Hilary Putnam founded machine-state functionalism, also called computational functionalism. He later turned its best argument against it. In Representation and Reality, he compared the move to jujitsu: use an opponent’s strength to throw him. The opponent happened to be his own earlier theory. Philosophers admire that sort of candor, preferably from a safe distance.
He did not hide or minimize his defection in a footnote. Chapter 5 bears the title “Why Functionalism Didn’t Work.” No euphemism there. The whole chapter says where the theory failed.
The reversal took more than twenty years. It began with a case against brain-state identity, gained a second edge from semantic externalism, and ended as a case against identifying mental states with computational states. The logic becomes easier to see if we take those steps in order.
The First Turn: One Mental State, Several Physical States
In the 1950s and 1960s, identity theorists proposed that mental-state types might prove identical with brain-state types. Pain, for example, might turn out to be C-fiber firing. Putnam thought the proposal tied minds too tightly to one kind of animal. Humans, octopuses, and possible extraterrestrials could all feel pain while possessing very different nervous systems.
His first argument can be stated in three steps:
- If pain were identical with one physical-state type, every creature in pain would occupy that physical-state type.
- Creatures with very different physical designs could all feel pain.
- Therefore, pain cannot be identical with any one physical-state type.
This argument does not deny that every actual pain has a physical realization. It denies a type identity. A human pain and an alien pain may each depend on physical activity without depending on the same physical kind.
Machine-state functionalism offered an attractive replacement. Define pain by the role it plays, not by its material. Ask what tends to cause it, what it tends to cause, and how it interacts with belief, memory, and desire. Different physical systems could occupy the same functional state, much as different computers can run the same program. Putnam had freed the mind from one material by giving it an abstract job description.
The Middle Turn: The World Helps Fix Content
In 1975, Putnam published “The Meaning of ‘Meaning.’” Its Twin Earth story added a different pressure. Imagine two internally matching people in 1750. One lives on Earth among H₂O. The other lives on Twin Earth among XYZ, a liquid that looks, tastes, and behaves like water. Their internal states match. Their words do not refer to the same substance. The Earthling’s “water” reaches H₂O; the twin’s reaches XYZ.
This result did not yet amount to Putnam’s full case against functionalism. The paper focused on linguistic meaning and reference. Still, it established a limit that any internal machine theory would have to respect: what a state concerns can depend on relations outside the thinker. Matching inner organization does not guarantee matching content.
By 1988, Putnam joined this externalist result to his older multiple-realization argument. The result was the promised jujitsu.
The Second Turn: One Mental State, Several Computational States
Machine-state functionalism had rejected the demand for one physical type per mental type. Yet it still treated mental states as computationally defined states. Putnam now asked why a shared belief should require a shared computational organization.
Consider two people who both believe that many cats live nearby. One counted paw prints in the garden. The other heard nightly cat fights. They may speak different languages, hold different background beliefs, and draw different inferences from what they believe. An alien could share the belief through a cognitive design unlike either human system.
Different routes into the belief would not settle the issue. A machine can reach the same state by several routes. A machine state gets its identity from its place in a transition structure: which inputs and internal states lead to it, and which outputs and further states follow from it. Differences in language, background belief, and habits of inference can change that entire pattern. Across sufficiently different thinkers, the computational state spaces may not even map neatly onto one another. Yet we may still have good reason to interpret the thinkers as sharing the belief.
The second argument has the same skeleton as the first:
- If believing that many cats live nearby were identical with one computational-state type, every possible thinker with that belief would occupy that type.
- Possible thinkers can share that belief while occupying states with different transition roles, perhaps within computational systems whose state spaces cannot be mapped onto one another.
- Therefore, the belief cannot be identical with any one computational-state type.
Putnam called functionalism’s contrary assumption its “Achilles’ heel.” It had denied one brain-state type for every belief across species, then quietly kept a one-state-per-belief picture within each organism. But learning histories, cultures, languages, and styles of thought vary within our species. Possible thinkers could vary far more. The same content can cross those computational differences.
