal states to be of utmost importance.
The most widely accepted contemporary formulation of functionalism has come to be called causal-theoretical functionalism, which depends on a commonsense (rather than computational) view of mental states (Kim 1996, 104-111; Baker 1995 see qualia citations). Causal-theoretical functionalism holds that the concept of a mental state is the concept of a state capable of causing certain kinds of behavior. Some functionalists hold that mental states are definable by a priori conceptual analysis (Baker 1995 see qualia citations).
Others hold that functional mental states are definable by empirical analysis and do not require conceptual analysis (Putnam 1975a, 1975b; Harman 1969 see qualia citations). For the purposes of my argument, however, I need not rely on any single version of functionalism or version of how functionalist mental states can be defined.
Rather, I need only define the conditions that are essential for any functionalist theory of mind. Based on the aforementioned characteristics and theoretical developments, I will present a general formulation of the types of functionalism relevant to my argument which I will call 'Functional Theory of Mind' (hereafter abbreviated as 'FTOM'):
FTOM: A theory of mind x is a functional theory of mind if x holds that (i) an internal state S of some system 
is a mental state in virtue of S's inputs, outputs, and relation(s) to other internal and external states of , (ii) the set of all internal mental states of constitutes a functional description of , and (iii) the functional description of can be realized in different types of substances and in different organizations of such substance(s).
I will briefly attempt to clarify the points in FTOM. In condition (i), inputs can be either perceptual information from the environment external to or inputs from other mental states internal to . Outputs can be behavioral responses, productions of gestures and utterances, or informational inputs to other mental states in .
Relations to other internal and external states refers to a state's interaction with other states that are either inside or outside . Condition (ii) is the claim that, given the total set of all the functionally defined states in , we have a complete functional description of the mental properties of . Condition (iii) is a statement of the multiple realizability of functional descriptions.
By reference in (iii) to the possibility of realization in different types of substances and in different organizations of such substance(s), functionalism, as I have generally formulated it, is metaphysically agnostic as to how a given functional description is realized. Hilary Putnam, one of the founders of functionalism, has written of multiple realizability that, "in principle, a machine. . ., a human being, a creature with a silicon chemistry, and a disembodied spirit could all work much the same way when described at the relevant level of abstraction" (1988, p. xii see qualia citations).
In other words, the most important feature of our minds is not the hardware in which it is realized but the functional description which the hardware instantiates. This means that it is possible that a functional description could be realized in any metaphysical system, be it dualistic, physicalistic, or pluralistic, so long as the metaphysical system allows for an implementation of the given functional description.
For example, three functionally identical systems, call them , 
, and 
, need not be realized in the same way. could be a normal adult human composed of biological material, could be an android composed of silicon and other non-organic compounds, and could be a 'disembodied spirit' if this were, in fact, a possible entity with causal relations between mental states. Each of the three entities would have identical psychological behavior and equivalent intelligence.
The point to be made here is that functional descriptions of the mental states of systems exist at a higher level of conceptual abstraction than the type of hardware or ‘stuff’ in which such systems are realized.
From 2000:
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Consider two systems, Al and George. Let’s say that both Al and George are physically identical and exist in two physically identical worlds. In this case, I think it easily follows that they are also functionally equivalent.
Also, I think it is rather uncontroversial that they are qualitatively equivalent. If one wants to make the case that it is possible that Al and George are not qualitatively equal, then this must rely on the assumption that qualia are an accidental feature of some systems.
This, I think, is a difficult assumption to maintain unless one wishes to entertain dualism. Given a physicalist framework, it is most plausible to maintain that Al and George, given their identical physical realization of function are, in every respect, equivalent.
Now, let’s reconstruct these two systems, given some differing properties. Let’s assume that Al and George are physically unequivalent, but functionally equivalent.
So, Al, let’s just say, is a robot, made from metal, silicon, and a vast array of wires, perceptual instruments, etc. And, let’s assume that George is a normal human.
And, they both exist in physically identical environments in physically identical worlds. So, we have the same functional systems realized or instantiated in two different types of hardware.
Now, let’s also say that Al the robot has no qualitative experience, but George the human does. Al, then, has all the same behavioral responses that George does.
Upon questioning Al about his feelings, his desires, even his phenomenal experience of the world, there will be no behavioral difference between Al and George. In this case Al is a prime candidate for the label of zombie. |
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A very important question concerning Al is whether or not Al can tell that he has no phenomenal experience of the world. Upon questioning Al about his subjective experience, if we are to maintain his functional equivalence to George, he will report having mental images of his wife whom he loves, of experiencing pain, of seeing red, and tasting jalepeño peppers.
We can imagine Al, upon being asked about the smell of a rose, pausing for a moment, looking up for a moment, and responding that he quite enjoys the sweet fragrance of roses. But, Al has no actual phenomenal experience of these things. If it is possible for Al to believe in his phenomenal experiences, but not actually have phenomenal experiences, then, I think there are some interesting consequences to consider. If this situation is not possible, then there are also some interesting consequences to consider.
Firstly, let’s consider the case in which the Al doesn’t know that he has no phenomenal experience. How would Al, if he actually had no phenomenal experience of the world, know that he actually had no phenomenal experience of the world?
