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).