Chapter on the Subject-Matter of a Bequest
A bequest remains valid even for something that may not lawfully be sold:
— such as a runaway slave, a stray animal, a bird in flight, a fetus still in the womb, or milk that is still in the udder2;
— and even for something non-existent, such as whatever a man’s slave-girl or tree may produce, whether for all time or for a stated period3.
If the thing subsequently comes into being, it belongs to the legatee, except for the slave-girl’s offspring, in which case he is entitled only to its value on the day it is born.

A bequest is also valid for property that carries no monetary value in law, like a dog whose use is permissible or oil that has become impure; it is valid for a single benefit, such as the service of a slave, the rent of a house, and the like; and it is valid for an unspecified item, as when one says “a garment,” in which case the legatee receives whatever the term properly denotes.

Where the customary usage of a term and its technical legal meaning differ, the legal meaning takes precedence4.