Hume’s guillotine

Part of a series which began with Fake news and the ethics of belief.

I complained last time about Clifford‘s apparent move from as ‘is’ to an ‘ought’.

This gives us an ideal opportunity to bring David Hume into the conversation.

David Hume (1711-1776) was a Scottish Enlightenment philosopher who many would argue was one of the greatest philosophers of all time.

A celebrated quote of his from A Treatise of Human Nature claims to have spotted a flaw in a lot of moral reasoning he had come across.

Hume's guillotine

He is talking about writers who seem to think they can deduce statements about what ought to be the case from statements about what is the case:

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.1

But these writers rarely think of explaining how they get from is to ought:

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.

He offers this advice: 

David Hume
Portrait of David Hume

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.

The issue Hume highlights here is now usually referred to as the is-ought problem. His conclusion has been labelled ‘Hume’s law’ or, more dramatically, ‘Hume’s guillotine’, which can be worded a number of ways, for example:

If a reasoner only has access to non-moral and non-evaluative factual premises, the reasoner cannot logically infer the truth of moral statements;2

or more formally:

If 𝜙 implies 𝜓, and 𝜓 is moral, then 𝜙 is moral.3

For my purposes I do not need as strong a version as this. I am however taking it a bit broader so as to cover all (categorical) prescriptive statements, not just moral ones. (That is assuming there are categorical oughts which don’t count as moral oughts.)

As far as strength is concerned I do not need to say it is impossible to deduce a (categorical) prescriptive statement (or belief) from ‘non-moral and non-evaluative factual’ – ie descriptive – premises. I only need to say that if you do try to deduce a prescriptive statement from one or more purely descriptive premises, then you must explain how you think this is possible.

I happen to think it is impossible to deduce a categorical prescriptive (eg moral) statement from purely descriptive premises. But I do not know how to prove it is impossible.

Read on.


1 David Hume, A Treatise of Human Nature, Volume II, Book III.

2 Wikipedia: Is–ought problem.

3 Campbell Brown, ‘Two Versions of Hume’s Law’, Journal of Ethics & Social Philosophy, Discussion Note, May 2015.

© Chris Lawrence 2021

5 thoughts on “Hume’s guillotine

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