The judgement in the case of Brian Blackwell, the eighteen-year-old who killed his parents with a claw hammer and a knife, is incoherent and therefore unjust.

On the one hand it was declared that Blackwell is "suffering from narcissistic personality disorder", but then the judge went on to say that he had been guilty of "gross callousness" and that he is "an arch-deceiver, a resourceful liar and a wholly manipulative young man".

This judgement is absurd. It is impossible for a man to be both perpetrator and victim in the same action. Either Blackwell is afflicted by some dread mental illness which renders him incapable of ordering his behaviour, or else he is morally responsible for what he has done. There are arguments to support either judgement, but to argue for both is plainly ridiculous.

The Blackwell case is only the latest horrific example of the medicalisation of morals. Is a habitual thief excused if declared to be suffering from kleptomania? Is attention deficit disorder just a euphemism for the unruly little brat? Am I afflicted by an eating disorder, or just greedy? The truth is, of course, that in all these cases it is impossible to decide since the evidence is based entirely on behaviour: that is, there are no spots, rashes, lesions or tumours accompanying the socially unacceptable behaviour. A thief or murderer is thus defined as criminal or victim entirely on the grounds of prejudice, whim or ideological preference.

This is altogether unsatisfactory, for it dehumanises the person by, quite literally, demoralising his motives and actions. The practical consequences for society as a whole are catastrophic, entailing as they do the conclusion that no-one is to be praised or blamed for any action. If Blackwell's manslaughter can be described as caused by a disease, then any action by anyone anywhere can be described in similar terms.

What is being proclaimed in judgements like the Blackwell case is psychological determinism: the idea that all our actions are determined by deep hidden causes in our brains over which we have no control. It leads directly to the abolition of right and wrong - those moral judgements without which society cannot function. To say that Blackwell was somehow the "victim" when he killed his parents is truly bizarre. It reminds me of the old joke about the social worker who sees the man on the Jericho road beaten up by the thief and left half dead. The social worker looks at the dying man and says: "The person who did this needs help." Now it's too near the truth for us to laugh about.

Where will this kind of absurd reasoning lead? One day in the not too distant future, someone will commit a foul and horrible murder. He will be described as "a victim of murder". Well, never mind this talk of the future: something very like that has already happened in the Blackwell case, hasn't it? And everybody will be excused all his ill deeds - except of course those accused of crimes against politically-correct decrees. No-one, for example, will ever be excused his "racism" because he is alleged to be "suffering from racism syndrome".

* Peter Mullen is Rector of St Michael's, Cornhill, in the City of London, and Chaplain to the Stock Exchange.