What evil is can be learned from the idea of good; for evil consists in a certain absence of good. Now by good we are to understand whatever is desirable; and as every creature desires its own being and perfection, we must admit that goodness belongs to being and to perfection of every nature; consequently that there is no being, or form, or nature of evil.
As the perfection of the universe requires the inequality of things in order that every gradation of good may be filled up, some having corporeal and others incorporeal being, so it requires also that there should be those in which there is the possibility of failure from goodness; whence it follows that some among them do fail - in which consists the idea of evil.
Evil is found in things under the form of corruption, which is a certain kind of evil, existing in good as its subject; for the taking away of good implies evil. In a merely negative sense, however, the absence of good has not the character of evil; otherwise things which are not after any mode would be evil; or a thing would be bad because it had not the good of something else, e.g. man would be bad because he could not run like a deer. Evil consists, therefore, in a subtraction of good by mode of deprivation; as loss of sight constitutes blindness. The form is, however, one and the same as the subject of the privation; that is to say. Being in potentia, whether it be, like materia prima, the subject of substantial forms and the opposite deprivations, or whether it be actual, absolutely, and in potentia only relatively; as a transparent body which is the subject of darkness and light. It is evident, therefore, that the form in virtue of which anything is actual constitutes some perfection and some good; therefore being, in potentia, is so far good, inasmuch as it bears relation to good; for being in potentia is the same as good in potentia. It remains true, therefore, that the subject of evil is good.
Nor can evil entirely destroy good. Good exists in a threefold manner: first, as form or quality, which is liable to be entirely taken away by its opposite, as light by darkness; secondly, as the subject, which is neither taken away nor diminished by evil; thirdly, as the capacity of the subject for actuality, which may be diminished by evil but not wholly taken away.
This diminution is not to be understood of quantity; it is the subtraction of certain forms and qualities by which, as internal dispositions, being is prepared for actuality. Thus the more such contrary dispositions are multiplied, the more the capacity for actuality is lessened; but were they multiplied to infinity, the capacity would be infinitely lessened, yet the remaining root, which is the substance of the subject, would not be taken away. Thus it is with sin; additions may be made by which the aptitude of the soul for grace is evermore diminished; yet this aptitude, which follows upon its nature, will not be taken away.
Evil, which is divided into penalty and guilt, may consist in the subtraction of form, or of some of its parts; such as the loss of a member, which is penal and contrary to the will; or it may consist in the withholding of operations which are due on the part of the will; this has the nature of guilt, i.e. when any one fails of perfect action in that whereof he is master according to the will.
Guilt has more of the nature of evil than pain has, not only of sense, but of loss. For evil signifies properly that of guilt, not of punishment: first, because goodness consists in actuality; and the goodness of a man consists in his making a good use of the things which he possesses, which things are used by the will. Thus a man is said to be good or bad according as his will is good or bad; and such badness consists in the inordinate action of the will, while punishment consists in the privation of some of those things of which the will makes use. A second reason is, that God is the Author of the evil of punishment, but not of that of guilt; and while the evil of punishment takes away some created good, the evil of guilt is opposed to the Uncreated Good, and so has more of the nature of evil than the former.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni