welcome covers

Your complimentary articles

You’ve read one of your four complimentary articles for this month.

You can read four articles free per month. To have complete access to the thousands of philosophy articles on this site, please

David Hume

Hume on Is and Ought

Charles Pigden considers Hume’s famous claim that you can’t deduce an ‘ought’ from an ‘is’.

According to David Hume, his Treatise of Human Nature “fell dead-born from the press, without reaching such distinction, as even to excite a murmur among the zealots.” Since Hume’s day his still-born baby has undergone a mighty resurrection and the murmur of commentators, whether zealots or otherwise, has risen to a continuous roar. One of the most talked-about paragraphs in that talked-about book occurs at the end of section 3.1.1, ‘Moral Distinctions Not deriv’d from Reason’:

“In every system of morality, which I have hitherto met with, I have always remark’d, 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 surpriz’d 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,’tis necessary that it shou’d be observ’d and explain’d; 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 … [I] am persuaded, that a small attention [to this point] wou’d 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 perceiv’d by reason.”

Hume’s idea seems to be that you cannot deduce moral conclusions, featuring moral words such as ‘ought’, from non-moral premises, that is premises from which the moral words are absent. The passage is summed up in the slogan ‘No-Ought-From-Is’ (or NOFI for short) and for many people it represents the take-home message of Hume’s moral philosophy. It is sometimes rather grandly referred to as Hume’s Law. But what exactly did he mean by it? Why did he think that his observation would ‘subvert all the vulgar systems of morality’? Is NOFI (or something like it) true? And what are the philosophical consequences?

This last question is particularly important since many philosophers think that NOFI supports non-cognitivism. This is the view that moral judgments are not genuinely or full-bloodedly true or false but that they serve to express emotions (emotivism) or to convey commands (prescriptivism). To say that promise-keeping is right is to say something like “Hurrah for promise-keeping!” or “Keep your promises!” Thus some philosophers think that once you accept NOFI, it is a few short steps to the view that there is no objective basis for ethics and that although we may disagree with the advocates of female circumcision or ethnic cleansing we cannot convict them of intellectual error. They are simply cheering for (or commanding) something that we are disposed to boo (or forbid). Other philosophers think that although NOFI does not support non-cognitivism, it is incompatible with naturalism, the view that moral judgments can be true or false as a result of natural facts about the world, such as facts about human flourishing or facts about what a sensible person would approve of under certain circumstances. So in the mid-Twentieth Century, No-Ought-From-Is became the focus for a furious debate. Some argued for No-Ought-From-Is in order to vindicate non-cognitivism, some tried to attack non-cognitivism by way of No-Ought-From-Is and some tried to refute No-Ought-From-Is in order to vindicate naturalism.

In my opinion all the parties to this dispute were making the same mistake. For in so far as it is true and provable, NOFI provides no support for non-cognitivism and no argument against naturalism. But to see why we have to go back to Hume.

What does Hume mean when he says that it seems inconceivable that a moral conclusion can be a deduction from premises that are entirely different from it? He could mean that you can’t get moral conclusions from non-moral premises by logic alone. Or he could mean that you can’t get moral conclusions from non-moral premises by means of logic plus analytic bridge principles. This is a difficult distinction, and one often slurred over by famous philosophers, so let me try to make it clear by means of an example. Consider the following inference, F: 1) Fritz is a bachelor; therefore 2) Fritz has no wife. Is this a valid argument? Well, in one sense ‘yes’ and in another sense ‘no’. It is not a logically valid argument, such that given its structure and whatever the meanings of the non-logical words, the premises cannot be true and the conclusion false. For there are many arguments with the same logical structure in which the conclusion is false but the premise is true. For example: 1) Obama is a Democrat; therefore 2) Obama has no trousers. But though the inference is not logically valid it is analytically valid, for given the meanings of ‘bachelor’ and ‘wife’ it is impossible for the premise ‘Fritz is a bachelor’ to be true and the conclusion ‘Fritz has no wife’ to be false. When a set of premises analytically entails a conclusion in this way, it is generally possible to convert the inference into a logically valid argument by adding in an extra premise, namely an analytic bridge principle, true by definition, which expresses the meaning links between the original premises and the conclusion. Thus the analytically valid inference F can be reformulated as the logically valid F’: 1) Fritz is a bachelor; 1a) a bachelor is a man who has no wife (and has never had a wife); therefore 2) Fritz has no wife.

