Few moral theorists would disagree that the fundamental principle of morality – perhaps of practical rationality itself – is “ Do good and avoid evil. ” Yet along with such an uncontroversial principle comes a major question: Can you fulfi l both halves satisfactorily across your life as a moral agent? We all have opportunities to perform acts that do good with no accompanying evil, but these are not as common as we might think. We can avoid evil by doing nothing, but doing nothing implies doing no good either. Clearly the fundamental principle does not require that you go about your life doing good on any and every possible occasion any more than that you sit on your hands and abstain from action out of fear of doing evil. The principle tells us to avoid evil, not to refrain from ever causing it. And the simple fact is that the complexities of life make it inevitable that, much of the time when we go about doing good, we will also be doing evil. Further, they are such that sometimes we can avoid evil only at the cost of not performing a good act which reasonable people would regard as at least permissible, if not sometimes obligatory. So how, as rational, morally responsible agents, are we to satisfy the fundamental principle in an adequate, harmonious fashion, given life ’ s exigencies? This is where the so - called ‘ doctrine of double effect ’ comes into play. Some call it a doctrine, infl uenced by the fact that Catholic ethicists and moral theologians have, since the Middle Ages, codifi ed and ratifi ed it as something akin to a doctrine of Catholic moral philosophy. Others call it the ‘ principle of double effect, ’ though it breaks down into a set of principles unifi ed by a common idea. Yet other writers see it simply as a kind of reasoning about certain types of hard case in ethics. Whatever the preferred nomenclature, the DDE, as I will call it here, is – for all its critics and the diffi culties it faces – a keystone of sound moral thinking, without which the fundamental principle would remain nothing but a high ideal with little consistent applicability. So consider a simple example...