How did Hilary Putnam contribute to the philosophy of mind?

Hilary Putnam made a profound impact on the philosophy of mind by introducing functionalism, often called "machine functionalism."1

Functionalism and Multiple Realizability

Putnam proposed that mental states should be defined by their functional or causal roles—how they relate to sensory inputs, other mental states, and behaviors—rather than by their physical composition. Drawing an analogy between the human brain and computers, he argued that the mind functions like software running on the brain's hardware. This led to his "multiple realizability" thesis, which asserts that the same mental function can be performed by different physical systems. Consequently, he argued that mental states cannot be uniquely reduced to specific physical brain states, challenging psychophysical reductionism.12

Later Revisions

In his later career, Putnam moved away from functionalism. He concluded that functionalism was too internalist, failing to account for how mental states are connected to the external world. Influenced by his semantic externalism, he argued that because mental states have semantic content, they cannot be characterized solely by internal functional or syntactic rules. He eventually abandoned the view, acknowledging criticisms such as the "inverted spectrum" argument, which suggests that functional similarity does not guarantee identical subjective experience.12