David Hume is famous for the “is-ought” problem, which comes from this famous passage, in his “Treatise on Human Nature”: In every system of morality, which I have hitherto met with, I have always remarked, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surprised to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not.
Hume infers from his insight that it is not reason but moral opinion that moves us to act, that reason is not the source of moral opinion. From this, he then further argues that moral opinion is a product of the passions – special emotions that arise out of the relations of ideas and impressions. In this essay, I will argue that Hume’s initial inference is correct, but that his subsequent inference is not.
…When in exerting any passion in action, we chuse means insufficient for the designed end, and deceive ourselves in our judgment of causes and effects. Where a passion is neither founded on false suppositions, nor chuses means insufficient for the end, the understanding can neither justify nor condemn it. It is not contrary to reason to prefer the destruction of the whole world to the scratching of my finger. It is not contrary to reason for me to chuse my total ruin, to prevent the least uneasiness of an Indian or person wholly unknown to me.