Why can't facts alone ever tell you what you ought to do?
Hume noticed that moral arguments slip from "is" to "ought" without saying how, and philosophers have been arguing about that step ever since.
▶ Start the storyBecause of a gap in logic that David Hume noticed in the 1730s. The is–ought problem asks whether moral statements about what ought to be can be inferred from objective statements about what is. Hume saw a significant difference between descriptive statements and prescriptive ones, and observed that moral writers reason about what is, then suddenly slip into claims about what ought or ought not to be, without explaining how they got there.
Hume's guillotine is the thesis that an ethical conclusion cannot be inferred from purely descriptive factual statements. Add Hume's fork, the idea that all knowledge rests either on logic and definitions or on observation, and a worry follows: "ought" statements seem to be known in neither way, so there might be no moral knowledge. Moral scepticism and non-cognitivism work with such conclusions.
A common reply, from ethical naturalists, is to bring in goals. A statement of the form "in order for agent A to achieve goal B, A ought to do C" can be factually verified or refuted. MacIntyre applies this to purpose: scissors that cannot cut paper are legitimately called bad because they cannot fulfil their purpose. But a poisoner whose victim survives might say "I ought to have used more poison", so the "ought" has only been pushed back to the goal.
"Is" (descriptive)
- Says how things are
- Checked by observation or logic
"Ought" (prescriptive)
- Says how things should be
- The gap: it seems to need something more than facts
Others bring in promises, as Searle did, or point to "thick" concepts such as kindness and cruelty, which Bernard Williams said express a union of fact and value. Whether any of these truly closes the gap is still debated.
Step 1: Pick a goal
"In order for A to achieve B"
Step 2: Check the facts
What does reaching B require?
Step 3: "A ought to do C"
Verifiable, relative to the goal
Step 4: The catch
Even a poisoner has goals; is that a moral ought?
Quiz me
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Recap
Hume's guillotine says you cannot validly derive an "ought" from purely descriptive "is" statements, and the main replies add goals, purposes or promises.
💡 A trick to remember it · Facts describe the map; "ought" needs a destination.
Surprising fact · Hume's first book fell "dead-born from the press", yet its small "is/ought" paragraph is still debated.
Connects to
- 📜 Why did Kant say you must not lie even to a murderer at your door?
- 🎭 What rules would you pick for society if you didn't know who you'd be in it?
- ⚖️ Could you work out what's right by adding up everyone's pleasure and pain?
- 🧭 Could you ever have chosen otherwise?
- Virtue ethics
- David hume
- Naturalistic fallacy
Sources (3)
No source, no claim. Every fact in this lesson (17 claims) cites at least one of these.