It is not necessary to attempt the hopeless task of denning liberty. Justinian adopts from Florentinus the definition: Liberty is the natural capacity {facultas) of doing what we like, except what, by force or law, we are prevented from doing. This definition no doubt expresses certain truths. Liberty is “natural”: slavery is iuris gentium. It is presumed that a freeman can do any act in the law: his incapacity must be proved. The reverse is the case with a slave. But, literally understood, it would make everybody free. As a matter of fact all persons not slaves are free, and as we have arrived at a more or less exact notion of Roman slavery we may leave the matter there.

The conception of manumission needs some examination. It is not in strictness transfer of dominium. A man has no dominium in himself or his members. Nor is it an alienation of liberty. The right received is not that of the master, and the rule that a man cannot give a better liberty than he has is intelligible without reference to such an idea. Nor is it a mere release from the owner's dominium: that is derelictio, from which manumission differs in several ways. Dereliction does not make the man free, it merely makes him a res nullius. Moreover manumission leaves many rights in the master, and there is no such thing as partial dereliction. If it had contained a dereliction, then, since derelictio is purely informal, a manumission which failed for lack of form would have been a dereliction. But this was not the case.