"The exchange of one fear or pleasure or pain for another fear or pleasure or pain, which are measured like coins, the greater with the less, is not the exchange of virtue. O, my dear Simmias, is there not one true coin, for which all things ought to exchange?—and that is wisdom; and only in exchange for this, and in company with this, is anything truly bought or sold, whether courage or temperance or justice. ...in the true exchange, there is a purging away of all these things, and temperance, and justice, and courage, and wisdom herself are a purgation of them."