The Necessity for Human Law and Its Limits

 From the foregoing it is clear that there is in man a natural aptitude to virtuous action. But men can achieve the perfection of such virtue only by the practice of a 'certain discipline.'--And men who are capable of such discipline without the aid of others are rare indeed.--So we must help one another to achieve that discipline which leads to a virtuous life. There are, indeed, some young men, readily inclined to a life of virtue through a good natural disposition or upbringing, or particularly because of divine help; and for such, paternal guidance and advice are sufficient. But there are others, of evil disposition and prone to vice, who are not easily moved by words. These it is necessary to restrain from wrongdoing by force and by fear. When they are thus prevented from doing evil, a quiet life is assured to the rest of the community; and they are themselves drawn eventually, by force of habit, to do voluntarily what once they did only out of fear, and so to practice virtue. Such discipline which compels under fear of penalty is the discipline of law. Thus the enactment of laws was necessary to the peaceful and virtuous life of men. And the Philosopher says (I Politics, 2): 'Man, when he reaches the perfection of virtue is the best of all animals: but if he goes his way without law and justice he becomes the worst of all brutes.' For man, unlike other animals, has the weapon of reason with which to exploit his base desires and cruelty.

 St Thomas Aquinas, Summa Theologiae, Prima Secundae, Qu. 95 Art. 1

Laws when they are passed should take account of the condition of the men who will be subject to them; for, as Isidore says (Etym. II, 10): the law should be ‘possible both with regard to nature and with regard to the custom of the country.’ But capacity to act derives from habit, or interior disposition: not everything that is possible to a virtuous man is equally possible to one who lacks the habit of virtue; just as a child is incapable fo doing all that a grown man can do. For this reason there is not the same law for children and for adults: there are many things permitted to children which are punished by the law, and even abhorred, in adults. Equally, it is possible to permit many things to those not far advanced in virtue which would not be tolerated in a virtuous man.

Now human law is enacted on behalf of the mass of men, the majority of whom are far from perfect in virtue. For this reason human law does not prohibit every vice from which virtuous men abstain; but only the graver vices from which the majority can abstain; and particularly those vices which are damaging of others, and which, if they were not prohibited, would make it impossible for human society to endure: as murders, theft, and suchlike, which are prohibited by human law.

Ibid., Qu. 96, Art. 2