2. Divine providence has endowed men with a natural tribunal of reason (naturale judicatorium rationis), to be the ruling principle of their proper activities. But natural principles are ordained to natural purposes. There are certain activities naturally suited to man, and these activities are in themselves right, and not merely by positive law.*
3. Where there is a definite nature, there must be definite activities proper to that nature: for the proper activity of every nature is consequent upon the nature. Now it is certain that men's nature is definite.* There must therefore be certain activities that in themselves befit man.
4. Wherever a thing is natural to any one, any other thing also is natural, without which the first thing cannot be had, for nature fails not in necessities. But it is natural to man to be a social animal. Those things therefore naturally befit man, without which the maintenance of human society would be impossible. Such things are the securing to every man of his own, and abstinence from wrongdoing. Some points therefore of human conduct are naturally right.
5. The use of lower creatures to meet the need of human life is a natural property of man. Now there is a certain measure in which the use of the aforesaid creatures is helpful to human life. If this measure is transgressed, as in the disorderly taking of food, it results in harm to man. There are therefore certain human acts naturally appropriate, and others naturally inappropriate.
6. In the natural order man's body is for his soul, and the lower powers of the soul for reason. It is therefore naturally right for man so to manage his body and the lower faculties of his soul as that the act and good of reason may least of all be hindered, but rather helped. Mismanagement in this regard must naturally be sinful. We count therefore as things naturally evil carousings and revellings and the disorderly indulgence of the sexual instinct, whereby the act of reason is impeded and subjected to the passions, which do not leave the judgement of the reason free.
7. To every man those things are naturally befitting, whereby he tends to his natural end; and the contraries are naturally unbefitting. But God is the end to which man is ordained by nature (Chap. CXV). Those things therefore are naturally right, whereby man is led to the knowledge and love of God; and the contraries are naturally evil for man.
Hence it is said: The judgements of the Lord are righteous, having their justification in themselves (Ps. xviii, 10).
Hereby is excluded the tenet of those who say that things just and right are the creation of positive law.
3.128 : How the Law of God relates a man to his Neighbour
3.130 : That the Divine Government of Men is after the manner of Paternal Government