Hear what Tully says more plainly still in the third book of his treatise on the Commonwealth, when discussing the reasons for government.

Do we not, says he, see that nature herself has given the power of dominion to everything that is best, to the extreme advantage of what is subjected to it?
Why, then, does God govern man, and why does the mind govern the body, and reason govern lust and passion and the other vicious parts of the same mind?
Listen thus far; for presently he adds, But still there are dissimilarities to be recognized in governing and in obeying.

For as the mind is said to govern the body, and also to govern lust, still it governs the body as a king governs his subjects, or a parent his children; but it governs lust as a master governs his slaves, because it restrains and breaks it.

The authority of kings, of generals, of magistrates, of fathers, and of nations, rules their subjects and allies as the mind rules bodies; but masters control their slaves, as the best part of the mind--that is to say, wisdom--controls the vicious and weak parts of itself, such as lust, passion, and the other perturbations.
For there is a kind of unjust slavery when those belong to some one else who might be their own masters; but when those are slaves who cannot govern themselves, there is no injury done.
XXVI.

If, says Carneades, you were to know that an asp was lying hidden anywhere, and that some one who did not know it was going to sit upon it, whose death would be a gain to you, you would act wickedly if you did not warn him not to sit down.