Anders Breivik pleaded not guilty on the opening day of his trial in Oslo. He admits the acts, but claims justification.

Mass killers don’t usually survive to stand trial; usually either they or the authorities (often both) are quite willing that they should end up dead. (France’s Mohammed Merah, killed a few weeks ago after a long siege in Toulouse, is a recent example.) Those that do survive, even if they are not actually barking mad, rarely have anything intelligible to say about their motives — recall Australia’s own Martin Bryant, who was found fit to stand trial after killing 35 people at Port Arthur.

So it’s rather a novelty to have an articulate killer in the dock able to explain himself. Two official psychiatric teams have come to conflicting views about his sanity, and the court will ultimately have to determine that question before sentencing him either to prison or to a psychiatric ward.

I’m no psychiatrist, so I have no idea whether Breivik meets any particular legal or clinical definition of insanity. But there is a sense in which he is clearly quite rational: his acts were well adapted to his goals, he carried them out calmly and efficiently, and he has been lucid in explaining them. Moreover, there is something disturbing in the idea of confining someone to a mental asylum for politically motivated acts; it brings back memories of some of the worst of Soviet practice.

It seems to me that we should listen to what Breivik has to say. One of the dangers of terrorism is its very inscrutability — we think of terrorists as beyond the reach of reason and therefore don’t pay attention to what makes them tick. Sometimes we’re told that it’s a sign of being “soft on terrorism” to even consider such things. But by ignoring terrorism’s motives we may miss the chance to prevent it happening again.

Breivik says — and there is no real reason to doubt him — that he considers himself at war. He told the presiding judge that he did not recognise the court’s authority, since it had “received [its] mandate from political parties which support multiculturalism”, and he had previously said that the massacre was “a preventative attack against state traitors”.

In short, Breivik has been told, by a raft of opinion leaders, that Islam and multiculturalism are a threat to Western civilisation, that left-wing politicians are not just mistaken but traitors to their societies, and that Norway and countries like it are facing an existential threat. And he believed them. He actually took their rhetoric seriously, and followed it through to its monstrous yet logical conclusion.

The same voices are widespread in Australia as well. As I said at the time: “The scariest thing about [the massacre] is how familiar Breivik’s rhetoric seems. His themes … are the common currency of right-wing pundits across most of the Western world.”

I’m not suggesting that those commentators are supporters of terrorism.

I have no doubt at all that Janet Albrechtsen, for example, was genuinely horrified at what Breivik did and has not the remotest intention of provoking such crimes. But she and others like her need to face up to the fact that an apparently rational man took those views as justification for murder on a horrific scale — and if one did, others might also.

That’s why we need to hear Breivik out, and we need to debate the implications of his views. Sweeping the whole issue under the carpet will only fan the flames of conspiracy theories — just as the United States might be a healthier society today if someone like David Koresh had had his day in court.

Norway is well suited to conduct this inquiry. Its reputation for tolerance and humane justice is the equal of anywhere in the world; Breivik will get a fair trial, no matter how traumatic it is for the survivors, because it’s necessary to demonstrate the nature of what it is that he’s attacking. As John McCain once said: ‘This isn’t about who they are. This is about who we are.”

And once we’re clear about where the angry rhetoric of anti-Muslim paranoia can lead, we might be less relaxed about our media and politicians who choose to give it a platform.