The notion of supervenience is that, “A set of properties A supervenes upon another set B just in case no two things can differ with respect to A-properties without also differing with respect to their B-properties. In slogan form, “there cannot be an A-difference without a B-difference”. (1) So, this notion when combined with physicalism is the assertion that all mental activity is reducible to physical causes. In other words, there is nothing that happens on a mental level without a supervening physical change in the brain. This does not allow for mental changes to affect the physical only the other way around.
Superveniences establish such a relationship between the mental and the physical, so that any change in the mental is caused by a change in the physical. Just as a shadow is dependent upon the position of the object causing it, so is the mental dependent upon the physical. (2)
Logically, the notion of supervenience physicalism is the default explanation of the mind from a Darwinian perspective. There is research; however, that goes against this notion (3,4). In other words, there is evidence that changes in the “mental state” affect the physical state of the brain.
Darwinists may purport determinism and physicalism in general arguments, but when applied to their own experiences and thoughts, they often balk. The essential problem for them from this framework is that their beliefs in Darwinism are nothing but illusory experiences supervened by physical processes in their brains. Their belief that they are rational scientists, or thinkers, is nothing but the result of complex biochemical processes, which results in the illusion that they have rationally selected among alternative worldviews or interpretations of data.
Many Darwinists, when faced with this type of argument, will retreat from a position of physicalism. Even Darwinists are distressed by, and most often don’t believe in, the notion that their mental processes are fully determined by physical substrates. In fact, they must retreat from this position, because it refutes their notion that they are rational in their abilities to evaluate evidence, or that they have freely chosen their worldviews.
(3). The Mind and The Brain
(4). The Spiritual Brain: A Neuroscientist’s Case for the Existence of the Soul