Thomas Aquinas, Compendium of Theology

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 (in eo quod quid) and difference according to what sort of thing it is (in eo quod quale quid).

We must note 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. In physical entities, moreover, 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. In moral entities, similarly, one end involves the privation of another end. Since the privation of a due perfection is an evil in physical entities, 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. In this sense, to be on fire is an evil for a log of wood. In the field of morality, likewise, 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, differ as good and evil. Consequently the corresponding contrary habits differ in good and evil as by specific differences, and as being contrary to each other. This is so, not on account of the privation from which evil receives its designation, but on account of the end which involves the privation.

This is the sense in which some philosophers understand Aristotle's assertion, that good and evil are genera of 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. This is clear in the case of white and black, sweet and bitter, and so on. But invariably, what is perfect, pertains to good, and what is deficient, pertains to evil.