In philosophy of mind, functionalism is the thesis that each and every mental state (for example, the state of having a belief, of having a desire, or of being in pain) is constituted solely by its functional role, which means its causal relation to other mental states, sensory inputs, and behavioral outputs.Block, Ned. (1996), "What is functionalism?" a revised version of the entry on functionalism in The Encyclopedia of Philosophy Supplement, Macmillan. (PDF online) Functionalism developed largely as an alternative to type physicalism and behaviorism.
Functionalism is a theoretical level between the physical implementation and behavioral output.Marr, D. (1982). Vision: A Computational Approach. San Francisco: Freeman & Co. Therefore, it is different from its predecessors of Cartesian dualism (advocating independent mental and physical substances) and Skinnerian behaviorism and physicalism (declaring only physical substances) because it is only concerned with the effective functions of the brain, through its organization or its "software programs".
Since a mental state is identified by a functional role, it is said to be realized on multiple levels; in other words, it is able to be manifested in various systems, even perhaps computers, so long as the system performs the appropriate functions. While a computer's program performs the functions via computations on inputs to give outputs, implemented via its electronic substrate, a brain performs the functions via its biological operation and stimulus responses.
Multiple realizability An important part of some arguments for functionalism is the idea of multiple realizability. According to standard functionalist theories, a mental state corresponds to a functional role. It is like a valve; a valve can be made of plastic or metal or other material, as long as it performs the proper function (controlling the flow of a liquid or gas). Similarly, functionalists argue that a mental state can be explained without considering those features of its underlying physical substrate, such as the brain, that do not affect its functional role; only the features relevant to its higher-level functions need to be considered. Because a mental state is not limited to a particular medium, it can be realized in multiple ways, including, theoretically, with non-biological systems, such as computers. A silicon-based machine could have the same sort of mental life that a human being has, provided that its structure realized the proper functional roles.
While most functionalist theories accept the multiple realizability of mental states, some functionalist theories, such as the functional specification theories (FSTs), reject this view. FSTs were most notably developed by David LewisLewis, David. (1980). "Mad Pain and Martian Pain". In Block (1980a) Vol. 1, pp. 216–222. and David Malet Armstrong.Armstrong, D.M. (1968). A Materialistic Theory of the Mind. London: RKP. According to FSTs, mental states are the particular "realizers" of the functional role, not the functional role itself. The mental state of belief, for example, just is whatever brain or neurological process that realizes the appropriate belief function. Thus, unlike standard versions of functionalism (often called Functional State Identity Theories), FSTs do not allow for the multiple realizability of mental states, because the fact that mental states are realized by brain states is essential. What often drives this view is the belief that if we were to encounter an alien race with a cognitive system composed of significantly different material from humans' (e.g., silicon-based) but performed the same functions as human mental states (for example, they tend to yell "Ouch!" when poked with sharp objects), we would say that their type of mental state might be similar to ours but it is not the same. For some, this may be a disadvantage to FSTs. Indeed, one of Hilary Putnam'sPutnam, Hilary. (1960). "Minds and Machines". Reprinted in Putnam (1975a).Putnam, Hilary. (1967). "Psychological Predicates". In Art, Mind, and Religion, W.H. Capitan and D.D. Merrill (eds.), pp. 37–48. (Later published as "The Nature of Mental States" in Putnam (1975a). arguments for his version of functionalism relied on the intuition that such alien creatures would have the same mental states as humans do, and that the multiple realizability of standard functionalism makes it a better theory of mind.
Types
Machine-state functionalism
The broad position of "functionalism" can be articulated in many different varieties. The first formulation of a functionalist theory of mind was put forth by Hilary Putnam in the 1960s. This formulation, which is now called machine-state functionalism, or just machine 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 Turing machines). Putnam himself, by the mid-1970s, had begun questioning this position. The beginning of his opposition to machine-state functionalism can be read about in his Twin Earth thought experiment.
In non-technical terms, a Turing machine is not a physical object, but rather an abstract machine built upon a mathematical model. Typically, a Turing Machine has a horizontal tape divided into rectangular cells arranged from left to right. The tape itself is infinite in length, and each cell may contain a symbol. The symbols used for any given "machine" can vary. The machine has a read-write head that scans cells and moves in left and right directions. The action of the machine is determined by the symbol in the cell being scanned and a table of transition rules that serve as the machine's programming. Because of the infinite tape, a traditional Turing Machine has an infinite amount of time to compute any particular function or any number of functions. In the below example, each cell 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 as specified by the following machine table:
State One State Two State ThreeB write 1; stay in state 1 write 1; stay in state 2 write 1; stay in state 31 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 is in state two and reads a 1, it will move one square to the right and go into state three. If it is in state three and reads a B, it prints a 1 and remains in state three. Finally, if it is in state three and reads a 1, then it will stay 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.
The above point is critical to an understanding of machine-state functionalism. Since Turing machines are not required to be physical systems, "anything capable of going through a succession of states in time can be a Turing machine".Putnam, H. (1967). "The Mental Life of Some Machines," in H.-N. Castaneda (Ed.), Intentionality, Minds, and Perception. Detroit, MI: Wayne State University Press, p. 183. Because biological organisms "go through a succession of states in time", any such organisms could also be equivalent to Turing machines.
According to machine-state functionalism, the nature of a mental state is just like the nature of the Turing machine states described above. If one can show the rational functioning and computing skills of these machines to be comparable to the rational functioning and computing skills of human beings, it follows that Turing machine behavior closely resembles that of human beings.Putnam, H. (1967). "The Mental Life of Some Machines," in H.-N. Castaneda (Ed.), Intentionality, Minds, and Perception. Detroit, MI: Wayne State University Press, pp. 179-180. Therefore, it is not a particular physical-chemical composition responsible for the particular machine or mental state, it is the programming rules which produce the effects that are responsible. To put it another way, any rational preference is due to the rules being followed, not to the specific material composition of the agent.
Psycho-functionalism A second form of functionalism is based on 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 psycho-functionalism.
The fundamental idea of psycho-functionalism is that psychology is an irreducibly complex 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, and further, that such a redefinition would not be desirable or salient were it achievable. 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 the kidney is to filter it and to maintain certain chemical balances and so on—this is what accounts 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