The Gap Hume Noticed

Hume is reading books on morality and notices something about how they are written.

For several pages the author proceeds in the ordinary way, making observations about human nature, about God, about what people do and what societies are like. These are statements about what is. And then, without warning, every sentence is joined by an ought instead.

The change is imperceptible, he says, and yet of the last consequence.

His complaint is precise and it is not a complaint about morality. It is that a conclusion about what should be done does not follow from premises that only describe how things stand, and that since these authors have not explained the step, they owe an explanation.

He is still owed one.

The first thing to get right is what this does and does not show, because it is regularly overstated in both directions.

It does not show that morality is subjective. It does not show that ought-statements are meaningless, or that you cannot reason about them, or that any moral claim is as good as any other. Hume was not a nihilist and the passage is not an argument for nihilism.

What it shows is that a particular inference is missing a premise. Somewhere in the argument something evaluative has to be supplied, and if it is not in the premises then it arrived silently, and the thing that arrived silently is doing all the work.

Which makes the gap enormously useful in practice.

Almost every argument you will hear in public makes this jump. A study establishes that something has a measurable effect, and a recommendation follows. A historical claim is established and a policy is inferred. Somebody explains what humans evolved to do and concludes that we should arrange society accordingly.

In each case the move is the same, and the useful response is not to reject the conclusion. It is to ask what premise would close the gap.

That premise is almost always the real disagreement. The facts are usually not in dispute; what is in dispute is the unstated evaluative claim that turns them into a recommendation, and asking for it out loud moves the argument to where it actually lives.

This is also why the word natural deserves the suspicion it gets.

Natural describes and recommends at the same time, and it slides between the two without announcing it. Establishing that something is natural, in the descriptive sense of occurring without human intervention, leaves entirely open whether it is good. Disease is natural. Whether the recommendation follows is exactly the question Hume asked somebody to answer.

Nobody has. What philosophy has done instead is spend three centuries proposing candidates for the missing premise and arguing about where it could possibly come from.

That argument is the rest of this book.