Why did Hilary Putnam argue that the mind is not defined by the material of the brain?

Hilary Putnam argued that the mind is not defined by the material of the brain through his doctrine of functionalism, which posits that mental states are characterized by their functional or causal roles rather than their physical substance.1

Putnam supported this view with the thesis of "multiple realizability," which asserts that the same mental function can be performed by different physical systems. He famously illustrated this by suggesting that the mind could theoretically be made of "Swiss cheese" without changing its functional nature. Drawing an analogy to computers, Putnam compared the brain to hardware and the mind to software; just as the same software can run on different hardware, mental states are not uniquely tied to a specific biological material.12

Consequently, Putnam argued that mental properties cannot be reduced to physical properties, as function and structure do not share a one-to-one correspondence. It is important to note that Putnam later abandoned this functionalist view, concluding that it failed to account for the semantic content of mental states and their necessary connection to the external world.12