It is hardly an exaggeration to say that, in the age of the classical lawyers, Roman commerce was mainly in the hands of slaves. The commercial importance of different slaves would of course vary greatly. The body-servant, the farm labourer, the coachman, have no importance in this connexion, and there were many degrees between their position, and that of a dispensator or steward, who seems often to have been allowed almost a free hand. The Digest gives us several striking instances. A slave might carry on a bank, with or without orders, the master's rights varying according as it was or was not with the peculium. A slave might be a member of a firm, and his master's notice to him, without notice to the other party, would not end the partnership. Even sale of the slave would not, in fact, end the firm: the new master would acquire the rights from the date of transfer, though as a slave's faculty is purely derivative the firm would be technically a new one.

A dominus can acquire or continue possession through a servus or ancilla. But possession differs from other rights in that it has an element of consciousness. A man may begin to own without knowing it, but he cannot ordinarily so acquire possession. Accordingly we learn that (apart from peculium) a man does not possess what his slave has received, unless and until he knows of it. When he learns the fact he possesses, and he is said to possess by his own animus and the slave's corpus.