What Is the Doctrine of Double Effect?

Question

What is the doctrine of double effect, and why is it one of the most important — and most contested — principles in moral philosophy? The answer is that it draws a line that our moral intuitions seem to draw, and explains why: the line between what we intend and what we merely foresee.

Quick Answer

The doctrine of double effect is a principle of moral philosophy that distinguishes the intended effects of an action from its merely foreseen side effects. It holds that an action with both a good and a bad effect may be morally permissible even though the bad effect is foreseen, provided four conditions are met: first, the action itself must be good or indifferent; second, the good effect must be intended and the bad effect merely foreseen, not intended; third, the good effect must not be produced by means of the bad effect; fourth, there must be a proportionate reason for permitting the bad effect. The doctrine was formulated by Thomas Aquinas in the thirteenth century to explain the permissibility of killing in self-defense: the defender intends to save their own life, and the death of the attacker is foreseen but not intended. It remains central to medical ethics: the physician who gives morphine to relieve the pain of the dying patient, foreseeing that it may hasten death, acts permissibly — the death is not intended — while euthanasia, which intends the death as the means to the relief, is not.

Historical Wisdom

The doctrine was formulated by Thomas Aquinas in the Summa Theologica (c. 1270), in his discussion of killing in self-defense. Aquinas argued that an act can have two effects, only one of which is intended, and that the permissibility of the act is judged by the intended effect: the defender intends to preserve their own life, and the death of the attacker is the unintended — though foreseen — side effect. The doctrine was developed and refined over the following centuries in the Catholic moral tradition, and it was given its modern formulation in the twentieth century. In 1967, Philippa Foot introduced the trolley problem to test the doctrine's limits — the distinction between doing and allowing, intending and foreseeing — and the debate over the doctrine has been one of the most productive in modern moral philosophy.

Philosophical Perspectives

The doctrine rests on a distinction that our moral intuitions seem to track: the distinction between what we intend and what we merely foresee. We judge the bomber who targets a school more harshly than the bomber who targets a munitions factory foreseeing civilian deaths — the death is the same, but the intention differs. The doctrine explains the difference: the second bomber does not intend the deaths, and if the military value is proportionate, the action may be permissible. The doctrine also explains the difference between killing and letting die: the physician who withdraws life support lets the patient die (the death is foreseen, not intended as a means), while the physician who administers a lethal injection kills (the death is intended as the means to the relief). The distinction is the ground of the medical ethics of end-of-life care.

The critics of the doctrine make two objections. The first is the psychological objection: intention is not a clean category — the agent who foresees the bad effect and proceeds anyway can be said to intend it, and the clean separation of the intended from the foreseen is a fiction. The second is the moral objection: why should the same harm be permissible when foreseen and impermissible when intended? If the harm is wrong, the foresight of it is responsibility enough; if it is not wrong, the intention should not matter. The defenders reply that the distinction tracks a real moral difference — the difference between the person who aims at the harm and the person who merely accepts it — and that the doctrine protects the agent's relation to their own action: the physician who never intends the death of the patient is a different agent from the one who does.

Lessons From Thinkers

From Thomas Aquinas: the double effect — an act with two effects is judged by the intended one. From Philippa Foot: the trolley problem tests the doctrine's limits — the distinction between doing and allowing. From the tradition: the doctrine protects the agent's intention — the physician who never intends the death is a different agent from the one who does.

Practical Application

The doctrine of double effect has direct practical application in medicine and beyond. For the physician: the relief of pain is intended, the hastening of death is foreseen — and if the pain is severe and the dose proportionate, the action is permissible; the distinction protects palliative care from the charge of euthanasia. For the policy-maker: the doctrine structures the ethics of war — the distinction between targeting combatants and accepting civilian casualties as the side effect of a legitimate strike. For the ordinary person: the doctrine is the reminder that our intentions matter morally — the same harm is different when it is aimed at and when it is merely accepted, and the person who acts well keeps the intention clean. The framework does not make the hard cases easy, but it draws the line that our intuitions already draw, and it makes the line visible.

Quotes

Thomas Aquinas states the foundation: "Nothing hinders one act from having two effects, only one of which is intended, while the other is beside the intention." The tradition's summary: "The death that is foreseen but not intended is different from the death that is intended — killing is different from letting die." And the medical application: "The relief of pain is intended; the hastening of death is foreseen; the physician who never intends the death is a different agent from the one who does."

Related Questions

• What Is Medical Ethics? — where the doctrine does its most important work.
• What Is the Trolley Problem? — the test of the doctrine.
• What Is Moral Responsibility? — the ground of the distinction.
• What Is Deontology? — the ethical framework of the doctrine.