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Functionalism is the dominant theory of mental states in modern philosophy. Functionalism was developed as an answer to the mind-body problem because of objections to both identity theory and logical behaviourism. Its core idea is that mental states can be accounted for without taking into account the underlying physical medium (the neurons), instead attending to higher-level functions such as beliefs, desires, and emotions.

According to functionalism, mental states are constituted by their causal relations to one another and to sensory inputs and behavioral outputs. Because these states are not limited to a particular physical medium, they can be realized in multiple ways, including, theoretically, within non-biological systems.

Varieties of functionalism

Machine-state functionalism

Artistic representation of a Turing machine

Functionalism comes in many different varieties. The first formulation of a functionalist theory was put forth by Hilary Putnam. This formulation, which is now called machine-state functionalism, was inspired by the analogies which Putnam and others noted between the mind and the theoretical "machines" or computers capable of computing any given algorithm which were developed by Alan Turing (called universal Turing machines).

In non-technical terms, a Turing machine can be visualized as an infinitely long tape divided into squares (the memory) with a box-shaped scanning device that sits over and scans one square of the memory at a time. Each square is either blank (B) or has a 1 written on it. These are the inputs to the machine. The possible outputs are:

  • Halt: Do nothing.
  • R: move one square to the right.
  • L: move one square to the left.
  • B: erase whatever is on the square.
  • 1: erase whatever is on the square and print a '1.

An extremely simple example of a Turing machine which writes out the sequence '111' after scanning three blank squares and then stops is specified by the following machine table:

State One State Two State Three
B write 1; stay in state 1 write 1; stay in state 2 write 1; stay in state 3
1 go right; go to state 2 go right; go to state 3 [halt]

This table states that if the machine is in state one and scans a blank square (B), it will print a 1 and remain in state one. If it is in state one and reads a 1, it will move one square to the right and also go into state two. If it is in state two and reads a B, it will print a 1 and stay in state two. If it's in state two and reads a 1, it will move one square to the right and go into state three. Finally, if it is in state three and reads a B, it prints a 1 and remains in state three.

The essential point to consider here is the nature of the states of the Turing machine. Each state can be defined exclusively in terms of its relations to the other states as well as inputs and outputs. State one, for example, is simply the state in which the machine, if it reads a B, writes a 1 and stays in that state, and in which, if it reads a 1, it moves one square to the right and goes into a different state. This is the functional definition of state one; it is its causal role in the overall system. The details of how it accomplishes what it accomplishes and of its material constitution are completely irrelevant.

According to machine-state functionalism, the nature of a mental state is just like the nature of the automaton states described above. Just as state one simply is the state in which, given an input B, such and such happens, so being in pain is the state which disposes one to cry "ouch", become distracted, wonder what the cause is, and so forth.


A second form of functionalism is based in the rejection of behaviorist theories in psychology and their replacement with empirical cognitive models of the mind. This view is most closely associated with Jerry Fodor and Zenon Pylyshyn and has been labeled psychofunctionalism. The fundamental idea of psychofunctionalism is that psychology is an irreducible science and that the terms that we use to describe the entities and properties of the mind in our best psychological theories cannot be redefined in terms of simple behavioral dispositions. Psychofunctionalists view psychology as employing the same sorts of irreducibly teleological or purposive explanations as the biological sciences. Thus, for example, the function or role of the heart is to pump blood, that of a kidney is to filter it and to maintain certain chemical balances and this is what counts for the purposes of scientific explanation and taxonomy. There may be an infinite variety of physical realizations for all of the mechanisms, but what is important is only their role in the overall biological theory. In an analogous manner, the role of mental states, such as belief and desire, is determined by the functional or causal role that is designated for them within our best scientific psychological theory. If some mental state which is postulated by folk psychology (e.g. hysteria) is determined not to have any fundamental role in cognitive psychological explanation, then that particular state may be considered not to exist. On the other hand, if it turns out that there are states which theoretical cognitive psychology posits as necessary for explanation of human behavior but which are not foreseen by ordinary folk psychological language, then these entities or states exist.

Analytic functionalism

A third form of functionalism is concerned with the meanings of theoretical terms in general. This view is most closely associated with David Lewis and is often referred to as analytic functionalism. The basic idea of analytic functionalism is that theoretical terms are implicitly defined by the theories in whose formulation they occur. In the case of ordinary language terms, such as "belief", "desire" or "hunger", the idea is that they get their meanings from our common-sense "folk psychological" theories about them. Such terms are subject to conceptual analyses which take something like the following form:

Mental state M is the state that is caused by P and causes Q.

