Ted Honderich: Security one week. Morality the next. It's enough to turn the stomach

It is cant to say Saddam is responsible for a war about to happen, when you are massing armies and about to attack

Tony Blair keeps telling us that he wouldn't be going to war if he didn't think it was moral. He wouldn't be doing it if his Christian conscience wasn't clear. He says he genuinely believes he's right. He's not doing it for ulterior or craven motives.

Well, I don't love him, but I never thought he was being consciously immoral. Did anybody? We didn't think he was being amoral either. We didn't think he was untouched by moral considerations, even if he does go on about national self-interest every once in a while. But the seeming needlessness and irrelevance of Mr Blair's disclosures about himself are not the main point.

The question is whether it is right to attack, invade and take Iraq. Whatever the right thing to do is, it can be done out of good, bad or indifferent personal motives. We all make mistakes. Saints can do the wrong thing and monsters the right thing. The First World War generals who let men go on dying in bloody latrines were more dim than evil.

Something else about the Prime Minister and his morality is unsettling. He has spent months campaigning for, and presumably thinking about, war against Iraq. We've heard a lot about terrorism, UN resolutions, weapons of mass destruction, and so on. Then, when there were going to be people marching in London one day, Mr Blair in Glasgow discovered there was a moral case for war – presumably about it being right. The moral case was about the probable, or possible, effects on Iraqis themselves of leaving Saddam in power. That raises a question: what did Mr Blair think he was doing before that day? Not trying to figure out the right thing to do?

Maybe he'll say now he was doing that all along, but didn't think of it in terms of morality. It won't be much of a reply. It will still seem that he is uncertain about what he has been, and is, involved in. Maybe uncertain of the fact that morality is absolutely inescapable. You can be as amoral or internationally realistic as you want, but it can't save you from moral judgement.

In any case, Mr Blair should have thought a little more about his moral case – that we can attack Iraq because, if we don't, Saddam will be free to do terrible things to his own people. That is just alarming. There is no parity between our doing something with the absolute certainty of killing and maiming thousands, and not doing it with only some probability, some chance, that some people will suffer as an indirect result.

This isn't all about Mr Blair as moralist. Does he have a grip on the nature of real reasons, including moral reasons? If you give something as a reason for attacking Iraq, what you do is point to a fact. Any such reason, by its very nature, is general. If you run into the same fact somewhere else, or have your nose rubbed in it, say with Israel and UN resolutions, you have the same reason for action there. If you say you haven't, then things follow as night follows day. Your fact isn't a reason with Iraq either. It can't be. If it was, it would be a reason with Israel.

One more thought about Mr Blair as moralist. If Iraq is attacked, it will not just be about fear of terrorism, let alone a clear and present danger. It will not just be about oil. It will not just be about our having the weapons of mass destruction to ourselves. It will not just be about American imperialism. Nothing, not even the lighting of a match and certainly not war, is the result of a single cause. To fail to see each of these reasons and to put each of them clearly, and to show how you weigh each of them, but instead to jump from one favoured item to another from week to week, is to fail in your obligation as a leader, maybe to fail culpably. It is to let down democracy.

It is cant to say the UN is in danger of destroying itself when you yourself are leading whoever you can against its legitimate practice and authority, in fact acting to weaken or undermine it. It is cant to say that it is Saddam who is responsible for a war about to happen, and you are not, when you are massing armies, condemning every concession as fraud, bribing poor governments to get votes for war, and are about to attack. Cant is no part of moral intelligence. It can turn the stomach. It should. And it does.

Ted Honderich is Grote Professor Emeritus of Philosophy, University College London