Institutio Oratoria

This much even the blind can see, as the saying is, and no one who is the possessor of sufficient wealth to satisfy his needs (and that does not imply any great opulence) will seek to secure an income by such methods without laying himself open to the charge of meanness. On the other hand, if his domestic circumstances are such as to require some addition to his income to enable him to meet the necessary demands upon his purse, there is not a philosopher who would forbid him to accept this form of recompense for his services, since collections were made even on behalf of Socrates, and Zeno, Cleanthes and Chrysippus took fees from their pupils.