Chapter 116 — In what way good and evil are differences of being, contraries, and genera of contraries

We have next to inquire how good and evil may be regarded as contraries and genera of contraries and differences constituting species of a sort, namely, moral habits. Each member of a pair of contraries is some kind of nature. For non-being can be neither genus nor specific difference, since genus is predicated of a thing according to what it is and difference according to what sort of thing it is.

One must know therefore that, as physical entities receive their species from their form, so moral entities receive their species from the end which is the object of the will and on which all morality depends. Now, in physical entities the presence of one form entails the privation of another, as, for instance, the form of fire entails the privation of the form of air or wood. In moral entities, similarly, one end involves the privation of another end. Since the privation of a due perfection is an evil, in physical things the reception of a form which implies the privation of the form that ought to be possessed is an evil; not, indeed, because of the form itself, but because of the privation its presence involves. For example, to be on fire is an evil for wood.

Similarly, in moral actions the pursuit of an end that entails the privation of the right end is an evil, not on account of the end itself, but because of the privation necessarily implied. It is in this way that two moral actions directed to contrary ends—and consequently the contrary habits as well—differ as good and evil, as though they differed by good and evil as specific differences contrary to one another. But this is so not on account of the privation from which evil receives its designation, but on account of the end that involves the privation.

This is the sense in which some philosophers understand Aristotle’s assertion that good and evil are genera of other contraries, namely, of moral contraries. But if we examine the matter closely, we shall find that in the sphere of morals, good and evil are differences rather than species. Hence it seems better to say that good and evil are called genera according to the opinion of Pythagoras, who reduced everything to good and evil as to supreme genera. This position does, indeed, contain some truth, in the sense that in all contraries one member is perfect, whereas the other is deficient, as is clear in the case of white and black, sweet and bitter, and so on. And what is perfect always relates to the notion of good, while what is deficient relates to the notion evil.