Is Hume claiming that you can’t get moral conclusions from non-moral premises by logic alone, that is, that there are no logically valid arguments from the non-moral to the moral? Or is he claiming that you can’t get moral conclusions from non-moral premises by logic plus analytic bridge principles, that is, that there are no analytically valid arguments from the non-moral to the moral? The first could be true and the second false. If you can’t get moral conclusions from non-moral premises by logic alone, it does not follow that you can’t get moral conclusions from non-moral premises by logic plus analytic bridge principles. For there might be analytic bridge principles, truths of meaning analogous to the claim that a bachelor is a man who has no wife (and has never had a wife), enabling us to move from non-moral premises to a moral conclusion via an analytically valid argument. For the second claim to be true there would have to be no analytic bridge principles linking the moral and the non-moral. The moral words would have to be indefinable or at least not definable in terms of natural properties or concepts. This was a thesis made famous by the Cambridge philosopher G.E. Moore (1874-1958) who thought it a fallacy, the ‘Naturalistic Fallacy’, to define the word ‘good’ in terms of anything else.

This was not Hume’s opinion. He was not denying that the moral can be defined in terms of the non-moral. He was merely denying the existence of logically valid arguments from the non-moral to the moral. This becomes clear once we note that Hume does not think that he has to argue for the apparent inconceivability of is/ought deductions. It is something he thinks he can take for granted. This is what we would expect if he were making the logical point since it would have been obvious to his readers. For it was a commonplace of Eighteenth Century logical theory that in a logically valid argument the matter of the conclusion – that is the non-logical content – is contained within the premises, and thus that you cannot get out what you haven’t put in. Thus if an ‘ought’ appears in the conclusion of an argument but not in the premises, the inference cannot be logically valid. You can’t deduce an ‘ought’ from an ‘is’ by means of logic alone.

So far from thinking that the moral concepts are indefinable, Hume actually defines the virtues in terms of the responses of an ideal spectator. Hume thought that we share a ‘moral sense’, a disposition to approve of some things and to disapprove of others, which operates in much the same way in most human beings so long as it is not clouded by misinformation or perverted by self-interest. To say that a trait is a virtue is to say that we would be inclined to approve of it at our unbiased best. “The hypothesis which we embrace is plain. It maintains, that morality is determined by sentiment. It defines virtue to be whatever mental action or quality gives to a spectator the pleasing sentiment of approbation; and vice the contrary. We then proceed to examine a plain matter of fact, to wit, what actions have this influence.” (Enquiries: 289). Thus NOFI is not incompatible with naturalism. Hume himself was a naturalist, since he supposed that there are moral truths which are made true by natural facts, namely facts about what human beings are inclined to approve of. There are many philosophers who think that Hume’s No-Ought-From-Is is somehow equivalent to Moore’s Naturalistic Fallacy, a confusion that is enshrined in many text books and encyclopedias of philosophy. This is simply a mistake.

But why did Hume boast that his logical observation would subvert all the vulgar systems of morality? Hume thought that our moral beliefs are based on feelings rather than reason, namely the feelings of approval and disapproval that we experience at our informed and unbiased best. But the orthodoxy of his day was that the truths of morality are demonstrable, that they can be derived by logic from self-evident axioms, that is, axioms evident to reason. For Hume no (non-trivial) moral claims are self-evident: they are evident only to human beings since only human beings have the right kind of feelings. By the end of 3.1.1, Hume thinks he has shown that no non-trivial moral truth is evident to reason. His principal argument (the Motivation or Influence Argument) goes like this:

1) Moral beliefs have an influence on [people’s] actions and affections.

2) Reason alone [that is beliefs derived from reason unaided by desire] can never have any such influence.


3) Morals… cannot be derived from reason.

