In the recent collection of essays entitled ‘Nuclear Weapons and Christian Conscience’, Elizabeth Anscombe asks how it is that Cathohcs have been able to accommodate themselves with so little strain of conscience to modern war; and the answer, in her own telling phrase, is ‘double think about double effect’. We all know what she means; but perhaps it is not always easy to see just where double effect thinking becomes ‘double think’.

The principle of double effect is in itself by no means so mysterious and subtle a doctrine as it is sometimes thought. It is no more in fact than a rational and systematic explanation of what goes on in our minds when we wonder whether or not to perform a certain action which we see will have both good and bad effects. Thus a man who sells sporting shotguns in a district where there is a danger of violence many wonder whether to carry on with his trade. If he does, he will be carrying on his normal and legitimate occupation, and supporting his family; but at the same time, he will be adding to the danger of human injury. How is he to make his decision? Catholic tradition would suggest four principles (though found with many variations) to help him make his choice. An act with two effects, one good and the other bad, is lawful, according to this tradition, on four conditions. First, the action, viewed in itself, must be good or at least indifferent; secondly, the agent must not intend the evil effect, but only the good; thirdly, the good effect must be produced as immediately as - that is not by means of - the bad; and, fourthly, there must be a sufficiently weighty reason for permitting the evil effect. Now if we apply the principles to the case of the man selling shotguns, they begin to look like ordinary common sense.