The condition of the mind depends directly on the condition of the body: fevers, food, sleep, drugs, and illness reliably alter thought, mood, and judgment. Because what is immaterial cannot be moved by matter, the regular dependence of thinking on physical causes shows the mind is itself a bodily process.
Differences in mental capacity between species and between individuals trace to differences in bodily organization, especially the size, structure, and quality of the brain and nervous system. Intelligence is not a gift of soul but an output of how the machine is built and fed.
Humans are distinguished from animals only by having more complex organization and more words, not by possessing a different kind of substance. Apes could in principle be taught language if their organs allowed, which would reveal them as men of a lesser development rather than a separate order of being.
Motion and sensation are inherent potentials of organized matter, not added by an external soul. A severed muscle still twitches, a decapitated animal still moves, and irritated flesh contracts: these show that living matter contains its own principle of movement.
Character, temperament, and morals can be inferred from physical constitution, since dispositions follow from the arrangement of the body. Climate, diet, and physical health shape not only health but also virtue, cowardice, and intellect.
Every function once attributed to an immaterial soul can be accounted for by the mechanics of the body, making the soul an unnecessary and unverifiable posit. The honest conclusion from observed physiology is that man is a self-regulating machine, however complex.