People all over the world react with visceral horror to attacks on civilians by Al Quaeda, by Palestinian suicide bombers, by Basque or Chechen separatists, or by IRA militants. As there now seems to be a pause in the spate of suicide bombings and other terrorist acts--if only momentary--perhaps now is a moment to grapple with a fundamental question: What makes terrorist killings any more worthy of condemnation than other forms of murder?

The special opprobrium associated with the word "terrorism" must be understood as a condemnation of means, not ends. Of course, those who condemn terrorist attacks on civilians often also reject the ends that the attackers are trying to achieve. They think that a separate Basque state, or the withdrawal of US forces from the Middle East, for example, are not aims that anyone should be pursuing, let alone by violent means.

But the condemnation does not depend on rejecting the aims of the terrorists. The reaction to the attacks of September 11, 2001 on New York and Washington and their like underscores that such means are outrageous whatever the end; they should not be used to achieve even a good end--indeed, even if there is no other way to achieve it. The normal balancing of costs against benefits is not allowable here.

This claim is not as simple as it appears because it does not depend on a general moral principle forbidding all killing of non-combatants. Similarly, those who condemn terrorism as beyond the pale are usually not pacifists. They believe not only that it is all right to kill soldiers and bomb munitions depots in times of war, but that inflicting "collateral damage" on non-combatants it is sometimes unavoidable--and morally permissible.

But if that is permissible, why is it wrong to aim directly at non-combatants if killing them will have a good chance of inducing the enemy to cease hostilities, withdraw from occupied territory, or grant independence? Dying is bad, however one is killed. So why should a civilian death be acceptable if it occurs as a side-effect of combat that serves a worthy end, whereas a civilian death that is inflicted deliberately as a means to the same end is a terrorist outrage?

The distinction is not universally accepted--certainly not by the major belligerents in World War II. Hiroshima is the most famous example of terror bombing, but the Germans, the Japanese, and the British as well as the Americans deliberately slaughtered civilian non-combatants in large numbers. Today, however, terrorism inspires widespread revulsion, which in turn helps to justify military action against it. So it is essential that the reason for that revulsion become better understood.

The core moral idea is a prohibition against aiming at the death of a harmless person. Everyone is presumed to be inviolable in this way until he himself becomes a danger to others; so we are permitted to kill in self-defense, and to attack enemy combatants in war. But this is an exception to a general and strict requirement of respect for human life. So long as we are not doing any harm, no one may kill us just because it would be useful to do so. This minimal basic respect is owed to every individual, and it may not be violated even to achieve valuable long-term goals.

However there are some activities, including legitimate self-defense or warfare, that create an unavoidable risk of harm to innocent parties. This is true not only of violent military or police actions but also of peaceful projects like major construction in densely populated cities. In those cases, if the aim is important enough, the activity is not morally prohibited provided due care is taken to minimize the risk of harm to innocent parties, consistent with the achievement of the aim.

The moral point is that we are obliged to do our best to avoid or minimize civilian casualties in warfare, even if we know that we cannot avoid them completely. Those deaths do not violate the strictest protection of human life--that we may not aim to kill a harmless person. On the contrary, our aim is if possible to avoid such collateral deaths.

Of course, the victim ends up dead whether killed deliberately by a terrorist or regrettably as the side effect of an attack on a legitimate military target. But in our sense of what we are owed morally by our fellow human beings, there is a huge difference between these two acts, and the attitudes they express toward human life.

So long as it remains an effective means for weak parties to exert pressure on their more powerful enemies, terrorism cannot be expected to disappear. But we should hope nonetheless that the recognition of its special form of contempt for humanity will spread, rather than being lost as a result of its recent successes.