The third condition is the one that does the real work, and it is the one most often lost when the doctrine is summarised as "intention matters." The question is not merely whether you wanted the harm, but whether the harm is how your plan works. If removing the harm would leave your plan intact, it is a side effect. If removing it would defeat your plan, it is a means — and the doctrine forbids it.
Aquinas set out the reasoning in discussing self-defence. A person defending themselves may intend to stop the attacker while foreseeing that the attacker may die; the death is not the goal, and the defence would be equally successful if the attacker survived. That is what makes it a side effect rather than a means, and Aquinas added that the force used must not be excessive — the proportionality requirement.
The doctrine explains the trolley problem's most stubborn feature. In the switch case, you divert the trolley to save five, and one person on the side track dies. If that person were somehow to step clear, your plan would still succeed — the death is a foreseen consequence, not the mechanism. In the footbridge case, you push a large man onto the track to stop the trolley. If he steps clear, nothing is saved. His being struck is your means. The doctrine says the first is permissible and the second is not, which matches what most people report feeling without being able to explain.
Where it mattersBeyond the thought experiment.
In medical ethics, the doctrine underlies the widely accepted distinction between administering pain relief in doses that may shorten life — where the intention is to relieve suffering and the shortening is foreseen — and administering a drug in order to end life. The physical outcome may be similar; the doctrine holds the intention and structure differ, and much law and professional guidance reflects that distinction.
In the ethics of war, it grounds the difference between targeting civilians and conducting a strike on a military target that will foreseeably kill civilians nearby. Critics note this is also where the doctrine is most open to abuse, since almost any consequence can be redescribed as unintended if one is willing to be creative about one's own purposes.
ObjectionsWhy it stays contested.
Closeness. The most technically serious problem. Suppose you need the large man to stop the trolley but do not, strictly, need him harmed — you need his mass in the path. Have you now intended only the blocking and merely foreseen the injury? If descriptions can be sliced that finely, the distinction becomes manipulable, and philosophers have not agreed on a principled way to fix the boundary.
Does intention affect permissibility? Judith Jarvis Thomson argued that intention bears on how we assess the agent — whether they acted from good or bad motives — but not on whether the act was permissible. On this view a doctor's private intentions cannot make an otherwise impermissible act permissible, which challenges the doctrine at its foundation.
Consequentialist rejection. If only outcomes matter, the distinction is beside the point: five deaths are worse than one however the one comes about. Defenders reply that this conclusion is itself the strongest evidence against pure consequentialism, since almost nobody is willing to accept that pushing the man is straightforwardly required.