Hilary Putnam was a strong proponent of functionalism.
Functionalism approaches the mind-body debate slightly differently. Rather than focus on what the internal constitution of our mind and body are, its focal point is what the function of mental states are and what role they play. Functionalism provides an answer to the question “what are mental states?” Its proposed answer is that our mental states can be reduced to a functional response to a stimulus. Pain is a response to injury. Belief is a response to sensory stimulation. Rather than the mind interacting with and controlling the body, as dualists believe, functionalists believe minds are merely “material systems” on a higher level than the body. It is no coincidence that fundamentalism gained traction in the latter half of the 20th century, around the same time technology and computers were invented. The theory sees the mind as one big computer. Brain processes involving neurons and chemicals are the hardware. But to interpret and process these processes, we need software, also known as, the mind. Putnam was a proponent of functionalism that promoted the idea of the mind and the body operating akin to a computer.
He attempted to reduce all mental processes to merely the response to both external and internal stimulus. We feel, believe, and think a certain way due to our inner response to experience, and other mental states. To illustrate his point, Putnam devised a "Twin Earth" scenario. On twin earth, everything looks, tastes, smells, and feels exactly the same as it does on earth. Water is wet, stone is cold and hard, gold is shiny and golden, etc. But on twin earth, everything has different molecular structures than earth. Water is not H2O, humans are not a complex construction of hydrocarbons, and the air is not made up of oxygen, carbon dioxide, and nitrogen. If we were transported to this earth, our beliefs about water, gold, air, and stone would remain the same because the internal and external stimuli would remain the same. This, he argues, applies to our mental states. They are functional in the sense that they respond to a stimulus and can be reducible to an input and an output. Many different inputs can generate the same output, but the core relationship remains the same- a causal relationship between stimulus and response. Functionalism does not speculate on what substance our mind is made up of. Taking suggestions from both the dualist and materialist camps, he wrote, “whatever our substance may be, soul-stuff, or matter or Swiss cheese, it is not going to place any interesting first order restrictions on the answer to the question” (Putnam: Philosophy and our Mental Life, 1975).
Searle’s Chinese Room John Searle’s argument against functionalism highlighted the dangers of explaining mental states in terms of stimulus and response. He argued that functionalism reduces human mental thought to a series of inputs and outputs. We present input X and get output Y. But this cannot adequately explain all human mental processes. He presented the scenario of himself in a room. Inside the room are a number of books and a pencil. People outside the room pass notes to him inside the room. On each note a question is written in Chinese. Searle has no understanding of Chinese, he does not know what is written on the notes, but within the books, he has directions of what characters to draw to produce an answer to each question. Searle used the Chinese room scenario to demonstrate the limits of the functionalist theory of the mind. According to functionalism, Searle in the Chinese room would be receiving inputs (stimuli) then producing the correct output, thereby presenting sufficient evidence for mental thought, despite not understanding Chinese and the answers he was providing. Searle sitting in the Chinese room was responding the same way to the questions as someone would that understands Chinese. Searle argues that just because two things respond the same way, to the same stimuli (i.e. they are functionally the same), does not mean they have the same mental states.
Mental states are a response to a stimulus.
Two things can give the same response to a stimulus but that doesn't mean they have the same mental states. Therefore, mental states cannot be reduced to a simple stimulus-response relationship.