Philosopher David Hume's famous statement.

You can't get ought from is.

Which is to say:

You can't infer 'P ought to be case' from 'P is the case'.


It is not logically valid to say "it is morally Good to do X because it is the case that Y".

This covers a lot of ground; you can't say something is morally right just because most people do it; you can't say that something is morally right just because we evolved that way; you can't say that something is morally right just because it makes the most people happy. For example, we might hold that throwing Christians to the lions is morally wrong -- even if everyone else is doing it, even if we evolved to kill our competitors, and even if it makes the populace happy.

Of course, many people do in effect claim that you can get ought from is -- utilitarians usually base their 'ought' on something akin to measuring happiness, most people are happy to defend things because "we evolved that way", and when in doubt, doing what everyone else is doing is actually a fairly decent heuristic for keeping the peace and helping your fellow humans.

It seems that this is sometimes called Hume's Maxim. I think that 'You can't get ought from is' is correctly called Hume's Law, and that 'one can only believe a testimony to the miraculous if its falsehood would be more miraculous than the fact which it endeavors to establish' is Hume's Maxim. Is this is wrong, or if they are in fact interchangeable, please let me know.

Node your homework

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:

  1. It is anchored in an appeal to the human passions; reasoning focuses on the consequences to the actor, therefore motivating by fear of consequences;

  2. OR

  3. 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."

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