Putnam’s Machine Functionalism

Hilary Putnam was a highly influential philosopher whose views impacted ideas and ways of thinking in philosophy of mind, even as his views evolved markedly over his remarkable career. Early in his work, he was a pioneering advocate of functionalism, the theory that mental states are generated by their functional roles—i.e., by what they do rather than by what they are made of (in contrast to identity theory). Functionalism contends that all mental states (like sensations, beliefs, emotions) are defined by their causal roles or functional states of the system, such that what really matters is not the physical composition or structure of the brain, not the neurobiology, but the causal relations between inputs (stimuli), internal states, and outputs (behavior). “The functional organization of a system is the set of causal relations among its sensory inputs, its motor outputs, and its internal states” (Putnam, 1975).

Machine-State Functionalism and Multiple Realizability

Putnam’s functionalism was sometimes called “machine-state functionalism,” which related to his computational theory of mind, which holds that the human mind is a computational system that is realized (i.e., physically operationalized) by neural activity in the brain. What then followed logically was his concept of “multiple realizability,” the idea that mental states can be realized in different kinds of physical systems (i.e., humans, animals, computers).

For example, the experience of pain, which relates to C-fiber neuronal firing in mammals, could be realized in other creatures (e.g., octopuses, aliens) with different biological structures, provided that their internal states played the same functional role. Putnam said, famously, “We could be made of Swiss cheese and it wouldn't matter” (Putnam, 1975).

In this way of thinking, Putnam analogized mental states to “software,” with the brain being the “hardware,” and just as a computer program can run on different kinds of computers, mental states (like pain) could “run” in different physical systems (e.g., human brains, animal brains, alien brains, AI). This multiple realizability empowered Putnam to reject identity theory (which claims that specific mental states are identical to specific brain states).

Later Critiques and Liberal Functionalism

To Putnam’s credit, he later distanced himself from a strict computational theory of the mind, rejecting hard-core functionalism, which he argued oversimplified mental phenomena and ignored the semantic and intentional aspects of thought. Semantic externalism is the idea that the meaning of our words and the content of our thoughts are not solely “in the head” but depend on our interactions with the external world. This challenged functionalism because functional states are conceived as purely internal brain states, detached from the environment.

Putnam also focused on the concept of “intentionality” (i.e., that mental states are “about” things), rejecting the idea of an “inner intentionality” as a mysterious, self-contained mental property. He argued for a more externalist view of meaning and thought, where the content of our minds is deeply intertwined with our interactions with the world. He said, “We find it tempting to imagine that the intentionality of our words is given in the experience of thinking itself… the 'illusion of inner intentionality' is an illusory notion that reference is a mysterious something that exists while we think” (Putnam, 1973).

He also recognized that functionalism cannot capture qualitative experience (qualia), nor could mental phenomena be exhaustively described by a single, fundamental physical theory. He said, “Functionalism is not a complete philosophy of mind” (Putnam, 1988).

Late in his career, he accepted a weaker form of functionalism, called “liberal functionalism,” which posits that consciousness and mental properties depend on “the right sort of functional capacities,” but with a broader understanding that recognizes the embodied nature of cognition and its interaction with the environment. He argued for a “pragmatic pluralism” where different conceptual schemes (including “mind talk”) are appropriate for different purposes and contexts, without necessarily being reducible to a single, underlying reality.

Putnam’s theory of consciousness moved toward an anti-reductionist, embodied, contextual approach. He consistently rejected substance dualism, but he struggled with qualia. He said, “The notion of a 'qualia' is a theoretical construct designed to do a certain job, but it is not clear that there is any job for it to do” (Putnam, 1999).

Putnam is to be admired for applying equal scrutiny to his own philosophical positions as to those of others, subjecting each position to rigorous analysis. As a result, he acquired a reputation—unfairly, I’d say—for frequently changing his positions (“Hilary Putnam,” 2025). He said, “The idea that there is a 'problem of consciousness' in the sense of a single, unified, philosophical problem that needs to be solved once and for all is, I think, a mistake” (Putnam, 1999).