Hobbes's account of self-concern hinges upon his psychology of man, which itself rests upon two crucial premises-first, that all voluntary action is egoistically motivated, and, second, that reason is simply the instrument we use to satisfy our desires. Stripped of its end-conferring power, reason is now impotent to determine self-interest, which is instead established by passion. In fact, our "good" consists entirely in whatever we happen to desire at a given moment. Since our desires constantly change, self-interest is in perpetual flux. This of courSe implies the non-existence of a summum bonum, which Hobbes regards as a mere philosopher's fiction.