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”.
Our mental processes are visible on MRI scans, indicating a physical explanation for each mental state, feeling, and emotion. Similarly, individuals who have suffered damage to the physical substance of our mental processes- the brain, also suffer from impaired mental function. When we suffer physical damage to the brain, we suffer reduced mental capacity. Many materialists believe this is irrefutable evidence that our mind is a purely physical entity. When a person develops Alzheimer's, a purely physical disease, they lose many of their mental states and the components that made them a person and an individual are gone.
Introspective Thought We understand colour in a way that is almost entirely non-physical. When I think of the colour purple, I can picture “purpleness” in my mind, but no part of my brain is physically turning purple.
If you damage the physical brain, you have impaired cognitive function. Therefore, our cognitive function is reducible to a physical process. Therefore, mental states are reducible to physical processes.
We do not see colour in physical terms. Therefore, not every mental state is reducible to a physical state. Therefore, there must be a non-physical element to our minds.