Flesh in the Age of Reason By Roy Porter With a preface by Simon Schama Penguin pound;25

“Know yourself,” urged the Ancient Greeks’ Oracle at Delphi. But what is our self? We can see and feel our body, but is that all there is? And if it is all, what is it?

Societies have come up with a range of answers to this question, including: flesh corruptible and a soul bound for glory (religious); imagination and action (Romantic); a stream of consciousness and a set of sensations and perceptions (modern); a set of neurons conditioned by environmental experience firing in a biochemical sponge (materialist).

Perhaps there has never been such a rich melting pot of theories as existed in England in the 17th and 18th centuries, including the answers above and more besides. Roy Porter in, sadly, the last volume of his fascinating history of ideas, traces how philosophical disputes and revolutions filtered down into the public’s everyday understanding, so that while people at the start of the 17th century commonly thought of themselves as immortal souls tied to decaying bodies, by the end of the 18th, a man of fashion bewailed that his perfectible body was let down by the bad moods and vagaries of his soul. That was the poet Byron, the biggest celebrity of his day and a compulsive dieter as well as a razor-tongued satirist and all-round bad boy.

Ever-increasing numbers of us feel in 2003 as Byron did in 1808 - “much better” for losing a few pounds. He lost more than three stone, and was disgusted by the whole “horror of digestion”, so that he wished he could leave off eating altogether.

The intellectual ferment known as the Enlightenment, out of which Byron burst like a champagne cork, still informs many of the ideas we share about what our bodies are and what relation they have to our “selves”. As Porter shows, still earlier systems of thought, like layers in archaeology, condition the soil out of which our current notions grow and the language in which we speak of them. We might think we think about ourselves in terms derived solely from our own experience and the discoveries of modern science, but it isn’t so. The merest skip through the terrain so richly explored by Porter reveals all kinds of previous world-orders and casualties from philosophical battles in our everyday conversations and scenarios.

For instance, oddities of medical history have percolated into everyday speech. Idioms such as “a gut feeling” can be traced back to a Dutch 17th-century theorist called van Helmont, who accounted for disease and developmental processes by locating different “souls” in different parts of the body, with an emotional soul controlling the pit of the stomach. This was one of several theories developed in response to discoveries like that of William Hervey (the circulation of the blood), which overthrew a millennium-old model of the body as composed of humours. Hervey in turn could only make that discovery because Descartes had thrown a terminal wrench into the workings of the older system. Suddenly, the body was a machine carnis, a meaty machine, not a dynamic balance of humours.

Yet, though the model is defunct, the humours too survive in our speech and, maybe, our thinking. If any of the humours (choler or blood, bile or liver, melancholy or spleen, phlegm or lungs) were underperforming, there would be “dis-ease”, as we still say today. While we may scoff at humours we still talk of being “hot-blooded”, of “a rush of blood”, of “bilious” temper. When the body was a soup of humours, who or what was stirring the soup? Aristotle (in common with Jewish thinkers) had felt that the animating principle ceased with the rest of the organism. The thinking agent and the breath of life were, if not one and the same, at least coterminous.

Christians, though, had been promised paradise, so there had to be some part of the person that was immortal. A medieval synthesis offered a tripartite soul, mirroring the Holy Trinity, with separate bits to go to heaven, control the emotions and run the body. The world changed.

With Rene Descartes (1596-1690), as Porter shows, we leap into modernity.

After redrafting algebra and geometry, Descartes applied himself to human identity, working from first principles. Where to start? Why, with yourself. And what do you know for sure? That you, there, are thinking, Cogito, ergo sum. What else thinks? Nothing we can know for sure, all else are automata, clockwork. Soul not in charge. Modern medicine became possible. If you fix the meaty machine, where does the blood go?

Descartes himself, unconvincingly writing that the mind consisted of the workings of the brain but that the soul guided the body through the pineal gland, was a materialist. Yet dualistically splitting off the body from the soul, he stimulated opposing attitudes. Porter unravels this legacy in medicine, literature and science, from both sides of the mind-body fence.

It’s a debate that is still running.

A raft of writers and artists rushed to embrace the soul and find its essence. Samuel Johnson’s immense learning and renown were vitiated by his terror of death and eternal damnation; William Blake held an incandescent conviction that the body was the merest delusion: there was scarcely an important British writer from 1600-1800 who failed to consider whether flesh is essential to our sense of self.

Porter ends his tour de force with Byron, whose sense of self is mostly fleshly. The body, from being a deathly cloth that the soul sheds in pilgrim’s progress to heaven, dying to live, only to be reunited on the day of judgement, instead, for Byron, is the means by which the soul in death merges with the process of nature. Living to die, perhaps.

As an introduction to early modern thinking and the impact of past ideas upon present lives, this book can find few equals and no superiors. Porter is a witty, humane writer with an extraordinary vocabulary and a sparkling sense of fun. Whether he is quoting from obscure medical texts or analysing scabrous diaries, dishing the dirt on long-dead bigwigs or evoking sympathy for human suffering, his grasp is masterly and his erudition appealing. I wish I could read it again for the first time: you can.