Double effect helps to avoid mistakes in moral reasoning in difficult cases. There must be two effects: good and bad. The effects must be proportional.
What four conditions must be satisfied before an act is permissible?
1. The nature of the act - action must be morally good or indifferent. Acts such as lying or intentionally killing an innocent person are never morally permissible.2. The meansend condition - the bad effect must not be the means by which the good effect is achieved.3. The rightintention condition: intention must only be to achieve the good effect. Bad effect must be only an unintended side effect. 4. Proportionality condition: good effect must at least be equivalent in importance to the bad effect
He allowed that, if a man was starving, it would be acceptable to steal rather than let him die in hunger. This achieves the primary precept of preservation of life