Hume's Guillotine

Pedro Schwartz for AdamSmithWorks

February 5, 2020
A rule that is hammered into every budding economist is that positive and normative economics should be kept apart. This injunction to separate norms and facts is one of the thorniest in the philosophy of knowledge and in ethics. Economists have always found it both necessary in theory and inapplicable in practice. 

‘Is’ and ‘ought’
This rule originates in a famous passage of David Hume’s Treatise on Human Nature (1739) that deserves to be read carefully before entering the treacherous question of the role of values in science and policy. He starts with a remark belonging to the logic of scientific reasoning.
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 surprised 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

Hume here seems to be querying the tendency of moralists to show that the ethics they defend is not a matter of their personal choice but something engraved by Nature in our conscience or decreed by a writ of Divinity, to guide us in our behaviour. He goes on to warn us that we should be aware of the importance of not falling into the trap of ethical absolutism by some kind of verbal sleight of hand.
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 should be observed and explained; and at the same time that a reason should be given, for what seems altogether inconceivable[.]

Hume then caps these remarks by proposing the mental hygiene of separating ‘ought’ from ‘is’ propositions, because that small precaution would save us from thinking that moral rules exist in reality or are revealed by reason. As he says, this would be enough to
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.[1]

This passage, therefore, introduces Hume’s theory of ethics, which was neither absolutist nor relativist. He neither thought that moral rules were universal and common to all humanity nor that good and bad were relative and wholly dependent on individual whim. Before I return to the 20th c. injunction to separate norms and facts and the problems it poses, I will say a few more words on Hume’s pragmatic philosophy of ethics. 

As Dennis C. Rasmussen[2] has explained, Hume, as well as Montesquieu, Adam Smith, and Voltaire, helped formulate a philosophy of Enlightenment that was not hegemonic in intent, contrary to what its many critics assert. They also formulated an ethic that was not based on Universal Natural Law or on the commands of Reason, as were those of Locke, on the one hand, and Kant, on the other. Morality for them was built on feeling or sentiment, not on the laws of nature or the dictates of reason. Judgments of good and bad had their roots in the natural impulses of men and women. As Hume put it, “self-love, affection between the sexes, love of progeny, gratitude, resentment” are universal primary passions. By distinguishing between the way moral standards are formulated, which may be common to humanity, and the content of morality, largely determined by society, these pragmatists avoided falling into mere relativism.[3] Thus, it was not a matter of ‘anything goes’. We would show little appreciation of Hume’s (and Smith’s) philosophy of ethics if we reduced it to a logical ploy to make moral judgment a matter of pure personal decision. Smith’s ‘impartial spectator’ within our breast saw things in a more detached fashion than we would if we followed our personal interests and passions. Though all four thinkers favoured liberal institutions and practices – security of property, limited government, religious toleration, freedom of expression, free trade, humane criminal laws[4] – they did not mean to impose them regardless of local traditions and the stage of development of other societies.[5] 

Hume is telling us that morality is more than following the prodding of Nature, even of Nature before the Fall, as the School of Aquinas maintained. He also rejected Kant’s ethics as the deduction by pure Reason of a categorical imperative that must be willed by all men independently of any ulterior motive or end. In Hume’s and Smith’s view those two extreme positions do not reflect how people react morally in real life. Humans are social animals and they internalise the rules by which they live from their group and its traditions. Hence, morality originates in sentiment and is clearly connected with fact. 

Looking at the question from the other end there is an inverse connection between value and fact: moral rules do influence what we do. This is more than a psychological phenomenon, than a question of what the mechanisms of action are. We are not only moved by instinct, habit, or interest, but also by what we think is right. Why did the captain of the Titanic decide to sink with her? Shame? Tradition? Honour? That decision was moved by a consideration of what is proper. 

In his Theory of Moral Sentiments Adam Smith did explicitly study the influence of custom and fashion on the sentiment of moral approbation or disapprobation.[6] However, he repeatedly criticised Bernard Mandeville’s indulgence for private vices, even if they resulted in public benefits. And he started his book with “Part I. Of the Sense of Propriety”, that is to say, with the study of the approval or disapproval which our own and others’ actions may meet from the judgment of mankind. His detailed analyses of virtue, duty, self-command also indicate that he was no relativist.

The  ‘naturalistic fallacy’
So much for Hume’s warning about the mistake of basing morality either on fact or on reason rather than on sentiment. Later philosophers have sharpened Hume’s razor or guillotine and have taken it to mean that no factual observation is relevant to the acceptance or rejection of a value judgement. This is precisely what I want to discuss. 