For example, the state of pain is caused by sitting on a tack (for example) and causes one to moan in pain. These sorts of functional definitions in terms of causal roles are claimed to be analytic and a priori truths about the mental states and propositional attitudes they describe. Hence, its proponents are known as analytic or conceptual functionalists. The essential difference between analytic and psychofunctionalism is that the latter emphasizes the importance of laboratory observation and experimentation in the determination of which mental state terms and concepts are genuine and which functional identifications may be considered to be genuinely contingent and a posteriori identities, while the former claims that such identities are necessary and not subject to empirical scientific investigation.

Homuncular functionalism

This form of functionalism was developed by Daniel Dennett and has been advocated by William Lycan. It arose in response to the challenge that Ned Block's China Brain and John Searle's Chinese Room thought experiments represented for the more traditional forms of functionalism. In attempting to overcome the conceptual difficulties that arose from the idea of a nation full of Chinese people wired together with each one carrying out the functional or causal role that would normally be ascribed to the mental states of an individual mind, for example, many functionalists simply bit the bullet and argued that such a Chinese nation would indeed possess all of the qualitative and intentional properties of a mind; i.e. it would become a sort of systemic or collective mind with propositional attitudes and other mental characteristics. Whatever the worth of this latter hypothesis, it was immediately objected against it that it entailed an unacceptable sort of mind-mind supervenience: the systemic mind which somehow emerged at the higher-level must necessarily supervene on the individual minds of each individual member of the Chinese nation, to stick with Block's formulation. But this would seem to put into serious doubt, if not directly contradict, the fundamental idea of the supervenience thesis: there can be no change in the mental realm without some change in the underlying physical substratum. This can be easily seen if we label the set of mental facts that occur at the higher-level M and the set of mental facts that occur at the lower-level M1. Given the transitivity of supervenience, if M supervenes on M1 and M1 supervenes on P (physical base), then M and M1 both supervene on P, even though they are (allegedly) totally different sets of mental facts.

Since mind-mind supervenience seemed to have become acceptable in functionalist circles, it seemed to some that the only way to resolve the puzzle was to postulate the existence of an entire hierarchical series of mind levels (analogous to homonculi) which became less and less sophisticated in terms of functional organization and physical composition all the way down to the level of the completely stupid and physico-mechanical neuron or group of neurons. The homunculi at each level, on this view, have authentic mental properties but become stupider and simpler as one works one's way down the hierarchy.

Functionalism and physicalism

There is much confusion about the sort of relationship that is claimed to exist (or not exist) between the general thesis of functionalism and physicalism. It has often been claimed that functionalism somehow "disproves" or falsifies physicalism tout court (i.e. without further explanation or description). On the other hand, most philosophers of mind who are functionalists claim to be physicalists--indeed, some of them, such as David Lewis have claimed to be strict reductionist-type physicalists.

Functionalism is fundamentally what Ned Block has called a broadly metaphysical thesis as opposed to a narrowly ontological one. That is, functionalism is not so much concerned with what there is as with what it is that characterizes a certain type of mental state, e.g. pain, as the type of state that it is. Previous attempts to answer the mind-body problem have all tried to resolve it by answering both questions: dualism says there are two substances and that mental states are characterized by their immateriality; behaviorism claimed that there was one substance and that mental states were behavioral disposition; physicalism asserted the existence of just one substance and characterized the mental states as physical states (as in "pain = C-fiber firings").

On this understanding, type physicalism can be seen as incompatible with functionalism, since it claims that what characterizes mental states (e.g. pain) is that they are physical in nature, while functionalism says that what characterizes pain is its functional/causal role and its relationship with yelling "ouch", etc. However, any weaker sort of physicalism which makes the simple ontological claim that everything that exists is made up of inorganic matter is perfectly compatible with functionalism. Moreover, most functionalists who are physicalists require that the properties that are quantified over in functional definitions be physical properties. Hence, they are physicalists, even though the general thesis of functionalism itself does not commit them to being so.

In the case of David Lewis, there is a distinction in the concepts of "having pain" (a rigid designator true in all possible worlds) and just "pain" (a non-rigid designator). Pain, for Lewis, stands for something like the definite description "the state with the causal role x". The referent of the description in humans is a type of brain state to be determined by science. The referent among silicon-based life forms is something else. The referent of the description among angels is some immaterial, non-physical state. For Lewis, therefore, local type-physical reductions are possible and compatible with conceptual functionalism. There seems to be some confusion between types and tokens that needs to be cleared up in the functionalist analysis.


