Hume

In my philosophical folklore post last week I asked about other tidbits of philosophical folklore, and commenter Ray Ingles gave one example:

The “is-ought fallacy” is another recurring ‘folk philosophy’ phrase – meaning “you can’t derive an ‘ought’ from an ‘is’”, after Hume.

This is a very interesting one, and it is undeniably common — even the exact phrase “you can’t derive an ‘ought’ from an ‘is’” returns something like 48000 hits on Google, and when you start adding variations, the number explodes. The principle is sometimes called ‘Hume’s Guillotine’, a label that seems to go back to philosopher Max Black in the 1960s. Others call it ‘Hume’s Law’, the source of which I have not been able to trace, although it does seem to be both more recent and less useful, given that there are plenty of other things that have also been called ‘Hume’s Law’. As is often the case with things that reduce to a slogan, it seems to be used in very different ways. Here are some various formulations that often get thrown around when talking about the ‘Is-ought fallacy’ or ‘Hume’s Guillotine’:

You can’t derive an ‘ought’ from an ‘is’.
You can’t derive an imperative from an indicative.
You can’t derive value judgments from factual judgments.
You can’t derive normative claims from factual claims.
You can’t derive evaluative claims from non-evaluative claims.

But oughts, imperatives, and value judgments are all very different things. ‘Ought’ statements, for instance, are generally indicative statements.  What adds to the confusion is that all of these, even if they are often true, seem to have obvious counterexamples, yet they are all treated as absolute statements. There are many intriguing puzzles here, and the question is sometimes even raised as to whether the use of the principle is self-defeating. As a friend of mine, James Chastek, once joked, “We can’t derive an ‘ought’ from an ‘is’; therefore we ought not to try.”

Perhaps we should go back and look at the source of this slogan, David Hume (1711-1776).

It may help to give some very general background. There were in Hume’s day two major approaches to ethics, which we might call moral rationalism and moral sentimentalism . They’d overlap in many of their details, but they had very different conceptions of what moral judgments were. On the moral rationalist view, moral judgments were perceptions of relations between ideas . On this account, cultivating good moral judgment is like learning mathematics. As you learn mathematics, you develop the ability to ‘see’ necessary and eternal relations between ideas; these were sometimes called ‘relations of equality’. Moral rationalism holds that cultivating good moral judgment is very much like this: you develop the ability to ‘see’ necessary and eternal relations between ideas, which were sometimes called ‘relations of perfection’. Moral claims are either true or false, and they are necessarily so. This was a very popular view in the eighteenth century, but the most widely read proponents of it were Nicolas Malebranche and Samuel Clarke, both of whom we know Hume read very closely.

Moral sentimentalism, however, denied that moral judgment was in itself a matter of reason. Rather, it was a matter of orderly sentiment . Whereas the moral rationalist thinks that moral judgments are like mathematical judgments, the moral sentimentalist thinks that moral judgments are like judgments of good taste. It’s important to understand that this did not mean that they thought that moral judgments are mere expressions of taste or arbitrary opinion; they thought that they were like judgment of good taste, taste that was informed, sympathetic, based on wide experience, capable of making fine distinctions. The person of good taste was held to have a better understanding of whatever they were discussing, capable of backing up their judgments with good reasons. In short, moral sentiments see the person of good moral judgment as a moral connoisseur, able to distinguish good and bad action in the same way that a connoisseur of wine is able to distinguish good and bad wine.

Hume is a moral sentimentalist, and the passage from which the “Hume’s Guillotine” derives occurs at the end of his attack on moral rationalism ( Treatise of Human Nature , Book III, Part I, Section I):

I cannot forbear adding to these reasonings an observation, which may, perhaps, be found of some importance. 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 surprized 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. But as authors do not commonly use this precaution, I shall presume to recommend it to the readers; and am persuaded, that this small attention would subvert all the vulgar systems of morality, and let us see, that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceived by reason.

This is a passage that has been interpreted in many different ways by Hume scholars. My own approach to interpreting the passage is to argue that context is important. Hume is not arguing about ethics in general. In fact, he goes on in the very next section to present an account of obligation, or ‘ought’, based on moral sentiment. Rather, he is finishing up his attack on moral rationalism. Indeed, he as much as says so. What conclusion does he want us to draw? “That the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceived by reason.” In other words: the claim only affects positions, like moral rationalism, that take moral judgments to concern relations perceived by reason . Hume’s own account of moral judgments is not affected by this because he is a moral sentimentalist, and thinks that obligations are neither relations nor perceived by reason, but are the objects of a kind of moral sentiment, something we feel. What is more, it is not put forward as a necessary or obvious truth, but as a challenge to the moral rationalist. It depends on the arguments that Hume has made in the rest of the chapter, in which he argued that nothing can have the features that moral rationalists like Clarke attribute to moral relations. Hume is here challenging the moral rationalist to prove him wrong. To be sure, it’s a challenge he doesn’t think the moral rationalist can meet, based on the arguments he’s already given — but it’s merely a challenge.

Regardless of whether this is the best way to interpret Hume, we can nonetheless see that there’s a great deal more complexity lurking beneath this apparently simple slogan. Ray Ingles, in the comment mentioned above, had noted some of that complexity:

And it’s sort of true. You can’t derive an ‘ought’ from an ‘is’. But you can derive an ‘ought’ from an ‘is’ and a goal .

For example, given the ‘is’-es of the rules of chess, and the goal to win the game, it follows pretty quickly that you ought not sacrifice your queen early in the game.


I wouldn’t put it exactly the same way, in part because I think statements of goals clearly fall under the ‘is’ category, but I think Ray’s approach to the matter is essentially right. We use the word ‘ought’ when we’re talking about decisions, plans, strategies — practical matters. So it makes sense to see ‘ought’ statements as identifying solutions to potential problems . If the problem is to build a bridge that won’t collapse in the wind, it follows from the claims of material science and engineering that there are things you ought to do and things that you ought not to do. Faced with the problem of designing an experiment that will test a hypothesis, a good scientist can derive from available facts how the experiment ought to be designed. When you are faced with the problem of how to act rationally, there are facts about reason that undeniably force the reasonable person to draw conclusions about what he or she ought to do.

Now, of course, given that the slogan seems to be used in so many different ways, it’s entirely possible that it’s being used correctly in a particular context . But the next time someone brings it up, press them to explain what they actually mean.

Articles by Brandon Watson

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