The Roman agricultural guide De Re Rustica mentions the classic principal-agent problem of slave labor. Since slaves are not properly compensated with the fruits of their labor, they require constant supervision. But since good supervision is difficult to obtain—and the supervisors themselves require supervision to do a good job—free labor is often preferable in the end. "a man's personal supervision never fails to yield a larger return from his land than does that of a tenant..On far distant estates, however, which it is not easy for the owner to visit, it is better for every kind of land to be under free farmers"