I owe nothing to one who wished to lend me money but did not do so, and in like manner I shall be the friend of one who wished but was not able to bestow a benefit upon me, but I shall not be under any obligation to him. I also shall wish to bestow something upon him, even as he did upon me; but if fortune be more favorable to me than to him, and I succeed in bestowing something upon him, my doing so will be a benefit bestowed upon him, not a repayment out of gratitude for what he did for me. It will become his duty to be grateful to me; I shall have begun the interchange of benefits; the series must be counted from my act.