Nero's palace, the Domus Aurea, lies under a hill in Rome, undiscovered for 500 years until the early Renaissance, when robbers broke through its domed ceiling to steal gold leaf.

Raphael and Michelangelo had themselves lowered by rope into the 300-room palace, and the designs and paintings they saw profoundly influenced their work. They had looked into the mind of ancient Rome and been awed.

Henry Perowne likes to imagine he looks into the mind of his patient each time he drills through the thick bone and slices into the dura protecting the kilogram of wet stuff that 'can make this bright inward cinema of thought'.

He knows that all he really sees is a roadmap in his own mind of places he dare not go, like the bad bits of New York, as he follows his microscope in search of a tiny tumour or blood clot or leaking vessel. But he feels confident - no matter the certainty of science eventually unlocking the brain's fundamental secrets - that the mind's mysteries will always be a wonder.

Saturday contains some elegant musings on the mind as Ian McEwan follows the neurosurgeon from awakening to exhausted collapse some 24 hours later. It is London on the day the city mobilised against Prime Minister Tony Blair's moves towards making war on Iraq, which runs as a motif through Saturday.

Perowne's opinion of Iraq has been formed by his treatment of an Iraqi professor of Sumerian civilisation's aneurysm. He saw the scars and learned of the torture that caused them. But he finds the carnival-like atmosphere of this protest against the horrors of war distasteful. Still, he tries to listen with earnest intent as his daughter, Daisy, harangues him for his antipathy towards those opposed to the war.

His day should, if all goes to plan, culminate in a celebratory dinner to mark the publication of his daughter's first book of poetry. Present will be his alcoholic father-in-law, a burned-out poet of no small fame, and his son, Theo, a jazz musician beginning to make his mark.

McEwan knows that life is a series of random events upon which we try to impose order. It never works because of the infinite variables at play. Perowne's day unravels after a vehicular tangle with a man whose external symptoms betray to the trained eye the onset of Huntington's disease, a terminal condition about which nothing can be done. As does this man, Baxter, who knows he has nothing to lose. Another variable kicks in.

There are consequences to actions, but some actions are beyond anyone's control, like the paranoia of Huntington's or the confusion of dementia or war with Iraq. McEwan has Perowne consider an aphorism of his son's: 'the bigger you think, the crappier it looks' but 'when I think closer in - you know, a girl I've just met ... or snowboarding next month, then it looks great. So this is going to be my motto - think small.'

McEwan seems often to look to the optimism of youth to sort through the cynicism that comes with life's experiences. In Saturday, he also offers reflections on why we do what we do.