Book II, Chapter V; translation by Andrew P. Peabody
Tusculanae Disputationes – Tusculan Disputations (45 BC)
Context: A: For what stronger proof can there be of its [philosophy's] uselessness than that some accomplished philosophers lead disgraceful lives?
M: It is no proof at all; for as all cultivated fields are not harvest-yielding [... ] so all cultivated minds do not bear fruit. To continue the figure – as a field, though fertile, cannot yield a harvest without cultivation, no more can the mind without learning; thus each is feeble without the other. But philosophy is the cultivation of the soul. It draws out vices by the root, prepares the mind to receive seed, and commits to it, and, so to speak, sows in it what, when grown, may bear the most abundant fruit.