A theory built entirely out of constraints has a problem the moment two of them apply at once, and it has a worse problem when obeying one produces a catastrophe.
The previous lesson built Kant's theory and left it with three unpaid bills: it says nothing about degrees, it gives no way of ranking duties against each other, and it appears to forbid lying to a murderer. This lesson pays them, or tries. What emerges is a version of deontology most working philosophers actually hold, which is considerably less rigid than Kant's and correspondingly less tidy.
The murderer at the door
In 1797 Benjamin Constant published an essay attacking the doctrine that it is a duty to tell the truth, using a case he attributed to a German philosopher: a murderer asks you whether your friend, whom he intends to kill, is hiding in your house. Constant's point was that a duty of truthfulness held without exception would make society impossible.
Kant replied the same year, in a short and startling essay, "On a Supposed Right to Lie from Philanthropy", and he did not soften. Truthfulness in statements is an unconditional duty. To lie is to wrong humanity in general, because it undermines the reliability of declarations on which all contracts and rights depend, so a lie is a wrong even when it harms no particular person.
His argument for the practical verdict is more interesting than the doctrine. If you tell the truth and your friend is killed, you are not responsible: you did what the law required and the murderer did the rest. If you lie, you take the consequences upon yourself, and Kant supplies the scenario in which this bites. You say your friend is not at home, believing him upstairs. Unknown to you he has slipped out of the back door. The murderer leaves, meets him in the street, and kills him. Had you told the truth, the neighbours might have caught the murderer searching your house, and your friend would have lived. You cannot know how a lie will run, and by lying you have made yourself answerable for whatever follows.
The argument has a clear weak point, and it is worth naming precisely: exactly the same unpredictability attaches to telling the truth. Kant's asymmetry does not come from the epistemology at all; it comes from the prior claim that a lie is a violation of duty and a truth is not, so responsibility transfers only in one direction. As a defence of the doctrine it is therefore circular, and as an argument from consequences it is unsound.
Modern Kantians almost all reject the verdict while keeping the framework. Christine Korsgaard's 1986 treatment is the standard one. The formula of humanity forbids treating people in ways they could not consent to, and consent has already been made impossible here by the murderer, who has stepped outside the conditions under which the kingdom of ends operates. In an ideal world of rational agents, lying is never necessary and never permitted; in a world containing someone who is using you as an instrument for murder, the ideal principle does not straightforwardly apply, and the right response is the one that best approximates respect for everyone's agency, which is to lie. This is a two-level structure of a kind, and it concedes what Kant would not: that what a duty requires can depend on what other people are doing.
Example. State Kant's argument as premises and identify which line the modern Kantian rejects.
(1) Truthfulness in declarations is an unconditional duty. (2) Anyone who acts within their duty is not responsible for the harms that follow. (3) Anyone who departs from duty becomes responsible for whatever follows from the departure. (4) Therefore you should tell the truth, and are blameless if your friend dies. The modern Kantian rejects (1), and the interesting part is the ground. They do not reject it by appealing to consequences, which would abandon the theory. They reject it by arguing that the duty's content is fixed by what respect for rational agency requires, and that respecting the agency of a murderer bent on using you is not achieved by handing him accurate information. Notice that (2) and (3) can then be left standing, which is why the repair is a repair rather than a defection.
Now you. Does the same reasoning license lying to a friend to protect their feelings?
Answer
No, and the difference is exactly what makes the repair principled rather than a licence. The murderer has forfeited the standing to be dealt with truthfully by making himself an agent of coercion whose project requires using you; your friend has done nothing of the kind. The relevant question, on Korsgaard's account, is whether the person could consent to the way you are treating them if they understood it, and a friend who found out you had lied about her business plan would object, while a murderer who found out you had misdirected him has no complaint that respects anyone's agency. This is a genuine constraint on the repair, and a reader should test any proposed exception the same way: name what the other party has done to change what is owed to them, and check that the answer is not merely that the truth would be inconvenient.
Ross and pro tanto duties
The tidier solution abandons absoluteness at the outset. W. D. Ross, in The Right and the Good in 1930, argued that Kant's error was to treat duties as exceptionless when they are better understood as claims that can be outweighed.
Ross lists seven, presented as a provisional inventory rather than a derivation: fidelity, which covers promises and truthfulness; reparation, for wrongs one has done; gratitude, for benefits received; justice, in the distribution of goods according to merit; beneficence, improving others' condition; self-improvement; and non-maleficence, not harming others, which he singles out as generally weightier than beneficence.
The key term is prima facie, which Ross regretted and which most philosophers now render as pro tanto: a genuine moral consideration with real weight, which can be outweighed by another without disappearing. When you break a trivial promise to save a life, the promise does not evaporate. It is overridden, and the residue shows: you owe an explanation and an apology, which you would not owe if the duty had simply lapsed. That residue is the best evidence Ross has, and it is good evidence, because no theory on which the promise ceases to apply can account for it.
