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The categorical imperative
1785 (Groundwork)· Idea or argument· Ethics & how to live
Well documentedSolid records, texts or physical evidence.
Kant's supreme moral rule: act only on principles you could will everyone to follow, and never treat people merely as tools.
See it on the Philosophy map →The idea
The idea: morality commands unconditionally, not 'if you want X, do Y.' 'Act only according to that maxim by which you can at the same time will that it should become a universal law.' Kant's example: borrowing money with a promise you won't keep. If everyone did that, no one would trust promises, so the plan defeats itself. Another version: treat humanity always as an end, never merely as a means. Main objection: rigidity; Kant forbade lying even to a murderer seeking your friend.
Why it matters
It grounds modern ideas of human dignity, consent and rights that cannot be traded away for better outcomes, and it is utilitarianism's chief rival.
Think of it like
Like a building code: it binds every owner equally, and no one gets an exception just because breaking it would be convenient this once.
Where it comes from
Set out in the Groundwork of the Metaphysics of Morals (1785) and Critique of Practical Reason (1788); the murderer case comes from his 1797 reply to Benjamin Constant.
Still debated
Whether the universal-law test gives clear answers in real cases, or lets cleverly worded bad rules pass and harmless ones fail.
Connections (4)
▸Immanuel Kant formulated The categorical imperativeWell documented
The connection
Kant sought a moral law binding on any rational being, resting on neither desires nor outcomes. Only a good will, acting from duty, is good without qualification. He gave the principle several forms: universal law, treating humanity as an end, and a 'kingdom of ends,' a community where everyone is both lawmaker and subject.
When and how
Königsberg, in the Groundwork of the Metaphysics of Morals (Riga, 1785), developed in the Critique of Practical Reason (1788) and Metaphysics of Morals (1797).
How we know
The texts themselves; students' notes of his ethics lectures, from the 1760s to the 1790s, show the idea taking shape.
Debates and open questions
Whether the several formulations are really equivalent, as Kant claimed, or express different moral ideas that can conflict.
▸The categorical imperative rivals Utilitarianism: the greatest happiness principleWell documented
The connection
These are modern ethics' two great rival answers. Kant: some acts are wrong whatever the outcome, and people must never be used merely as means. Utilitarians: the right act maximizes total well-being. Test case: lying to a murderer to save a friend. Kant forbids it; a utilitarian requires it. Mill attacked Kant directly in Utilitarianism (1863).
When and how
Kant's Groundwork (1785) and Bentham's Introduction (1789) appeared four years apart, independently; Mill's critique came in London in 1861–63.
How we know
Mill argues Kant's examples show only that universal immorality has bad consequences; a 2020 survey found philosophers split roughly in thirds between these two and virtue ethics.
Debates and open questions
Whether they truly conflict; Derek Parfit's On What Matters (2011) argued the best versions converge, their defenders 'climbing the same mountain on different sides.'
▸The categorical imperative inspired John Rawls, A Theory of JusticeWell documented
The connection
Kant held that moral rules must be ones every rational person could will, treating each person as an end, never merely as a means. Rawls built that respect into his theory: the veil of ignorance forces impartial choice, and he called the original position a 'procedural interpretation' of Kant's idea of autonomy.
When and how
A Theory of Justice (1971), section 40, 'The Kantian Interpretation of Justice as Fairness'; developed in his 1980 Dewey Lectures, 'Kantian Constructivism in Moral Theory'.
How we know
Rawls says so explicitly in section 40, and he taught Kant's ethics at Harvard for decades; those lectures were published as Lectures on the History of Moral Philosophy (2000).
Debates and open questions
Some Kant scholars say Rawls used a thinned-out Kant, swapping pure reason for rational choice behind the veil; Rawls later presented his theory as political rather than Kantian at root.
▸The categorical imperative is tested by The trolley problemMostly agreed
The connection
Kant's formula of humanity forbids using a person merely as a means. That seems to explain the trolley verdicts: diverting the trolley kills one as a side effect, while pushing the man off the footbridge uses his body as a tool. But Kantians struggle to explain why diverting is permitted at all.
When and how
Foot (1967) and Thomson (1976, 1985) set up the cases; nonconsequentialists such as Frances Kamm have analyzed their variants in detail since the 1990s.
How sure are we?
The means versus side-effect reading is standard in the literature, and Joshua Greene's brain-imaging studies linked refusals to push with duty-based, 'deontological' intuitions.
Debates and open questions
Kant never discussed such cases; whether his ethics actually permits switching the trolley, and whether any single principle explains all the variants, is debated.