P38: Whatever so disposes the human Body that it can be affected in a great many ways, or renders it capable of affecting external Bodies in a great many ways, is useful to man; the more it renders the Body capable of being affected in a great many ways, or of affecting other bodies, the more useful it is; on the other hand, what renders the Body less capable of these things is harmful.

Dem.: The more the Body is rendered capable of these things, the more the Mind is rendered capable of perceiving (by IIP14). And so what disposes the Body in this way, and renders it capable of these things, is necessarily good, or useful (by P26 and P27), and the more useful the more capable of these things it renders the Body. On the other hand (by the converse of IIP14, and by P26 and P27), it is harmful if it renders the body less capable of these things, q.e.d.