If, as the thought experiment has dictated, that Al is a functionally identical to George, who has phenomenal experience of the world, then his reactions, at least externally, must be identical to George’s. But what about Al’s internal behavior? Can Al introspect his non-phenomenal mental states? Well, given that introspection serves some functional purpose, which it does do, then we must assume that Al can report his internal mental states, even though he has no phenomenal awareness of such states.
From 2001:
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Consider the following five subscript variables:
XE represents the properties of the physical environment of some system X
XP represents the internal physical properties of some system X
XF represents the functional properties of some system X
XB represents the behavioral properties of some system X
XQ represents the qualitative properties of some system X
Now, let’s consider two systems, A and B.
Situation 1: Assume that AE = BE and AP = BP. From this, can we conclude anything about the other properties of systems A and B? Given the current assumptions about system A and system B, if it is the case that AF 
BF, then the functional properties of system A and system B would not be determined by their identical physical properties.
It is difficult to imagine that physically identical systems would not also be functionally identical, as functions are instantiated or realized in some physical way. If these ways of being realized, namely AP and BP, are the same, then this should entail AF = BF as well.
Behaviorally, it would be, again, difficult to imagine how, given their identical physical external and internal environments, there could be any differential behavior between system A and B. However, AB and BB may be dependent upon the natire of AQ and BQ, a discussion of which follows.
Given the current assumptions about the two systems, what can we determine about the nature of their qualitative properties? If it is the case the AQ BQ, then it follows that there is no causal relationship between a system’s physical properties (which are instantiations of its functional properties) and its qualitative properties.
This would leave qualitative properties as accidental properties of a system that could only be accounted for in some dualistic metaphysics. I think it would be difficult to defend the view that two physically identical systems would experience the world in different ways.
And, even if it were the case that this is so, we have could have no epistemic justification for believing that AQ BQ and we would have no plausible account of the causal mechanisms which would bring about the qualitative divergence in these two systems. Though not impossible, a highly problematic dualistic or pluralistic account would have to be provided in order to account for dissimilarity in the qualitative experiences of systems A and B.
Given this, I can claim that AB = BB, in light of the difficulties in explaining how the qualitative properties of some system would differ from a physically identical system in a physically identical environment.
Therefore, I will make the somewhat uncontroversial claim that, given AP = BP and AE = BE, it follows that AF = BF, AB = BB, and AQ = BQ. (foot: Any other interpretation has the burden of explaining how the qualitative properties came to be, and how such properties causally interact with the physical properties)
Situation 2: Let’s assume that AE = BE and AF = BF (as in the first situation), but AP BP. This is possible because two systems with the same function can be realized in a multitude of different physical ways. (foot: For example, the function of the human eye is to transduce light that strike the retina. This same function could be accomplished by using an array of atrificial photoreceptors.) Given that these two systems are physically different, but have the same function, what can we conclude about their other relevant properties; namely, their behavioral properties and qualitative properties?
Could it be the case, given the current conditions on system A and B, that AQ BQ?
One view, conscious inessentialism (CI), takes a strong position in regard to this question. According to CI, "…for any intelligent activity i, performed in any cognitive domain d, even if we do i with conscious accompaniments, i can in principle be done without these conscious accompaniments." (Owen Flanagan, 1992. Consciousness Reconsidered. (Cambridge: MIT UP)).
Relevant Cases:
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AE = BE or AE BE entail nothing about A and B
Assume that, for all future cases, AE = BE
AP = BP entails AF = BF, AB = BB, AQ = BQ
AP BP entails no relationship
AF = BF or AF BF entail nothing else about A and B
AB = BB or AB BB entail nothing else about A and B
AP BP will be all the interesting cases, then.
Assume that, for all future cases, AE = BE and AP BP
AF = BF MAY entail something about behavior/qualia, but probably not
AB = BB can anything be shown by this? Beginnings of zombiehood
AQ = BQ qualitative states are NOT accidental… disprove zombiehood?
AF BF and AB = BB POSSIBLE?
AF BF and AB = BB BECOMING ZOMBIE?
AF = BF and AB BB two things with same function, no other similarities |
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The eight possibilities:
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AF = BF and AB = BB and AQ = BQ
AF = BF and AB = BB and AQ BQ
AF = BF and AB BB and AQ = BQ
AF = BF and AB BB and AQ BQ
AF BF and AB = BB and AQ = BQ
AF BF and AB = BB and AQ BQ
AF BF and AB BB and AQ = BQ
AF BF and AB BB and AQ BQ |
Searle Mention:
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There may be some resistance to this thesis, but such protest can be made only on empirical grounds. In principle at least, there is no reason why the same function could not be found in potentially infinite modes of physical realization.
Some philosophers, such as John Searle, have argued on theoretical grounds, that biological matter has certain ‘causal properties’ that can not be adequately simulated in other non-biologically postulated systems.
Even though this may be the case, there is no a priori justification for this claim. Exactly what the ‘causal properties’ of biological matter are, and how they differ from the ‘causal properties’ of silicon, metal, oxygen or any other physical substance is a purely empirical and non-theoretical matter.
