Should all vices be crimes? This a question St. Thomas addresses explicitly in his Summa, wherein he objects to the criminalization of most vices on the ground that it would make criminals of most people. St. Thomas argues that “human law rightly allows some vices, by not repressing them” and with good reason:
[Virtuous conduct] is not possible to one who has not a virtuous habit, as is possible to one who has. Thus the same is not possible to a child as to a full-grown man: for which reason the law for children is not the same as for adults, since many things are permitted to children, which in an adult are punished by law or at any rate are open to blame. In like manner many things are permissible to men not perfect in virtue, which would be intolerable in a virtuous man.
Now human law is framed for a number of human beings, the majority of whom are not perfect in virtue. Wherefore human laws do not forbid all vices, from which the virtuous abstain, but only the more grievous vices, from which it is possible for the majority to abstain; and chiefly those that are to the hurt of oth-ers, without the prohibition of which human society could not be maintained: thus human law prohibits murder, theft and such like.
What St. Thomas is teaching is nothing less than the non-aggression principle: that human law should limit its punishments to acts that hurt other people or their prop-erty. He goes on to assert that criminal punishment “belongs to those sins chiefly whereby one’s neighbor is injured.”
St. Thomas also cites God’s own unwillingness to prevent earthly evils, often times because the cure would be worse than the disease:
Human government is derived from the Divine gov-ernment, and should imitate it. Now although God is all-powerful and supremely good, nevertheless He al-lows certain evils to take place in the universe, which He might prevent, lest, without them, greater goods might be forfeited, or greater evils ensue. Accordingly in human government also, those who are in authority, rightly tolerate certain evils, lest certain goods be lost, or certain greater evils be incurred: thus Augustine says (De Ordine ii, 4): “If you do away with harlots, the world will be convulsed with lust.”
Elsewhere, St. Thomas repeats this truth that suppressing vices can lead to greater evils: “[Human law] does not lay upon the multitude of imperfect men the burdens of those who are already virtuous . . . Otherwise these imperfect ones, being unable to bear such precepts, would break out into yet greater evils.”
As St. Thomas foresaw (along with St. Augustine), the criminalization of other moral evils–drunkenness, drug abuse, sexual immorality and other vices which are not directly harmful to others or their property—is unjust and results in greater sins.