Lesson · 4 min Steady

Is and Ought

In one line Facts alone do not hand you an ought; name the principle you add.

Hume makes a similar cut in morals. A description of what is, however complete, does not by itself hand you what ought to be. The ought was added by a feeling or by a further principle.

The addition is allowed. Hiding it is the mistake. Smuggling the “should” into the word “is” lets a value wear the costume of a fact. Show the principle. Then it can be argued, kept, or refused.

Two people can agree on every measurement and still disagree about what should be done. The measurements settle what is. They do not finish the ought. A louder reading of the same facts does not finish it either.

Go deeper

Hume’s remark that writers slide from “is” to “ought” without noticing is now called his guillotine. Some philosophers take it to show that morals cannot be derived from facts alone; others say that facts about human needs and flourishing do bear on what we ought to do.

Either way, the habit of spotting the hidden step is useful.

The list and the seal

A report lists the wages, the hours, and the injuries. Someone says the list has already proved what the shop ought to pay.

Hume would keep the list. He would also ask for the seal that was pressed on afterward: the principle that turned those facts into a duty. The seal can be a good one. It is not already inside the list.

Try this today When someone says “the numbers prove we must,” ask what principle turns them into a must.

Cite or share this lesson

A plain reference you can paste into notes or an essay:

Quest2Reason. “Is and Ought.” https://quest2reason.com/learn/lesson/is-and-ought. Accessed 13 November 2026.

Sources

David Hume, A Treatise of Human Nature, Book III, Part I, Section I — A description of what is does not, by itself, deliver what ought to be. The added principle should be shown. Paraphrase. The English text is public domain.
Stanford Encyclopedia of Philosophy, “Hume’s Moral Philosophy”, section 5 — Places the is–ought passage in the Treatise (3.1.1) and says its interpretation is much disputed.