The Principle Of Double Effect

In RulesAreMadeToBeBroken, RaySchneider says: "It all reminds me of the moral decision making principles taught by the good Jesuits ... ThePrincipleOfDoubleEffect is one of the most interesting and most misunderstood."

Can someone explain the rule?

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I'm a Dominican, not a Jesuit, but as I understand it the principle states that, If an act has both good and bad (foreseeable) consequences, it is permissible when the following hold: 1) the act is not itself bad; 2) the bad consequence is not intended; 3) the bad consequence is not the means of achieving the good consequence; and 4) the good consequence sufficiently outweighs the bad consequence.

Consider, for example, the question of whether I should hard-code an array size. This might have the good consequence of simplifying my code, or maybe making it a little faster; a bad consequence is the maintenance cost.

1. Hard-coding array sizes is not, in itself, bad, or else defining 3 dimensional points would be bad.
2. I wouldn't do it intending to increase maintenance cost.
3. My code doesn't become simpler because of the increased maintenance cost.
4. Is simple code worth the increased maintenance cost? Well, in the abstract, who can say? Here is where straight reason yields to prudence, and why RulesAreMadeToBeBroken.

-- TomKreitzberg