The problem with this argument is that it is clearly invalid (and not, as some commentators say, clearly valid). It is obviously the case that moral beliefs have an influence on conduct, and it may be true that beliefs derived from reason can have no such influence without the aid of a pre-existing desire. But it would only follow that our moral beliefs are not derived from reason if our moral beliefs influenced us without the aid of pre-existing desires, and that claim is dubious in the extreme. Nonetheless, Hume thinks he has proved that no non-trivial moral proposition is self-evident. Does that show that the orthodoxy is wrong and that the truths of morality aren’t demonstrable? No. For it might be possible to derive the truths of morality from self-evident but non-moral premises, a possibility suggested by philosophers such as Hobbes, Spinoza and Locke. The point of No-Ought-From-Is is to foreclose this option. If no moral claims are self-evident and if you can’t logically derive moral claims from non-moral claims (such as claims about the being of God or observations concerning human affairs) then morality is not demonstrable, the orthodoxy is false and the vulgar systems have been subverted.

But is No-Ought-From-Is true? Not quite. It is an instance of the logical principle that in a valid inference there can be no matter in the conclusion that is not contained in the premises, and as the New Zealand logician Arthur Prior pointed out this is not strictly correct. However, what we can show is that if you have non-logical words in the conclusion of a valid inference that do not appear in the premises they will be vacuous in a certain sense and that in a logically valid argument you can’t get anything non-vacuous out that you haven’t put in. This gives us No-Non-Vacuous–Ought-From-Is which is close enough to the original NOFI to sustain Hume’s key arguments. So for simplicity’s sake we will stick with No-Ought-From-Is in its original form.

One question remains. Does NOFI support non-cognitivism? Well, if non-cognitivism is correct, No-Ought-From-Is is true too. If moral claims are covert commands or disguised exclamations like ‘Hurrah for chastity!’ then they can’t be the conclusions of a logically valid inference. For in a logically valid inference if the premises are true the conclusion must be true likewise. But if non-cognitivism is correct, moral claims are neither true nor false which means that they cannot be true. However this is an argument from non-cognitivism to No-Ought-From-Is. Hence it cannot establish non-cognitivism. Is it possible to go the other way and argue from No-Ought-From-Is to non-cognitivism? Perhaps. How about this?

1*) You can’t derive an ought from an is, moral conclusions from non-moral premises.

2*) The best explanation of 1*) is:

a) that in a logically valid argument you can’t get anything (non-vacuous) out that you haven’t put in; and

b) that since moral judgments are covert commands or disguised exclamations, moral words are fundamentally different from non-moral words (they aren’t ‘descriptive’ or fact-stating).

Therefore, probably:

3*) Moral judgments are covert commands or disguised exclamations.

This is an ‘inference to the best explanation’, a logical move which relies on the principle that the best explanation for a well-established fact is probably true. Hence if NOFI is a well-established fact and non-cognitivism is part of the best explanation, non-cognitivism is probably true. But there is a better (because simpler) explanation of NOFI, namely that that in a valid inference you can’t get anything (non-vacuous) out that you haven’t put in. This logical point explains NOFI all by itself. Thus non-cognitivism is not part of the best explanation of NOFI, which means that No-Ought-From-Is provides no support for non-cognitivism. Therefore the objectivity of ethics has not been disproved.

Why then is No-Ought-From-Is important? The answer is that it isn’t as important as many philosophers take it to be. Like many truisms it acquires its importance by being denied. If someone proclaims that the sun won’t rise every day unless we rip out the hearts of sacrificial victims it is worth insisting on the truism the sun rises every day whatever we do. Similarly, if someone proclaims that they can logically deduce moral conclusions from non-moral premises, it is worth insisting on the truism that you can’t get an ‘ought’ from an ‘is’. Otherwise perhaps not.

© Dr Charles Pigden 2011

Charles Pigden is an Associate Professor of Philosophy at the University of Otago in New Zealand. He edited and contributed to a recent book called Hume, ‘Is’ and ‘Ought’ (Palgrave Macmillan, 2010).

This site uses cookies to recognize users and allow us to analyse site usage. By continuing to browse the site with cookies enabled in your browser, you consent to the use of cookies in accordance with our privacy policy. X