it started with Thomas Aquinas treatment of homicidal self-defense. According this principle, its morally permissible to preform an action that has two effects.
1_the act, considered in itself and apart from its consequences, is good, or at least morally permissible.
2_The bad effect can’t be avoid if the good effect is to be achieved.
3_The bad effect isn’t the means of producing the good effect but only a side effect.
4_The criterion of proportionality is satisfied, in that the good effect and the bad effect are more or less equally balance in importance.