Thomas Aquinas’ Theory of Double-Effect

The theory of double-effect is a development from Aquinas’ justification of killing in self-defense: ST II-II, 64, 7).

  1. One may never intend intrinsically evil actions, even for a good end (anti-Utilitarian).
  2. A good action, or at least a morally neutral action, can have two effects: a good intended, and an evil, not intended, but tolerated.
  3. Proportionality: the good to be achieved must outweigh the evil tolerated; one cannot allow significant evil effects for a trivial reason.

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