  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 scratching of a finger (Book II, Part III, Section III, Paragraph 6). The argument can be rendered for our purposes as a valid deductive argument from three premises: (1) If moral knowledge is possible, then some moral judgments are beliefs. (2) Our moral judgments by themselves necessarily give us some motivation to act, even without the accompaniment of already existing desires. (3) A belief by itself, unaided by already existing desires, can never give us any motivation to act. Therefore, moral judgments are not beliefs. Therefore, moral knowledge is impossible. Since the argument is manifestly valid in form, we need to examine the premises to see if we should be persuaded.