Darwin's theory of evolution.
The materialist argument proposes that humans are made out of purely physical matter. There is no non-physical aspect to our being, nor is there a non-physical mind which exerts force over our bodies. Within the neurons and fibres of our brain, we are able to experience everything known as life. Emotions, feelings, and mental states can be reduced to brain activity and the release and production of chemicals. When we say, “I feel pain”, we are essentially saying, “I am experiencing c-fibre stimulation”, in exactly the same way as when we say, “a noise is loud”, we are simply saying, “the airwave signal has a large amplitude”.
The process of evolution is an entirely physical process. It is therefore sensible to assume that the creation of mental states is a purely physical process as well. If not, then other animals and plant life would also possess non-physical minds. The same argument applies to the gestation of a foetus. A foetus grows into a human by the division and multiplication of billions of tiny physical cells. At what point does the non-physical soul develop?
The Philosophical Zombie While no philosophers assert that zombies exist, the philosophical zombie has been employed as a thought exercise to refute the claims of materialists. In the nineteenth century, following a cascade of scientific breakthroughs, materialism began to gain favour as an epistemological argument. Thinkers like G.F. Stout and David Chalmers have since challenged the notion of evolution and Darwinism as evidence of materialism's strength. Imagine a zombie world, exactly like this one, with the same physical laws, conditions, and species. Each of us has a zombie twin, with identical physical characteristics, that interacts with the world in the exact same way we do. Except our zombie twin lacks non-physical consciousness. In the zombie world, there are no conscious experiences. Would the zombies act the same way as we do? Would they feel pain? Would they see colour? If they wouldn't, then materialism as a position in the mind-body debate is untenable.
The brain was created through evolution. Evolution was a physical process. Therefore, the brain and the corresponding mental states are all physical processes.
When we think about the colour blue or brain does not go blue. Therefore, mental states are not merely physical states.