‘His brain just broke.’ These were the words of an American defence lawyer in a recent trial involving violent rape. The argument was that undiagnosed paranoid schizophrenia left the defendant unable to form intent. Whatever the verdict in this particular case, the burgeoning field of ‘neuro-law’ means that we will be hearing more defence pleas that boil down to a claim that ‘my brain made me do it’. Here are a few observations for anyone inclined to blame somebody’s brain for his or her behaviour.

The brain is usually blamed for actions that attract moral disapprobation or legal sanction. People do not normally deny responsibility for good or neutral actions such as pouring out a cup of tea. This is a pick ‘n’ mix approach to human action and intent, and grounds, I would say, for treating the ‘my brain made me do it’ plea of mitigation with some suspicion.

There are events that clearly owe their origin to the brain alone – for example, the twitching associated with an epileptic fit. And then there are actions that do not begin and end with the brain. While someone is clearly not responsible for having epilepsy, can the same be said if that person drives illegally, against medical advice and causes a fatal crash?

My-brain-made-me-do-it as a defence sucks out the agency in our actions, turning us into little more than a bundle of synapses and nerve impulses: our nervous system determines us absolutely. In the end there is no agency, and no toe-hold for the first-person (or indeed any person) viewpoint. The brain is ownerless – it is no-one’s brain – and the self that appropriates it vanishes into a boundless, personless, net of causes and processes.

But why stop at the brain? Since the brain is causally wired into nature at large, ‘my brain made me do it’ actually means that ultimately ‘the Big Bang’ made me do it. Neuro-determinism quickly slides into determinism tout court. It is one long chain of knee-jerk responses undeflected by agency, from the Big Bang to the Big Crunch. Individual responsibility is lost in this. Why should a brain make me do anything? Why should this impersonal bit of matter single out me? It is as if I am trying to have a particular brain as both first-person – as my brain – and third-person – as a material cause with all the innocence of material causes.

The neuro-mitigation of blame, therefore, has to be used critically and treated largely with suspicion except in those instances where there is clear evidence of grossly abnormal brain function or abnormal mental function due to clear-cut illness which may have its origin in brain disease. We must temper mercy with scepticism if we are not going to allow our image of humanity to be effaced in a tidal wave of determinism. ‘I did it’ may sometimes be hard to admit. But it is far more human than saying ‘my brain made me do it’.

Click here to read SImon Myerson's response: The courts are not keeping pace with science

A Battle of Ideas debate on "My brain made me do it" will take place on Sunday, October 28 at 14.00

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Professor Raymond Tallis is professor of medicine at the University of Manchester and author of The Hand: A Philosophical Inquiry into Human Being