If there are acts you may not perform whatever the outcome, they have to come from somewhere other than the good they produce, and the most ambitious attempt to find that source starts from nothing but the requirement that your reasons hang together.
That is Immanuel Kant's project in the Groundwork of the Metaphysics of Morals of 1785, and it is worth taking seriously even by readers who end up rejecting it, because it is the only major theory that tries to derive substantive moral conclusions without assuming anything about what is valuable. The previous lesson left consequentialism unable to explain constraints. This lesson builds a theory that has nothing but constraints, and the one after it deals with the mess that creates.
Starting from the good will
Kant opens with a claim that sounds extravagant and is doing precise work. Nothing in the world, or out of it, can be called good without qualification except a good will.
The argument is by elimination. Intelligence, courage, wealth, and even happiness are good in ordinary circumstances and bad in the hands of a villain: a courageous and resourceful criminal is worse than a cowardly one, so those qualities cannot be unconditionally good. What is left is the quality of the willing itself. A good will is not good because of what it achieves, since achievement depends on luck and circumstance; it is good in itself, in the way a jewel is, shining by its own light even if it accomplishes nothing.
This yields an immediate and much misunderstood distinction. An act done in conformity with duty may be done for any reason: the shopkeeper who does not overcharge inexperienced customers may simply be protecting his reputation, and his honest dealing has no moral worth even though it is the right action. An act done from duty is done because it is right. Kant's hard case is the man who has lost all taste for life and preserves it anyway because he sees that he must; his act has moral worth precisely because no inclination supports it.
Kant is not saying that acting from inclination is wicked, nor that the good person should feel nothing. He is saying that if you want to isolate what moral worth consists in, you have to find a case where nothing else could explain the act, and the joyless man is that case. Friedrich Schiller's satirical couplet, that he gladly serves his friends but does so with pleasure and is therefore troubled that he is not virtuous, is funny and misses the point.
Hypothetical and categorical
Everything else follows from a distinction about the form of practical requirements.
A hypothetical imperative tells you what to do given an end you have: if you want to pass the exam, study. Its authority is borrowed from the end, and it evaporates if you drop the end. Nobody who abandons the exam is still bound to study.
A categorical imperative tells you what to do regardless of what you want. Its authority is not borrowed. Kant's claim is that moral requirements must have this form, because a requirement that lapsed as soon as you stopped wanting something would not be a moral requirement at all: "do not torture people if you want to be well thought of" is prudence, and if it were the whole story then the person who does not care about being well thought of has no reason not to torture. Philippa Foot challenged exactly this in a 1972 paper, arguing that morality might be a system of hypothetical imperatives after all, and that the appearance of categoricity comes from etiquette-like conventions of speech. That challenge is live, and the rest of Kant's system depends on it failing.
Given the distinction, Kant asks what content a categorical imperative could have. It cannot get its content from any end, since an end would make it hypothetical. All that is left is its form: that it be a law, meaning something universal. Hence the first formulation: act only in accordance with that maxim through which you can at the same time will that it become a universal law.
Working the test
A maxim is the principle you would be acting on, stated as an intention: in circumstances C, I will do A, in order to achieve E. The test asks you to imagine the maxim as a universal law of nature, holding for everyone always, and see whether something goes wrong.
Kant identifies two ways it can go wrong, and keeping them apart is the whole technical content of the theory.
A contradiction in conception occurs when the universalised maxim cannot even be coherently thought. Kant's example is false promising. The maxim is: when I need money, I will borrow it on a promise to repay that I have no intention of keeping. Universalise it. Everyone in need makes lying promises, so promises to repay carry no information about repayment, so nobody accepts them, so the institution of promising to repay does not exist, so the maxim cannot be acted on: there is no promise for me to make falsely. The maxim destroys the very practice it depends on for its success. This yields a perfect duty, one that admits no exceptions and no discretion.
A contradiction in the will occurs when the universalised maxim can be conceived without incoherence but cannot be willed by a rational agent. Kant's example is refusing all beneficence. The maxim is: I will never help anyone in need. A world in which nobody helps anyone is perfectly conceivable, and would function. But you cannot consistently will it, because you are a finite creature who will certainly need help, and in willing the universal law you would be willing away the assistance you cannot rationally renounce. This yields an imperfect duty, one that requires adopting an end, here the happiness of others, without specifying which acts discharge it, so there is latitude in how and when.
The four examples in the Groundwork fill out the grid: against suicide and against false promising as perfect duties, to develop one's talents and to help others as imperfect ones.
Example. Apply the test to riding a train without paying, on the maxim: when I can avoid the fare without being caught, I will.
