An argument of David Hume provides a more direct threat to the possibility of moral knowledge based on the fact that morals excite our passions and motivate us to act. If morals are based on reason so that they consist in true or false ideas, they would have to be in themselves incapable of having this direct influence on our actions (Hume, Treatise, Book III, Part I, Section I, Paragraph 6.) As he famously said, it is not contrary to reason to prefer the destruction of the whole world to the sc