In other words, if government were totally useless, it would not be.

The duty of Allegiance would be no duty, but for the advantage of it, in preserving peace and order among mankind.
[Hume is here supposing that men enter into society on equal terms; he makes no allowance for the exercise of the right of the stronger in making compulsory social unions.

This, however, does not affect his reasoning as to the source of our approbation of social duty, which is not usually extended to tyranny.] When political societies hold intercourse with one another, certain regulations are made, termed Laws of Nations, which have no other end than the advantage of those concerned.
The virtue of Chastity is subservient to the utility of rearing the young, which requires the combination of both parents; and that combination reposes on marital fidelity.

Without such a utility, the virtue would never have been thought of.

The reason why chastity is extended to cases where child-bearing does not enter, is that _general rules_ are often carried beyond their original occasion, especially in matters of taste and sentiment.
The prohibition of marriage between near relations, and the turpitude of incest, have in view the preserving of purity of manners among persons much together.
The laws of good manners are a kind of lesser morality, for the better securing of our pleasures in society.
Even robbers and pirates must have their laws.