Universalise it and ask which contradiction, if any, arises. If everyone who could evade the fare did, the fare system collects nothing from anyone able to evade it, the service cannot be funded on fares, and either it closes or it becomes free. Either way there is no fare to evade, so the maxim cannot be acted on in the universalised world: a contradiction in conception, and therefore a perfect duty not to do it. This is the structure Kant thinks all free riding has, and it is worth noticing how different the verdict's basis is from the consequentialist one. A consequentialist says one evaded fare costs the railway almost nothing, so the act is nearly harmless and may well be permissible. Kant says the act is impermissible whatever it costs, because what makes it wrong is that you are making an exception of yourself, helping yourself to a practice on terms you could not extend to everyone.
Now you. Now try the maxim: when I am short of money, I will sell my labour to whoever pays best. Does the test forbid it?
Answer
No, and seeing why shows what the test does and does not catch. Universalise it: everyone short of money sells their labour to the best payer. That world is conceivable and functions, indeed it describes a labour market, so there is no contradiction in conception. Can it be willed? Yes; nothing in a rational agent's ends is destroyed by it. So the maxim passes both tests and the act is permissible. The comparison with the fare case is instructive: what makes free riding fail is not that it involves self-interest but that it depends on others not doing the same. A useful shorthand is that the test detects parasitic maxims, ones whose success requires that they not be universal, and it is silent about maxims that are merely selfish.
What the test gets wrong
The universal law formula has two well-known failure modes, and any honest presentation includes them.
The first is false positives generated by irrelevant detail. Take the maxim: when I want to relax, I will play tennis at four on Thursday afternoons. If everyone played tennis at four on Thursday afternoons, there would be no courts free and nobody to sell the balls, so the maxim seems to fail the conception test, and tennis is forbidden. Something has gone wrong. The standard diagnosis is that the maxim has been described at the wrong level: it should read "when I want to relax, I will pursue a recreation", and the specific time is a circumstance of acting rather than part of the principle. The trouble is that Kant gives no rule for describing maxims at the right level, and without one the test can be made to deliver almost any verdict by adjusting the description. This is the standard objection, and it is serious.
The second is false negatives. The maxim "when someone threatens my life, I will kill them if I can" universalises without contradiction, and so does "when I encounter someone of a group I despise, I will refuse to trade with them". Universalisability catches parasitic maxims and misses maxims that are consistently and universally cruel. Kant needs a further formulation to reach those, and he supplies one.
Example. A shopkeeper acts on the maxim: when a customer cannot tell the difference, I will sell the inferior item at the price of the better one. Work the test, then state the maxim at a different level of description and see what changes.
Universalised, every seller substitutes the inferior item whenever a buyer cannot tell. Buyers then know that price carries no information about quality, so nobody pays the higher price, so there is no higher price to obtain by substitution: a contradiction in conception, and the act is forbidden. Now redescribe the maxim as "when trading, I will maximise my return", which universalises perfectly well and is permitted, and as "on Tuesdays I will sell tinned goods", which is trivially permitted. The three descriptions are all true of the same act, and only one of them delivers the right verdict. What separates them is that the middle one omits the feature the wrongness depends on, the exploitation of the buyer's ignorance, while the last adds features that are irrelevant. The working rule most Kantians use is that a maxim must include exactly the circumstances and the end that explain why the agent is doing it, and no more, which is workable in practice and is not a rule Kant gives.
Now you. Does the universal law test forbid a doctor from becoming a doctor if everyone becoming a doctor would leave nobody to grow food?
Answer
No, and this is the clearest case where the popular "what if everybody did it" test and Kant's test come apart. Kant's question is not what would happen if everyone did it, which is a question about consequences, but whether the maxim could be coherently thought or willed as a universal law. The maxim "when I have an aptitude for medicine and wish to be useful, I will train as a doctor" universalises without contradiction: a world in which everyone with an aptitude for medicine becomes a doctor is perfectly conceivable, and nothing in the maxim is destroyed by its own universality, because the maxim was never conditional on other people doing something different. Contrast the free rider, whose maxim explicitly requires that others pay while he does not. The distinction is the whole content of the conception test, and a reader who has it can dispose of most popular misuses of Kant in a sentence.
The formula of humanity
Kant claims the second formulation is equivalent to the first, which nobody quite believes, and it is the one most readers find compelling: act so that you use humanity, in your own person as well as in the person of any other, always at the same time as an end and never merely as a means.
The word doing the work is merely. You use the bus driver as a means to get across town, and there is nothing wrong with that. What is forbidden is treating a person purely as an instrument, in a way that bypasses their status as a rational agent with ends of their own.
