On Marriage and Concupiscence

Forasmuch, then, as the good of marriage could not be lost by the addition of this evil, some imprudent persons suppose that this is not an added evil, but something which appertains to the original good. A distinction, however, occurs not only to subtle reason, but even to the most ordinary natural judgment, which was both apparent in the case of the first man and woman, and also holds good still in the case of married persons today. What they afterward effected in propagation—that is the good of marriage; but what they first veiled through shame—that is the evil of concupiscence, which everywhere shuns sight, and in its shame seeks privacy.

Since, therefore, marriage effects some good even out of that evil, it has whereof to glory; but since the good cannot be effected without the evil, it has reason for feeling shame. The case may be illustrated by the example of a lame man. Suppose him to attain to some good object by limping after it, then, on the one hand, the attainment itself is not evil because of the evil of the man's lameness; nor, on the other hand, is the lameness good because of the goodness of the attainment. So, on the same principle, we ought not to condemn marriage because of the evil of lust; nor must we praise lust because of the good of marriage.