Man can sin against nature in two ways. First, when he sins against his specific rational nature, acting contrary to reason. In this sense, we can say that every sin is a sin against man's nature, because it is against man's right reason.
Thomas AquinasGod himself would not permit evil in this world if good did not come of it for the benefit and harmony of the universe.
Thomas AquinasThe highest manifestation of life consists in this: that a being governs its own actions. A thing which is always subject to the direction of another is somewhat of a dead thing.
Thomas AquinasLove works in a circle, for the beloved moves the lover by stamping a likeness, and the lover then goes out to hold the beloved inreality. Who first was the beginning now becomes the end of motion.
Thomas Aquinas