Who is so ungrateful as the man who has so completely excluded and cast from his mind the benefit that ought to have been kept uppermost in his thought and always before him, as to have lost all knowledge of it? It is evident that he has not thought very often about returning it if it has faded into oblivion. In short, the repaying of gratitude requires right desire and opportunity and means and the favor of Fortune; but he who remembers shows sufficient gratitude without any outlay. Since this duty demands neither effort nor wealth nor good fortune, he who fails to render it has no excuse in which he may find shelter; for he who has thrust a benefit so far from him that he has actually lost sight of it never could have wished to be grateful for it. Just as tools that are in use and are every day subjected to the contact of our hands never run any risk of becoming rusty, while those that are not brought before the eyes, and, not being required, have remained apart from constant use, gather rust from the mere passing of time, so anything that our thought repeatedly busies itself with and keeps fresh does not slip from the memory, which loses only that which it has over and over again failed to regard.