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. This change is imperceptible; but is, however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, it is necessary that it should be observed and explained; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it. (Treatise 3.1.1)
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. Passions may indeed arise from relations of ideas and impressions, but there is no good reason to presume passions, though necessary, are sufficient to produce a moral opinion.
…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. It is as little contrary to reason to prefer even my own acknowledged lesser good to my greater, and have a more ardent affection for the former than the latter. A trivial good may, from certain circumstances, produce a desire superior to what arises from the greatest and most valuable enjoyment; nor is there any thing more extraordinary in this, than in mechanics to see one pound weight raise up a hundred by the advantage of its situation.In short, a passion must be accompanyed with some false judgment in order to its being unreasonable; and even then it is not the passion, properly speaking, which is unreasonable, but the judgment.