Example. A therapist's patient discloses a credible intention to seriously harm a named person. Which of Ross's duties are engaged, and what does his framework deliver?
Fidelity is engaged, and strongly: confidentiality is a promise, made explicitly at the start of treatment, and the whole practice depends on it. Non-maleficence is engaged in an unusual way, since the harm would be done by the patient rather than by the therapist, so what is really engaged is beneficence towards the third party, plus a duty of justice. Ross's framework says that fidelity is a real duty that does not disappear, that non-maleficence is generally the weightier kind of consideration, and that here the duty proper is to warn, because a serious threat to life outweighs a promise of confidence. It also says the residue is real: the therapist has broken a promise, owes the patient an account of why, and should have made the limits of confidentiality clear in advance, which is exactly what professional codes now require. The framework has not calculated anything, and it has organised the case correctly and identified what remains owed to the person whose claim was outweighed. Notice that the law reached the same place by a different route in Tarasoff in 1976, and that neither the court nor Ross could state a rule that would settle the next case.
Now you. A junior researcher discovers that a paper from their own laboratory contains fabricated data. Which duties conflict, and what does the residue look like?
Answer
Fidelity to colleagues and to an implicit undertaking of loyalty, gratitude towards a supervisor who trained them, justice towards everyone whose work or treatment will rest on the false result, and non-maleficence towards patients if it is clinical research. Ross's structure says the last two are weightier: the harm from a false result propagating is serious and falls on people who cannot protect themselves, while the claims of loyalty and gratitude, though real, are claims of a lighter kind and are in any case weakened by the fact that the colleague has done the wrong. So the duty proper is to report. The residue shows in what remains owed even so: raising it internally first if that is safe and effective, reporting the finding rather than denouncing the person, and not extending the accusation beyond what the evidence supports. A theory that simply said "report" and stopped would have missed all of that, and the residue is where most of the practical difficulty in real whistleblowing actually sits.
The objection is the obvious one. Ross gives no ranking and no method: when fidelity conflicts with non-maleficence, what settles it? His answer is that judgement settles it, that the conflict is a matter for perception rather than calculation, and that moral knowledge here is like knowing which of two propositions is more probable when no rule applies. Critics call this an admission of defeat. Sympathisers reply that the alternative theories only appear to have a method, since consequentialism's arithmetic runs on judgements about value that are no better grounded, and that a theory which is honest about where judgement enters is preferable to one that hides it. Either way, this is where most contemporary non-consequentialists actually stand.
Thresholds
Between Kant's absolutism and Ross's weighing sits a third position, threshold deontology, which most people hold without knowing it has a name.
The claim is that constraints are not merely weighty considerations but genuine prohibitions, which nonetheless give way when the stakes pass some very high threshold. Torturing one person is forbidden to save five lives, and permitted to save a city. Michael Walzer's "supreme emergency" doctrine, defending the bombing of German cities in 1940 and 1941 but not after, is a threshold view applied to war.
Two objections are standard. The first is arbitrariness: no one can say where the threshold sits, and there is no principle to derive it from. The second is sharper and structural. If the threshold is at 1,000 lives, then at 999 the act is absolutely forbidden and at 1,001 it is permitted, which makes an enormous moral difference turn on two deaths, when the same two deaths make no difference anywhere else on the scale. Attempts to smooth the discontinuity, by letting the constraint's strength scale with the stakes, tend to turn the view back into a weighted consequentialism where constraints are just heavy considerations, which is Ross.
Nobody has solved this. What can be said for the view is that it matches how almost everyone reasons, and that a theory whose competitors are absolutism and pure aggregation is entitled to some patience.
The doctrine of double effect
The most powerful tool in the non-consequentialist kit is a distinction not between doing and allowing but between what you intend and what you merely foresee.
The doctrine has its origin in Thomas Aquinas's discussion of killing in self-defence in the Summa Theologiae, where he observes that one act can have two effects, of which only one is intended, and that the killing of the assailant is outside the defender's intention. Its modern formulation sets four conditions. The act itself must not be wrong independently of its consequences. The bad effect must not be the means by which the good effect is achieved. The agent must intend the good effect and merely foresee the bad one. And there must be proportionality between the two.
The second condition is the one that does the discriminating, and the classic pair shows it. A pilot who bombs a munitions factory knowing that civilians nearby will die intends the destruction of the factory; the deaths are a foreseen side effect, and if the factory were somehow destroyed with no deaths the mission would have succeeded completely. A pilot who bombs a residential district in order to break civilian morale intends the deaths; if the civilians all survived, the mission would have failed. The outcomes may be numerically identical. The doctrine says the first can be permissible and the second cannot.
The same structure runs through medical ethics, where it is doing daily work rather than sitting in a seminar. Palliative sedation that hastens death while relieving pain is permitted, on the grounds that the relief is intended and the shortening foreseen, while administering the same drug in order to end the patient's life is not. Removing a cancerous uterus from a pregnant woman is permitted, while crushing the skull of a fetus to save the mother has traditionally been forbidden, because there the death is the means.
