One act, two effects: why intent changes everything

Aquinas argued a single act can have two effects: one you intend, one you merely foresee. Killing an attacker in self-defense is permissible because the death is a foreseen side effect of saving your life, not the means to it. This distinction still shapes just-war theory and medical ethics today.

— Thomas Aquinas, Summa Theologica — Summa Theologica, II-II, Question 64, Article 7, c. 1265-1274
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