What is it that is so appalling about acts of terrorism? The answer may seem obvious, but the question is worth dwelling on, because it takes us to the heart of humanist ethics. It is not just a matter of the number of people killed. The terrorist attacks on New York and Washington on 11th September stunned the world by their sheer scale, but it was not the scale of them that made them distinctively acts of terrorism. Other terrorist killings might take only one or two lives. What is distinctive of terrorism is that it is the deliberate killing or maiming of innocent people with the aim of spreading fear in the civilian population and thereby forcing governments to change their policies. 'Innocent' is a key word here which needs unpacking. To say, for instance, that the thousands of people killed in the World Trade Center were 'innocent' does not mean that they had all led morally impeccable lives. They were probably a typical cross-section of humanity: the good and the bad and everything between. They were engaged in many different kinds of work — financial dealers, administrators, secretaries, canteen workers, cleaners, plus the firemen and rescue workers who came on the scene and were then killed by the collapse of the towers. Some of these kinds of work you may think disreputable, others were ministering to human needs in a way to which no one could object. The point is that the attack took no account of any of this. The victims were killed simply because they were there. The attack was indiscriminate in the sense that it was not a response to who they were or what they were doing. They were not, at the time they were killed, a direct threat to anyone. They were simply lives to be taken, as a means of spreading fear.

Contrast this with, say, the classic case of killing in self-defence. Killing another human being is always in one sense a terrible thing to do, but in such a case, if the intended victim can preserve her own life only by killing her attacker, then it may be the necessary and the right thing to do. If the attacker is deliberately aiming at the life of the intended victim and is responsible for what he is doing, then killing him in self-defence is not the killing of an innocent person. It is the appropriate response to what he is doing. This contrasting case helps us to see in what sense indiscriminate terrorist killing is the killing of the innocent. Terrorist attacks are a violation of human dignity. That is a grandiose-sounding phrase, but in the present context it can be given a relatively precise sense. Human beings are not mere objects, to be used for the purposes of others. They are, as the philosophical vocabulary has it, 'persons', active beings with purposes of their own. They may do things which are terrible, they may do things which are wonderful, but to treat them in the light of what they themselves are doing — even in the case of killing the attacker — is to treat them as human beings. In the famous words of the philosopher Immanuel Kant, it is to treat them as ends in themselves and not simply as means to one's own ends. The philosophical vocabulary may sound puzzling and obscure, but I think it points to something which is essential to a humanist morality.

It is against that background that I now want to say something about the present so-called "War on Terrorism" and the bombing of Afghanistan. Some have said that the bombings are, in their turn, acts of terrorism, and as such are just the same as the attacks on New York and Washington. That is a simplification, but I think that it points to an important truth. The bombings do not precisely fit the definition of terrorism. I think we can allow that their purpose is not to induce fear in the civilian population as a deliberate means of undermining the ruling regime. We can allow that the intended targets are military ones and that the purpose is to destroy the regime's military infrastructure, as a prelude to the use of ground troops both against terrorist groups based in Afghanistan and in support of Afghan opposition forces. No doubt there are other purposes also in play. No doubt President Bush and Mr Blair, and their governments, are motivated also by a desire to demonstrate that they are not 'soft' on terrorism and that they are visibly 'doing something', and thus to appease the demand for some form of retaliation on the part of their electorates. But that does not by itself make the bombings acts of terrorism.

However, we also know that some at least of the military targets are in or near centres of population. Some of them, such as airports, can be used for either civil or military purposes. We know that civilians have been killed, and that more will be killed, as direct victims of the bombing. We also know that many civilians are fleeing the targeted areas, adding to what was already a huge refugee problem. Millions of Afghans were already, before the bombing started, facing starvation this winter as a result of the combination of years of drought and civil war. The bombings have not only added to the numbers who are displaced and cannot feed themselves, but have also made it much more difficult to get aid to those who need it. We do not know how many people will die, but we know that the bombing will directly or indirectly bring about the deaths of large numbers of innocent civilians.

Spokesmen who have defended the bombing have said that the civilian casualties are a regrettable but unavoidable side-effect of the attacks on military targets. There is a traditional moral distinction made between killing which is intended and killing which is foreseen but unintended, and the significance of this distinction is a matter of long-standing philosophical debate. How far will it take us in the present context? There are indeed cases where the distinction is highly pertinent. For example, the fourth airliner on September 11th was apparently brought down when some of the passengers decided to tackle the hijackers. They probably knew that the plane would crash, killing everyone on board. They did not of course intend that outcome. Their intention was to prevent the terrorists from hitting another target with the aircraft and causing many more deaths and vastly greater destruction, and they knew that the only way to prevent this was to do what would probably lead to their own deaths and those of their fellow-passengers. Deliberately to bring down an airliner with the intention of killing all the passengers would have been an appalling action. As it was, the attempt to overpower the hijackers, knowing the likely but unintended outcome, was an admirable and courageous action.

Sometimes, then, the distinction between intended consequences and foreseen but unintended consequences makes a big difference to the morality of an action. The distinction does not, however, give carte blanche to anyone to do what will lead to the deaths of any number of innocent people, provided only that the deaths are foreseen rather than intended. In the case of the bombing of Afghanistan, it is not good enough just to say that the civilian casualties are unintended and therefore have to be accepted. They could have been avoided, if the American and British governments had decided on a different course of action. They cannot be defended as the only way of achieving justice, or of preventing an even worse outcome.

We know what a genuinely discriminate action in this situation would have been. If there were evidence that some of those responsible for the terrorist actions were still alive, and if the evidence were good enough to be presented in a court of law, then an attempt could be made to capture the suspects and bring them to trial before an impartial international court. Admittedly it is difficult to see how this could be done without some use of military force to apprehend the suspects, but military action of that kind would be a direct and appropriate response to the acts of the terrorists. It may yet be done. It could have been done without killing innocent civilians.

Most military action in a war is less discriminate than that. If the armed forces of a country are defending it against an invading force, then their operations must be directed against the enemy forces quite generally. There is a sense in which this could be described as 'killing the innocent'. Most of the enemy forces are likely to bear no significant responsibility either for the decision to start the war, or for their own participation in it. Most of them are likely to be conscript soldiers, with little choice but to fight. Still, to fight against them is to respond to them as soldiers. If they are killed, they are at any rate killed in virtue of what they are doing. Though any killing in war is difficult to justify, ways of fighting which discriminate between combatants and non-combatants are not on a par with terrorism.

Bombing, however, is by its very nature indiscriminate. Some bombing is quite unequivocally terrorist bombing. The bombing of cities by both sides in the 1939-45 war, for instance, was deliberately intended to undermine the morale of the enemy by killing civilians. But even if bombing is aimed at military targets, it cannot be truly discriminate. 'Smart' bombs and 'smart' missiles are not that smart. They cannot distinguish between combatants and civilians. Even if they are successfully directed at a relatively precise military target, they are likely to kill everyone in or near the target. Some of them will miss their target and hit residential areas instead. In other cases targets will have been wrongly identified, and what was thought to be a terrorist camp, for instance, will turn out to be a village. To treat the resultant civilian casualties as 'regrettable but inevitable' is to regard innocent lives as expendable. It is not literally the same as terrorism, but it is the morality of the terrorist.