QuestionsWhat makes something right?Treat humanity always as an end, never merely as a means
What makes something right?Duty

Treat humanity always as an end, never merely as a means

The formula of humanity (categorical imperative)

Immanuel Kant 1724 · 1804Set the limits of reason; act from duty.
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What it claims

Act so as to treat humanity, whether in your own person or in that of anyone else, always at the same time as an end, and never merely as a means.

Why

Each of us sees ourselves as an end in itself, and every rational being does so for the same reason: all have absolute worth.

The same idea through history
Immanuel Kant1785Second formula of the categorical imperative, after that of universal law. Kant applies it to the same four examples: suicide, the false promise, talents left idle, refusing to help.
Immanuel Kant1785The false promise, already judged by the universal law test: with the formula of humanity, one sees at once that the lender is used as a mere tool.
Christine Korsgaard1986Christine Korsgaard reads the formula this way: deceiving or coercing someone means preventing them from being able to consent to what is done to them. That is the root of every wrong done to others.
In short

"Treat humanity always as an end, never merely as a means" is an argument by Immanuel Kant (Groundwork of the Metaphysics of Morals, 1785, AA IV 428-430, 1785) on the question "What makes something right?" (answer: duty). Its claim: Act so as to treat humanity, whether in your own person or in that of anyone else, always at the same time as an end, and never merely as a means. According to AskPhilosophy's logic check (Z3 solver), the conclusion does follow from its 3 premises. AskPhilosophy presents 3 objections, 2 of them with a documented reply, and the original text behind each premise, checked word for word.

Groundwork of the Metaphysics of Morals, 1785, AA IV 428-430
Valid

An object has value only because someone wants it; a person has value in itself. Kant draws from this one of the most famous sentences in philosophy: treat humanity, in oneself as in others, always also as an end and never merely as a means. We may make use of others (the baker, the bus driver), but never as mere tools.

P1Every human being necessarily thinks of their own existence as an end in itself, not as a mere tool serving something else.
P2Every other rational being thinks of its existence in the same way, for the same reason, which also holds for me.
P3 · implicit premiseWhat every rational being must think, for a reason valid for all, really holds for all: rational nature is therefore an end in itself for everyone, not only in its own eyes.
P4What is an end in itself has absolute worth: unlike things (beings without reason, which have only relative worth), a person must never be used merely as a means.
Therefore: act so as to treat humanity, whether in your own person or in that of anyone else, always at the same time as an end, and never merely as a means.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P3, one can grant that everyone values themselves in their own eyes, even for similar reasons, without this binding me to others: a clear-sighted egoist thinks exactly that.
The argument is valid: P1 and P2 give a reason everyone has to regard themselves as an end in itself, P3 makes it valid for all, P4 draws the ban on using someone as a mere tool. The weak point is P3: Kant himself says he states this step here as a postulate, proved later. Criticism also targets the meaning of "merely as a means" and the restriction to rational beings (P4).
Analogy. A hammer is for driving nails; when it breaks, we throw it away. An employee also provides a service, but we have no right to "throw him away" or deceive him: he has his own aims, which we must respect even as we benefit from his work.

Objections

3

Each objection is a conversation: click to see who replies and where the debate stands.

Derek Parfit 2011 · attacks the conclusion

What does "merely as a means" mean? We use others all the time. Parfit argues that treating someone merely as a means is neither necessary nor sufficient for acting wrongly: one can wrong someone without using them, and a person who sees others only as tools can perform perfectly permissible acts. So the formula does not, on its own, give the criterion of wrongdoing.

Kant replies
Kant 1785 · Kant's criterion (1785)
Kant himself gives the criterion with the example of the false promise: the person I want to use through this promise cannot possibly agree to my way of acting towards him. To treat someone merely as a means is to act in a way he could not consent to, and whose aim he could not share.
The debate goes on (our summary) Parfit discusses precisely this criterion of possible consent (in the preceding chapter of his book) and proposes to refine it: what matters is what everyone could rationally consent to. Kantians like Korsgaard see this as confirming Kant's idea; Parfit sees it as a step towards a theory reconciling Kant with consequences.
Jeremy Bentham 1789 · attacks premise 4

Why should the boundary of respect run through reason? A grown horse or dog is more rational than a newborn infant. The question is not: can they reason? nor: can they talk? but: can they suffer? An ethics that makes animals mere "things" resembles the laws that treated slaves as things.

Kant replies
Kant 1797
Cruelty to animals is contrary to duty, but to the duty towards oneself: it dulls in us compassion for their suffering, a disposition very useful to morality towards other people. Painful experiments made merely for speculation are to be abhorred. Gratitude towards an old horse or dog is indirectly part of our duties.
The debate goes on (our summary) Critics find this detour fragile: if torturing a dog is wrong only because it hardens us, then torturing a dog unseen and with no effect on us would not be wrong. Some present-day Kantians, like Korsgaard, try to show that Kant's ethics can recognise direct duties towards animals.
Joshua Greene 2008 · attacks the conclusion

In the trolley dilemma, almost everyone refuses to push a man off a footbridge to stop the trolley and save five people: that would be using him merely as a means. For the utilitarian, five lives count for more than one. Greene, relying on brain imaging, argues that this refusal comes mainly from an automatic emotional reaction, which Kantian theory merely justifies after the fact.

no reply
No reply possible: objection raised in 2008, after the author's death (1804).
The debate goes on (our summary) Many philosophers, including Selim Berker (2009), reply that knowing which part of the brain produces a judgement does not tell us whether the judgement is right: a moral argument is still needed to say which reactions mislead us. And Kantians recall that utilitarianism would also allow taking a passer-by's organs to save five patients.
What today's voices say
Criticize it
Peter SingerRejects the idea that it is always wrong to use someone as a means, and uses Parfit's example of the collapsing building.