The objections are serious. Intentions are hard to identify and easy to redescribe, a difficulty sharpened by Jonathan Bennett's observation that the terror bomber can claim to intend only that the civilians appear dead for long enough to break morale, which seems to launder the intention without changing anything. There is also a question about why the agent's own mental state should determine what is permissible for them, when the victim is equally dead either way. And the doctrine's verdicts sometimes look like the answer arriving before the reasoning. Its defenders reply that intention is central to how we assess agents everywhere in law and life, that the redescription trick fails because the terror bomber's plan requires the deaths to be real, and that the doctrine's medical applications are precisely where a bright line is most valuable.
Example. Apply the four conditions to a doctor who gives a dying patient a morphine dose large enough to control pain, knowing it will probably shorten life by hours.
The act, administering an analgesic, is not wrong in itself, so the first condition is met. The pain relief is not achieved by means of the shortening of life; the drug relieves pain directly and the respiratory depression is a separate effect of the same dose, so the second is met. The doctor intends the relief and foresees the shortening, which the second condition supports rather than merely asserting, since a dose that relieved pain without depressing respiration would satisfy her completely. And a few hours of life against severe pain at the end of it is a proportionate trade, meeting the fourth. So the doctrine permits it, which is also the settled position of medical bodies and, in most jurisdictions, of the law. Change one thing, that the doctor's aim is to end the patient's life and the pain relief is incidental, and the same injection becomes impermissible on this doctrine, which is what makes euthanasia a separate legal question from palliative care.
Now you. In Thomson's loop variant, the trolley is diverted onto a side track that curves back to the main line, and it is only the body of the one man that stops it reaching the five. What does this do to the doctrine?
Answer
It puts real pressure on it. In the ordinary switch case the death of the one is a foreseen side effect: if he miraculously escaped, the five would still be saved. In the loop case his body is the means, since without it the trolley continues round and kills the five, so the death is intended in exactly the way the doctrine forbids, and it should be as impermissible as pushing the man off the bridge. Yet most people report the loop case as feeling like the switch case, not like the bridge. Thomson introduced the variant in 1985 to make trouble for the whole enterprise, and the trouble is real. Three responses exist: accept that the loop is impermissible and treat the common intuition as error; deny that the man's body is strictly the means, arguing that what stops the trolley is the mass and the death is incidental to it, which is the Bennett redescription problem in reverse; or conclude that intention is not the operative factor and look for another, which is what led Thomson and others towards accounts based on what is done to whom rather than on what is in the agent's head.
What the trolley literature establishes
The trolley cases have generated more work than any other thought experiment in ethics, and it is worth being clear about their status.
Foot introduced the first case in 1967, in a paper about abortion, to illustrate the difference between what an agent does and what an agent allows. Judith Jarvis Thomson developed it in 1976, replacing the driver with a bystander at a switch and adding the man on the footbridge, and in 1985 added the loop. The pair of switch and footbridge is the engine: identical numbers, near-universal disagreement in the verdicts, and the question of what makes the difference.
The empirical work confirms the pattern is real and widespread. Marc Hauser and colleagues, testing large online samples in 2007, found approval of diverting the trolley running around 89 percent and approval of pushing the man around 11 percent, with the gap holding across countries, religions and levels of education. In the same study, a majority of respondents could not produce a justification for their own pair of answers, which is a genuinely important finding: the judgements are stable and the reasons offered for them are not.
The 2018 Moral Machine study, which collected close to 40 million decisions from participants in 233 countries and territories on autonomous vehicle dilemmas, found broadly shared preferences for sparing more lives, humans over animals, and the young over the old, with substantial regional variation in how strongly each was held.
What none of this establishes is which answer is correct. A survey measures what people judge, and the previous lessons' warnings about framing effects apply in full. What it does establish is that the switch and footbridge verdicts are not artefacts of a few philosophers' intuitions, so a theory that treats them as identical owes an explanation of a very robust discrepancy, and a theory that separates them owes an account of the difference that survives the loop.
Where deontology stands
The honest summary is that the theory in its Kantian form does not survive contact with conflicts, and the repairs are all partial.
Ross gives up systematicity and keeps everything else, which is why he is popular and why his position is sometimes accused of being a list rather than a theory. Threshold views keep constraints and cannot locate the threshold. Double effect gives a real criterion and creaks under redescription and under the loop. None of this refutes the family: a theory with real problems can still be closer to the truth than an alternative with different problems, and the constraints these theories are trying to capture remain something consequentialism cannot deliver at all.
What both families share is a picture in which morality is a matter of getting the right verdict on an act. There is an older tradition that thinks this is the wrong question altogether, and asks instead what kind of person to be, on the grounds that most of moral life consists not in deciding hard cases but in noticing what is at stake and caring about it. That is the next lesson.