“Ought” and “Is” Revisited

It’s nearly as predictable as if it were a law of nature: Every few years, someone argues with me online that Hume’s Law (that one cannot derive an “ought” from an “is”) is wrong. And usually, the challenge comes from an atheist who is convinced that they must set this law aside in order to defend moral realism — and thus answer critics who say that atheism cannot justify morality.

There are two basic points such people should learn about this. First, that Hume’s Law is a simple matter of logic; in the sense Hume was talking about, an “ought” cannot be derived from an “is,” period. Second, that in itself this does not show there are no moral truths. It doesn’t even show that ethical naturalism (the view that there are “natural” — and thus in principle scientifically discoverable — moral truths) is false. Anyone who wishes to maintain that there are moral facts discoverable by science is therefore welcome to attempt to do so some other way, in spite of Hume’s Law.

All that this law does is point out that, if none of the premises in an argument contain the term “ought” (or some equivalent, or something that implies it), then the conclusion cannot meaningfully contain it either. And that’s just a straightforward consequence of the nature of deductive logic: You cannot derive a conclusion that isn’t in some way already contained in the premises. For example, it’s impossible to validly conclude that Socrates is mortal if none of the premises you start with mentions Socrates in any way. Similarly, if none of your premises mentions “oughts” in any way — if they are all “is” statements — then you cannot validly conclude an “ought.” It’s as simple as that. If you want to argue against Hume’s Law, at least understand that you are arguing against the nature of deductive logic itself.

So why do so many people think Hume was wrong? The main reason seems to be that they are unclear what his law is really about. Thus, for example, my recent online challenger argued that what we ought to do is obviously determined by our nature — and thus that oughts are derivable from what in fact is our nature. This is an old argument. Here’s Ayn Rand saying essentially the same thing in her 1963 essay “The Objectivist Ethics”:

“…the fact that living entities exist and function necessitates the existence of values and of an ultimate value which for any given living entity is its own life. Thus the validation of value judgements is to be achieved by reference to the facts of reality… So much for the issue of the relation between ‘is’ and ‘ought’.”

But this merely assumes that we ought to value our lives. And that is to already start with a hidden “ought” in the premises. Rand hasn’t derived morality from non-moral facts alone.

Richard Carrier, in “Moral Facts Naturally Exist (and Science Could Find Them)” — published in Loftus’s anthology The End of Christianity, though not endorsed by him — attempts a similar derivation of values from facts. His argument is that it is an objective fact that we desire certain things (e.g., that our cars run well) and also that there are objectively correct and incorrect ways of achieving such desires (e.g., in order for a car to function properly, its oil needs to be changed regularly). “And,” he tells us, “wherever both [of these things] are an empirically demonstrated fact, the imperative they entail is an empirically demonstrated fact.” Thus, that you should change your car’s oil when needed “is factually true independent of human opinion or belief.”

The rather obvious flaw here is that the “ought” in the conclusion only follows if it is the case that you also ought to desire your car to run well — or, once again, if there already is an “ought” in the premises. The mere fact that you do desire it doesn’t cut it. And it’s very easy to show why not. Suppose, for example, that Manson desires to murder a bunch of people, and that in order to do so, he needs to acquire some weapons. Then by the above logic, it is “factually true independent of human opinion or belief” that Manson should acquire weapons. Which means that when the authorities put him in jail (thus taking away his ability to go get guns and knives), they did something factually wrong!

I’m sure Manson would agree. But that certainly doesn't make it true.

(Hume’s Law is often conflated with the naturalistic fallacy, which therefore is also denied by Sam Harris, Richard Carrier, and others. Next time, I’ll point out a reason atheists in particular might want to defend this fallacy.)



Franz Kiekeben is a former lecturer in philosophy and the author of two books on atheism, The Truth about God, and Atheism: Q & A. He has also written for Skeptic magazine and published academic articles on determinism and on time travel.