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Hume's Law says that one cannot derive what ought be (or ought not be) from what is. There is nothing about the factual state of the world that makes it necessarily 'correct' or 'ideal.' And there is nothing about the way things are that can tell us about how they should be.
Take, for example, an action regarded widely as something one ought not commit: incest. If we evaluate the morality of incest based on its incidence in the human population, we might find that it is statistically unusual. Reasoning that one ought to be as most humans are, we would conclude that it ought not be done. But now suppose that suddenly, a religious movement encouraging incest emerged, and the incidence of incest in the population skyrocketed, eventually becoming extremely common. It seems ludicrous that an increase in the incidence of incest could make it something that one 'ought' to do. But this necessarily follows from is-to-ought reasoning.
What's more, apparent 'reason-derived morality' is in fact not purely reason-derived. Pure reason further fails as a moral guide on one of the following two grounds:
- It is anchored in an appeal to the human passions; reasoning focuses on the consequences to the actor, therefore motivating by fear of consequences;
- It aborts the reasoning process altogether, by ultimately saying that 'certain activities are inherently immoral and that's just that.' This, though potentially true, offers no guidance for making moral decisions. One would still be left with the task of gauging the 'inherent' morality of an action.
Hume's Law rejects the notion that reason alone can serve to guide moral decision and action. But it does not altogether preclude reason from having a role in moral decision-making. It's just that moral knowledge must at least have an additional ingredient besides reason. Hume calls this other ingredient a "moral sense."