Kant's underlying argument is that rational nature exists as an end in itself. Everything else in the world has value because someone values it, and so has a conditional worth; a rational being is the source of value rather than a repository of it, and cannot be assigned a price without absurdity. Hence the distinction between price and dignity: whatever has a price can be replaced by something equivalent, while whatever is above all price, and admits of no equivalent, has dignity.
The formula gives an operational test that has proved more durable than the universalisation one. To treat someone merely as a means is to act towards them in a way they could not, in principle, consent to, because you have concealed from them the very thing they would need to know in order to decide. Deception and coercion are the paradigms, and this is why both are so central to Kantian ethics: the deceived person cannot share your end because they do not know it, and the coerced person cannot share it because their agreement was not theirs to give. Notice how directly this answers the transplant case of the previous lesson. The surgeon uses the healthy man's body as a resource, on a plan he could not possibly consent to. The five patients' need does not enter, because the wrong is done to him and is not a quantity to be traded off.
Example. A researcher enrols patients in a trial without telling them that half will receive a placebo, reasoning that the knowledge would distort the results and that the trial will benefit thousands.
The formula of humanity forbids it, and the reasoning is exact. The patients are being used as instruments in a project whose actual nature is hidden from them, and the concealment is precisely of the fact they would need in order to decide whether to participate. They cannot share the researcher's end because they have not been told what it is. That the trial benefits thousands is irrelevant, because the wrong consists in what is done to these people and not in a balance of goods. Notice that this is not an objection to placebo trials as such: a trial in which participants are told they will be randomly assigned and may receive a placebo is entirely permissible, because they have consented to that arrangement in full knowledge, and the deception of the individual patient is replaced by a procedure they endorsed. Informed consent, which is now the governing principle of research ethics worldwide, is a direct institutional descendant of this formula.
Now you. A friend asks whether you like her new business plan. You think it is doomed, and you say it is promising, to spare her feelings. Which formulation does this offend, and how would each theory so far assess it?
Answer
The formula of humanity, most clearly. You have decided what she is fit to hear and managed her accordingly, which substitutes your judgement for hers about a matter that is hers to decide, and she could not consent to being handled that way. The universal law formula also condemns it, though less crisply, since a universal practice of reassuring lies would make reassurance uninformative and thus useless, which is the parasitic structure again. A consequentialist assessment depends entirely on the effects: if she will lose her savings, the kind lie is the harmful act and the theory condemns it too, which is worth noting because it shows the theories often converge and the interesting cases are where they do not. A virtue theorist, in a later lesson, asks a different question again: what a good friend does here, which is likely to be neither a comfortable lie nor a blunt verdict but a truthful answer delivered with care, and that reframing is the tradition's characteristic contribution.
Autonomy and the kingdom of ends
The third formulation gathers the others. Act as though you were through your maxims a lawgiving member of a kingdom of ends: a community of rational beings, each of whom legislates the moral law and each of whom is subject to it.
This is where Kant's account of freedom sits, and it connects directly to the previous course. For Kant, to be free is not to be uncaused; it is to act on a law you give yourself rather than being pushed around by inclinations you did not choose. Heteronomy, acting from desires that happen to be in you, is a kind of unfreedom even when nothing external constrains you, and autonomy is obedience to a law of your own making. The moral law and the free will turn out to be the same thing described twice.
Two consequences of this are worth carrying forward. Moral requirements apply equally to everyone, because they are derived from rational agency alone and every rational agent has that in the same degree, which grounds a strong doctrine of equal moral status with no reference to anyone's capacities or usefulness. And the theory is genuinely non-consequentialist all the way down: the reason not to use a person is not that a world with more using in it is worse, but that this person is not the kind of thing that may be used.
What the theory buys, and what it costs
Set against the previous lesson's scoreboard, the gains are large. Kantian ethics has constraints, and they are not bolted on: the wrongness of killing, deceiving and coercing follows from the same formula. It explains the intuition about the transplant surgeon exactly, and explains why the numbers do not help. It grounds equal moral status, and it delivers the whole modern apparatus of consent. It never demands that you sacrifice your projects to an aggregate, since your own humanity is an end too.
The costs are equally large and mostly show up as rigidity. The theory says nothing about degrees, so a small lie and a great betrayal both violate a perfect duty. It has no way to prioritise, so a duty of beneficence with no specification of how much yields little practical guidance. And it appears to forbid lying to a murderer at the door, which Kant, asked exactly that question, confirmed in print.
That last is where the tradition had to do repair work, and it is the subject of the next lesson: what happens when two duties both apply, whether any duty is genuinely absolute, and whether the distinction between what you intend and what you merely foresee can carry the weight it is asked to bear.