"Nothing is more usual in philosophy, and even in common life, than to talk of the combat of passion and reason, to give the preference to reason, and to assert that men are only so far virtuous as they conform themselves to its dictates. Every rational creature, 'tis said, is oblig'd to regulate his actions by reason; and if any other motive or principle challenge the direction of his conduct, he ought to oppose it, 'till it be entirely subdu'd, or at last brought to a conformity with that superior principle.'' -David Hume
David Hume famously demonstrated the is/ought fallacy, in which it's said that you cannot derive what one ought to do from what is. This means that I can't logically say that "If Tim sees Jim drowning, he ought to help him" unless one takes it as implied that one ought to help a man who is drowning, in which case we've derived an ought from an is and an ought. One effect of this fallacy is making reason a "slave of the passions," as Hume put it, in our day to day lives. Only if we already have an idea of what we ought to do, can we then use reason to determine the best course of action to do as much, or otherwise determine what else we ought to do.
It would then seem that we're ultimately bound to act in accordance with our innate desires, and whatever memes we've picked up to create for us new desires. Whereas one can reasonably determine that a greater desire trumps a lesser desire, no purely rational system can be created so as to provide a set of oughts that will hold true for all individuals, unless we are to hold to the fact that all individuals share the same basic desires. It seems to me that such isn't the case though.
It was proposed by the pop philosopher Ayn Rand that all humans value life, and thus we can determine what one ought to do based upon that. Nevertheless, it strikes me as obvious that all men don't value life, given that folks often take their own lives. It could also be suggested that all people value happiness, and thus we ought to work to maximize happiness. Nevertheless, this once again just doesn't seem to be the case, as evidenced by the claims of many folks on this board.
So it would seem that Hume was correct in his suggestion that:
"Reason is, and ought only to be, the slave of the passions, and can never pretend to any other office than to serve an obey them" -David Hume
Well... insofar as we take that "ought only to be" as an opinion derived from his own passions. :)