The English philosopher G.E. Moore, in his 1903 Principia Ethica that so inspired the Cambridge Apostles, contributed to turning Hume’s ‘is – ought’ warning into an absolute prohibition. [7] Moore enjoined us not to commit what he called the ‘naturalistic fallacy’. The fallacy he denounced was that of believing that the quality of ‘good’ is as observable and factual as is  the quality of ‘yellow’. ‘Good’ is undefinable. What we call ‘good’ is for Moore the result of a pure intuition or decision, quite unconnected with any observable quality of the act or rule being judged. An example from aesthetics may clarify the shortcomings of Moore’s concept. When we value a picture or a sculpture, we must be able to do more than exclaim ‘oh!’ or ‘fie!’; or even ‘I like it’ or ‘dislike it’. We can relate it to its time and fashion, or to the work of other artists or sculptors, or to the technical ability shown, or to the biography of the author, or to who her patrons were. All these considerations will heighten our appreciation of the piece. In some way therefore value and fact are connected. But how?

For Hume and Smith, it is ‘we’ who judge the propriety of an act or character. For the arch-romantic it is ‘I’ who decides what is proper or improper. Moore’s attempt to define what was really valuable, to wit, “States of mind […], the pleasures of human intercourse and the enjoyment of beautiful objects”[8] was rather frivolous: it led directly to the ridiculing of Eminent Victorians (1918) by Lytton Strachey. Bertrand Russell may have been moved to uncontrollable laughter when he read it in gaol but I think the mirth was displaced.

Amartya Sen’s solution
As Hume and Smith see it, we people do not choose our values in that frivolous way. What we do value, and what rules we obey, or debate whether to obey, are given to us in the society or group in which we live. It is simply not true that values cannot ever be argued about. For Lionel Robbins, “If we disagree about ends it is a case of thy blood or mine […] But, if we disagree about means, then scientific analysis can often help us to resolve our differences.”[9]

To solve the apparent impossibility of relating values and facts, as set out by Robbins and many after him, Amartya Sen proposes to distinguish basic and non-basic values.[10] A basic value is one that no reasoning or attention to fact will make the holder budge from, even to the point of coming to blows. Here the absolute separation between normative and positive is applicable: martyrdom is not an unthinkable gesture in any society. If the value is non-basic then it is possible to argue about it. Thus, the proposer of a minimum wage might be ready to reconsider, if it was shown that setting a legal floor to wages could increase inequality by discriminating against the poorest. Values become non-basic when facts reveal they clash with other values held by the person in a moral quandary. In this case, the attempt to raise the incomes of the poorest is in fact revealed to be counterproductive regarding the desire to afford them a chance to find employment.[11] 

In a situation where questions can be discussed calmly, Sen implies, it could be the case that most values asserted might turn out to be non-basic. If we go back to the distinction above between the formulation of moral standards and their content, the tendency of humanity is to formulate their moral beliefs in absolute terms, as if all were basic values. But in a civilised setting we usually are ready to examine our values critically, even if in the end we hold by our fundamental beliefs. Thus, discussion in the field of morality bears resemblance to that in the field of science: we throw up moral hypotheses and discuss them by (1) adverting to other hypotheses that we hold, and (2) checking their consequences with new facts. To do this properly we must make a clear distinction between normative and positive statements.


[1] Hume, David (1739): A Treatise of Human Nature. Book III, Part I, Section I, in fine. Numerous editions.

[2] Rasmussen, Dennis C. (2014, 2017): The Pragmatic Enlightenment: Recovering the Liberalism of Hume, Smith Montesquieu, and Voltaire. Cambridge University Press.

[3] Rasmussen (2017), pg.19.

[4] Rasmussen, pgs. 81-82).

[5] If I may mention a present-day example, the belief that democracy could be introduced by force in Iraq or other Middle-Eastern nations was a mistake that none of the four would have committed.

[6] Smith, Adam (1759, 1781): The Theory of Moral Sentiments

[7] Moore, George Edward (1903): Principia Ethica. Cambridge.

[8] A most interesting discussion of the personality and philosophy of G.E. Moore can be found in ch. 6, “My Early Beliefs”, vol. I of Robert Skidelsky’s biography of John Maynard Keynes. Hopes Betrayed, 1883-1920. MacMillan. The above quotation is on pg. 139. Keynes, of course, was one of the young Apostles conquered by Moore.

[9] Robbins. L. (1932): An Essay on the Nature and Significance of Economic Science, pg. 132. MacMillan. As quoted in Sen (1970).

[10] Sen, Amartya K. (1970): Collective Choice and Social Welfare. Oliver and Boyd. Ch. 5.

[11] This is so also when we deal, not with an individual decision, but with a whole social system: see for example how facts kill values in the story of the Khrushchev reforms told by Francis Spufford, in Red Plenty. Faber and Faber, 2010.

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