Putnam's Twin Earth thought experiment

Hilary Putnam is also responsible for one of the main arguments used against functionalism: the Twin Earth thought experiment, which was originally intended as an argument against semantic internalism. The experiment is simple and runs as follows. Imagine a Twin Earth which is identical to Earth in every way but one: water is not H2O, it's a substance XYZ. It is absolutely critical, however, to note that XYZ on Twin Earth is still called 'H2O' even though it is a different substance (i.e. the one we call 'XYZ' on Earth). Since these worlds are identical in every way but one, you and your Twin Earth Doppelganger see exactly the same things, meet exactly the same people, have exactly the same jobs, and behave exactly the same way. In other words, you share the same inputs, outputs, and relations between inputs and outputs. But there's one crucial difference. You know (or at least believe, if we wish to make a weaker claim or avoid epistemological issues) that water is H2O. Your Doppelganger knows that water is XYZ. Therefore, you differ in mental states though the causal properties that define your mental states are identical.

Most defenders of functionalism initially responded to this argument by attempting to maintain a sharp distinction between internal and external content. The internal contents of propositional attitudes, for example, would consist exclusively in those aspects of them which have no relation with the external world and which bear the necessary functional/causal properties that allow for relations with other internal mental states. Since no one has yet been able to formulate a clear basis or justification for the existence of such a distinction in mental contents, however, this idea has generally been abandoned in favor of externalist causal theories of mental contents (also known as informational semantics). Such a position is represented, for example, by Jerry Fodor's account of an "asymmetric causal theory" of mental content. This view simply entails the modification of functionalism to include within its scope a very broad interpretation of input and outputs to include the objects that are the causes of mental representations in the external world.

Inverted Spectrum

A main criticism of functionalism is the Inverted Spectrum scenario. This is where a person is born with a complete opposite spectrum of seeing light. "Normal" people would see Red Orange Yellow Green Blue Violet, where the inverted spectrum would Violet being Red, Orange being Blue, and so forth. When presented with a strawberry, a normal and inverted case would be asked to define the "redness" of the strawberry. Their inputs and outputs would be the same, so the mental state should too. A solution is the Qualia of what they where saying. By saying "it is red" they are refering to different reds in their mind.

Meaning holism

Another common criticism of functionalism is that it implies a radical form of semantic holism. Block and Fodor (1980) referred to this as the damn/darn problem. The difference between saying "damn" or "darn" when one smashes one's finger with a hammer can be mentally significant. But since these outputs are, according to functionalism, related to many (if not all) internal mental states, two people who experience the same pain and react with different outputs must share nothing in common in any of their mental states. A possible solution to this problem is to adopt a moderate (or molecularist) form of holism. But even if this succeeds in the case of pain, in the case of beliefs and meaning, it faces the difficulty of formulating a distinction between relevant and non-relevant contents without invoking the analytic-synthetic distinction.

Searle's Chinese Room

Main article: Chinese room

The Chinese room argument by John Searle is a more direct attack on the claim that thought can be represented as a set of functions. The thought experiment asserts that it is possible to mimic intelligent action without any interpretation or understanding through the use of a purely functional system. It attacks the idea that thought can be equated with following a set of syntactic rules.

As noted above, many functionalists responded to Searle's thought experiment by suggesting that there was a form of mental activity going on at a higher level than the man in the Chinese room could comprehend (the so-called "systems reply"). But this response runs into a more general difficulty: if functionalism ascribes minds to things that don't have them, then it is obviously too liberal. In response to this problem, Lycan (1987) suggested that much of human physiology be included in functional characterizations. But this, in turn, leads to a problem of chauvinism. The question of where exactly to draw the line here does not constitute an objection so much as a challenge to functionalists to come up with clearer ideas on what is mental and what is not.


  • Levin, Janet, "Functionalism", The Stanford Encyclopedia of Philosophy (Fall 2004 Edition), Edward N. Zalta (ed.). (online)
  • Block, Ned. "What is functionalism" in Readings in Philosophy of Psychology, 2 vols. Vol 1. (Cambridge: Harvard, 1980).
  • Brown, Curtis. 'Functionalism in Philosophy of Mind. 2000.
  • Mandik, Pete. Fine-grained supervience, cognitive neuroscience, and the future of functionalism. 1998.

See also

Wikibooks Consciousness studies has more about this subject:

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