Given the sin of impiety through which they [the Romans] sinned against the divine nature [by idolatry], the punishment that led them to sin against their own nature followed.... I say, therefore, that since they changed into lies [by idolatry] the truth about God, He brought them to ignominious passions, that is, to sins against nature; not that God led them to evil, but only that he abandoned them to evil.
Thomas AquinasIf there were some solitary or feral man, the passions of the soul would be sufficient for him; by them he would be conformed to things in order that he might have knowledge of them. But because man is naturally political and social, there is need for one man to make his conceptions known to others, which is done with speech. So significant speech was needed if men were to live together. Which is why those of different tongues do not easily live together.
Thomas AquinasPerfection of moral virtue does not wholly take away the passions, but regulates them.
Thomas AquinasIf a thing can be done adequately by means of one, it is superfluous to do it by means of several; for we observe that nature does not employ two instruments [if] one suffices.
Thomas AquinasIn order for a war to be just, three things are necessary. First, the authority of the sovereign... Secondly, a just cause... Thirdly... a rightful intention.
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