A functionalist can answer by replacing one state with a class. Put every computational state that counts as believing that cats live nearby into the same box. The states need not match internally; they need only belong to the right equivalence class.
Now ask what puts them in the box. The rule must decide when cat, meew, and an alien signal concern the same animals. It must separate an unfamiliar belief from a familiar belief expressed in an unfamiliar way. In short, it must interpret the thinkers before it can sort their computational states. The proposed reduction then moves in a circle. It explains shared content by membership in a class, but identifies the class by deciding which systems share the content.
A functionalist may insist that some objective equivalence relation exists even if no finite thinker can find it. Putnam need not prove otherwise. Machine-state functionalism needs a rule stated in computational terms. An endless list built from prior acts of interpretation would only record our semantic verdicts. It would not reduce them to computation.
The problem runs in the other direction as well. Twin Earth gives us internally matching systems with different contents. If their internal physical organization matches, their internal computational organization can match too. Yet one “water” state concerns H₂O and the other concerns XYZ. No formal transition inside either system settles that difference. The environment does.
The two arguments work in opposite directions. Multiple realization challenges necessity. Twin Earth challenges sufficiency:
- Same content, different computational states: no particular internal computational-state type is necessary for that content.
- Same internal computational state, different content: no particular internal computational-state type is sufficient for that content.
The first point defeats the demand that every instance of a belief share one computational type. The second defeats the claim that occupying a computational type fixes what the belief concerns. Together they block the one-to-one identity proposed by simple machine-state functionalism.
What the Argument Does Not Show
The conclusion needs a fence. Putnam’s argument rejects the constitutive identity just defined. It does not deny that brains compute, that computational models explain parts of cognition, or that an artificial system might think. The modest claim that computation contributes to thought remains untouched.
Nor did Putnam dispose of every wide form of functionalism. A theorist may let the relevant role extend through the body, environment, social practice, and causal history. Putnam considered such a sociofunctional rescue. It accepts much of his externalism and abandons the picture of an isolated program running inside one skull. It may still count as functionalism. But formal computation alone no longer fixes the content. The world-involving relations now carry part of the load.
Putnam also offered a separate argument about implementation. Under a permissive mapping, an ordinary physical system can count as realizing any finite automaton. The philosopher David Chalmers later required the system’s causal structure to mirror the formal transitions. That reply may secure objective computation and block the cheap mappings. Grant it. The semantic problem remains: a genuine computation does not determine its own reference merely by running.
My View
Putnam supplies the negative result: formal computation cannot by itself tell us what a state concerns. My positive account goes beyond his. A world-involving producer-consumer history—through learning, selection, or both—may fix a representation’s accuracy conditions: what it concerns and what would make it mistaken. That can establish content at the level of a mechanism without a separate proprietor. Whether the larger system thereby believes, understands, or acts depends on a further question: how the state participates in the system’s integrated cognitive organization.
Neither a world-involving history nor cognitive integration requires carbon. An artificial mechanism could acquire non-derived content, and an artificial system might think by means of it. Silicon may qualify; an abstract program, considered only as a formal transition structure, does not.
Multiple realization first freed mental states from one physical material. Properly followed, it also frees them from one computational form.
-gts
This essay is part of Mind, Matter, and Meaning, an AI-assisted philosophy project by Gordon Swobe exploring consciousness, meaning, and artificial minds. Learn more about the project and read the book.
References
Putnam, Hilary (1967). “Psychological Predicates.” In W. H. Capitan and D. D. Merrill (eds.), Art, Mind, and Religion, 37–48. Pittsburgh: University of Pittsburgh Press. Reprinted as “The Nature of Mental States” in Mind, Language and Reality: Philosophical Papers, Volume 2. Cambridge: Cambridge University Press, 1975.
Putnam, Hilary (1975). “The Meaning of ‘Meaning.’” In Keith Gunderson (ed.), Language, Mind, and Knowledge, Minnesota Studies in the Philosophy of Science 7, 131–193. Minneapolis: University of Minnesota Press.
Putnam, Hilary (1988). Representation and Reality. Cambridge, MA: MIT Press.
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