The principle that says it is morally allowable to perform an act that has at least two effects, one good and one bad. It may be used under the following conditions: 1. the act to be done must be good in itself or at least morally indifferent; by the act to be done is meant the deed itself taken independently of its consequences; 2. the good effect must not be obtained by means of the evil effect; the evil must be only an incidental by-product and not an actual factor in the accomplishment of the good; 3. the evil effect must not be intended for itself but only permitted; all bad will must be excluded form the act; 4. there must be a proportionately grave reason for permitting the evil effect. At least the good and evil effects should be nearly equivalent. All four conditions must be fulfilled. If any one of them is not satisfied, the act is morally wrong.
An example of the lawful use of the double effect would be the commander of a submarine in wartime who torpedoes an armed merchant vessel of the enemy, although he foresees that several innocent children on board will be killed. All four required conditions are fulfilled: 1. he intends merely to lessen the power of the enemy by destroying an armed merchant ship. He does not wish to kill the innocent children; 2. his action of torpedoing the ship is not evil in itself; 3. the evil effect (the death of the children) is not the cause of the good effect (the lessening of the enemy's strength); 4. there is sufficient reason for permitting the evil effect to follow, and this reason is administering a damaging blow to those who are unjustly attacking his country.