The Problem of Mental Causation
One of the most obvious problems faced by a substance dualist view of the mind is the following: How does the mind, which is supposed to be a nonphysical, spiritual substance, interact causally with the physical body? Causal interactions between body and mind clearly do take place. According to Thomas Huxley, mental properties of brain states are causally inert. Huxley believes humans are not qualitatively different from other animals. Humans, like other animals, are 'conscious automata'. If a certain number of depressed persons swallow the chemical compound that is the active ingredient in Prozac, a certain proportion of those persons will experience improvement in their depressed mental state. Mental properties enter into ceteris paribus laws that can back causal explanations. Mental properties are, again, different properties than the disjunctive physical properties on which they supervene. Where a given higher-level property reduces to a physical property, no problem about causation arises.