He who is drawn to something desirable does not desire to have it as a thought but as a thing.
Thomas AquinasReasoning is compared to understanding as movement is to rest, or acquisition to possession.... Since movement always proceeds from something immovable, and ends in something at rest, hence it is that human reasoning, in the order of inquiry and discovery, proceeds from certain things absolutely understood--namely, the first principles; and, again, in the order of judgment, returns by analysis to first principles, in the light of which it examines what it has found. Now it is clear that rest and movement are not to be referred to different powers, but to one and the same.
Thomas AquinasSecondly, man sins against nature when he goes against his generic nature, that is to say, his animal nature. Now, it is evident that, in accord with natural order, the union of the sexes among animals is ordered towards conception. From this it follows that every sexual intercourse that cannot lead to conception is opposed to man's animal nature.
Thomas Aquinas