Historical Origins

In its traditional form, the PDE has four conditions:

1. The act to be done must be good in itself or at least indifferent.

2. The good intended must not be obtained by means of the evil effect.

3. The evil effect must not be intended for itself, but only permitted.

4. There must be proportionately grave reason for permitting the evil effect. (Fagothey)

Most trace the origins of this traditional four-part PDE and its two-part contemporary successor to St. Thomas Aquinas's (1224-1274) discussion of killing in self-defense (Aquinas; Mangan). Aquinas notes that the Christian tradition had, until his time, almost universally forbidden killing in self-defense. This prohibition probably stemmed from a teaching of St. Augustine (354-430) in De Libero that Christians should not kill others to save themselves because bodily life is that which "they ought to despise" (I, 5 PL 32, 1228). In his justification of killing in self-defense, Aquinas invoked what later became the essential conditions of the PDE. He argued that:

A single act may have two effects, of which only one is intended, while the other is incidental to that intention. But the way in which a moral act is to be classified depends on what is intended, not what goes beyond such an intention. Therefore, from the act of a person defending himself a twofold effect can follow: one, the saving of one's own life; the other the killing of the aggressor. (Ilallae, q.64, a.7)

Implicit here is the crucial distinction upon which the PDE depends, namely intention/foresight. An act of self-defense is classified as such provided that it is the saving of oneself and not the killing of the aggressor that is intended. If the killing was intended (intendere), and not merely foreseen (praeter intentionem), then, for Aquinas, the act would properly be classified as homicide.

It would seem that both conditions one and three of the traditional PDE might be elicited from this passage. Condition three forbids the intending of an evil effect for itself(as an end). Yet if acts are to be classified according to what is intended, then a violation of condition three (intending the evil as an end) will also be a violation of condition one (the act will be classified as bad in itself). Furthermore, condition two, that the good intended not be obtained by means of the evil, though not explicitly stated, can be understood as a plausible explication of conditions one and three as one who intends an end may also be taken as intending the means to his or her end.

Not to have intended evil is a necessary, but not sufficient, condition of justified self-defense for Aquinas. In the same section he offers a second condition:

An act that is properly motivated may, nevertheless, become vitiated if it is not proportionate to the end intended. And this is why somebody who uses more violence than is necessary to defend himself will be doing something wrong. (IIaIIae, q.64, a.7)

What became the fourth condition of the traditional PDE, the proportionality principle, can be elicited from this passage. Though it is not obvious from this passage, nor from the broader context of Aquinas's work, that proportionate is meant to refer to the measure of good and bad effects, later moralists interpreted the condition in this way.

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