The Art of Medicine

We recognize the substantial difference—which is proper and derived from the genus of the substance—from the primary action that the species being defined produces, and toward which all other actions conspire. This is not because we wish actions to be the differences themselves, but because they reveal to us the differences of things, namely the forms from which they are produced and named. B All these things will become clearer by an example. The primary action of a human is reasoning, toward which all other actions of a human conspire. For we live for its sake; we are nourished for the protection of life; for the sake of nutrition, we eat, drink, and seek food, drink, and other necessities of life through the actions of hands, legs, feet, eyes, tongue, and other parts of the body, all of which serve the principal action of the human, namely reasoning, as has been said. However, as we have already C warned, such actions are not substantial differences, but they reveal the differences, namely the forms from which the differences themselves are named. Furthermore, although actions have the beginning of motion from the form, some refer more to the body, some more to the form, and some equally to both. For "to walk" seems to belong more to the body, "to reason" to the mind, and "to sense" to both; yet all actions of this kind proceed from the form and reveal differences.