The Art of Medicine

Some of which are proper, to which the others are referred and conspire, such as "rational"; others are common, such as "sentient." The proper alone constitutes a definition with the genus; the common does not, but only when joined with others. In the same manner, parts of substances (for, according to Aristotle, they are substances) obtain substantial differences, taken from the primary action. Just as because the primary action of the hand is grasping, it takes its difference from the faculty of grasping. However, among the parts of substances, some immediately constitute the thing and are called substantial, such as matter and form; if one of these is missing, the thing will not exist at all. Consequently, the differences derived from these are substantial, such as "animate body." But those taken from matter are not revealed by actions, because matter is not the principle of action, but they are perceived by the senses, such as "corporeal." Other parts are referred to integrity, consisting indeed of matter and form, but some of them can be absent from the thing, such as foot, hand, or eye; their differences, as we have said, are substantial—not indeed of the whole, except by accident and in the second place, but of the parts whose differences they are. For example, "to walk" is the proper difference of the legs, by which they are distinguished from other parts; but we call a man a "walking animal," transferring the difference from the part to the whole.