What is necessary, on the part of the giver, for alienation to be valid

A thing becomes ours by secondary acquisition either through the act of man or by operation of law. After the introduction of ownership, it is of the law of nature that men, who are the owners of property, should have the right to transfer the ownership, either in whole or in part. For this right is present in the nature of ownership, at least of full ownership.

Two matters only are to be noted, the one affecting the giver, the other the receiver. In the case of the giver, a mental act of will is not sufficient, but together with it, either words or other external signs are required, because a mere mental act, as I have said elsewhere, does not meet the requirements of human society.

The requirement that delivery of the property take place arises from municipal law. But because this has been received by many nations, it is improperly called a principle of universal law (naturae societas humanae). Thus we see that, in some places, it is the custom to require a declaration in the presence of the people, or before a magistrate, and insertion in the public records; and it is quite certain that all these formalities arise from municipal law. But the act of will, which is expressed by a sign, must always be understood to be the act of a rational will.