The depths of the mind are quite close to the surface

In 1900, the three most important scientific entities of the 20th century were already unveiled. Max Planck gave us the quantum, Hugo de Vries, Carl Correns and Eric Tschermak each rediscovered Mendel's experiments with peas that would lead to our understanding of the gene, and in The Interpretation of Dreams Sigmund Freud outlined his theory of the unconscious. A striking coincidence.

As the century matured, the quantum and the gene began to help synthesise the sciences, but the unconscious - while impressing painters and poets - found fewer adherents in the hard sciences.

As knowledge about the quantum and the gene proliferated and deepened, generating elaborate technologies, notions of the unconscious remained shifting and vague. Each psychoanalyst - Freud, Carl Jung, Jacques Lacan, for example -offered his own version, while the technology generated, psychoanalysis, gradually attracted notoriety for its inefficacy and not a few abuses. This emphasised that the reality the unconscious purported to deal with was somewhat different - very different - from that shared by the other two concepts.

Consequently, a history of the unconscious runs the risk of being a narrative of something that does not exist. It is a brave man who would undertake such a task. Guy Claxton is not only brave but, with a double first in natural science from Cambridge and a doctorate in psychology from Oxford, he is a 21st-century universal man. He has produced a curious book, as notable for its gaps as for its contents.

The contents fall into two main sections. The first comprises a diligent survey of the many references in historical sources to "oddities" of human behaviour that, Claxton claims, anticipate the idea of the unconscious.

Thus he discusses the "invisible forces" that the ancients thought shaped our lives, as in Plato's tripartite model of the psyche, Augustine's idea of memory as a "vast, immeasurable sanctuary" or Pascal's view that "the heart has its reasons of which reason itself knows nothing".

The later sections consider a number of recent cognitive experiments - in subliminal perception, for example, implicit memory or blindsight - that Claxton presents as evidence for layers of consciousness. In one experiment, people who had been shown the word "ocean" were quicker at spotting the word "ship" than those who had not been "primed".

While I found the historical sections fascinating, I was not entirely convinced of their relevance. One could write a history of Prester John, the southern torrid zone or Paradise full of "intimate" allusions. This would not prove that these entities exist.

So far as the later section is concerned, one cannot doubt the experimental results as far as they go. Reaction times and perceptions can be altered by our being primed in some way. But do these influences last a lifetime, as Freud claimed? And are these experimental results any advance, in principle, on acknowledging that there are different mental states that we all accept, such as coma, sleep, forgetting? Implicit memory and subliminal perception are a long way from what most people understand by the Freudian unconscious.

Which brings us to the strangest and most important omission in the book - the author's failure to get to grips properly with Freud and the ferocious but convincing criticisms made of him in the past 15 years. Claxton partly covers himself by saying there is no need to go over ground covered in Henri Ellenberger's magisterial The Discovery of the Unconscious (1970) but that is less than satisfactory.

One of the points of Ellenberger's book was to show that, in matters such as childhood sexuality, repression or displacement activity, Freud was nowhere near as original as he claimed - these ideas were introduced in 19th-century Germany well before The Interpretation of Dreams appeared.

More recent scholarship has accused Freud of far worse crimes than unoriginality. Authors such as Frank Cioffi and Alan Esterson have proved that Freud suppressed inconvenient clinical details and committed other unscientific practices little short of fraud. This is the man whose concept of the unconscious was not a form of implicit memory but a motivating force throughout life, helping to determine character, and with its own associated pathology, which it was the business of psychoanalysis to uncover and treat. That there is no mention of Cioffi or Esterson in this book is unfortunate.

To be fair, Claxton is clearly uneasy about Freud. Instead, he offers us a version of the unconscious that, he implies, is a much smaller entity than the one Freud concocted. It accounts for oddities in human behaviour rather in the way that astrology is held by some to account for oddities in human behaviour -unpredictable in its manifestations and fitfully effective.

One of the problems is that Claxton himself shows a pathology of sorts - what we might call metaphorical myxomatosis. His book is so overpopulated, overrun with metaphorical descriptions of irrationality in the past that this seems to have rubbed off on him - he cannot resist the metaphor of depth, of things buried "deep" inside the mind.

What can this mean? Does information we have forgotten (repressed) lie at the geometrical centre of the brain, where the pineal gland is, say? We do not mean this, do we, not literally. Daniel Dennett has pointed out that when we "see" a tiger in our mind's "eye" we don't really "see" it in the sense that we can count the stripes. Half a century after Ludwig Wittgenstein and Gilbert Ryle, Claxton is still talking about the unconscious as "a region of the mind", and of the mind's "depths".

Until we break free of such language, we are going nowhere.

Peter Watson is the author of A Terrible Beauty: The People and Ideas that Shaped the Modern Mind . His latest book, Ideas: A History from Fire to Freud , will be published in May.