In short, repayment requires gratitude, time, opportunity, and the help of fortune; whereas, he who remembers a benefit is grateful for it, and that too without expenditure. Since gratitude demands neither labour, wealth, nor good fortune, he who fails to render it has no excuse behind which to shelter himself; for he who places a benefit so far away that it is out of his sight, never could have meant to be grateful for it. Just as those tools which are kept in use, and are daily touched by the hand, are never in danger of growing rusty, while those which are not brought before our eyes, and lie as if superfluous, not being required for common use, collect dirt by the mere lapse of time, so likewise that which our thoughts frequently turn over and renew never passes from our memory, which only loses those things to which it seldom directs its eyes. III. Besides this, there are other causes which at times erase the greatest services from our minds. The first and most powerful of these is that, being always intent upon new objects of desire, we think, not of what we have, but of what we are striving to obtain. Those whose mind is fixed entirely upon what they hope to gain, regard with contempt all that is their own already. It follows that since men's eagerness for something new makes them undervalue whatever they have received, they do not esteem those from whom they have received it. As long as we are satisfied with the position we have gained, we love our benefactor, we look up to him, and declare that we owe our position entirely to him; then we begin to entertain other aspirations, and hurry forward to attain them after the manner of human beings, who when they have gained much always covet more; straightway all that we used to regard as benefits slip from our memory, and we no longer consider the advantages which we enjoy over others, but only the insolent prosperity of those who have outstripped us. Now no one can at the same time be both jealous and grateful, because those who are jealous are querulous and sad, while the grateful are joyous. In the next place, since none of us think of any time but the present, and but few turn back their thoughts to the past, it results that we forget our teachers, and all the benefits which we have obtained from them, because we have altogether left our childhood behind us: thus, all that was done for us in our youth perishes unremembered, because our youth itself is never reviewed. What has been is regarded by every one, not only as past, but as gone; and for the same reason, our memory is weak for what is about to happen in the future.