Science is still central to the book, especially the disciplines which Pinker calls "the sciences of mind, brain, genes and evolution" (cognitive science, neuroscience, behavioural genetics and evolutionary psychology). In early chapters, Pinker briefly summarises the main findings of these disciplines and shows how they have effectively undermined the idea that human nature is infinitely flexible.

This idea - the "blank slate" of the title - can be traced back to John Locke, the 17th century English philosopher who thought that the mind of the newborn child was like "white paper void of all characters, without any ideas". This view, known as empiricism, eventually triumphed over the older theory, going back to Plato, that the newborn mind was already richly structured.

The most famous piece of hardwired machinery is Chomsky's language-acquisition device, which Pinker described at length in The Language Instinct. Now, Pinker outlines many more special-purpose modules, from devices for understanding other minds, to modules for acquiring certain kinds of fears and food preferences and for recognising biological species. And this is just the tip of the iceberg. Judging by Pinker's list, the baby's mind is an exquisitely complex set of components.

But Pinker's main concern is no longer merely to expound and argue for particular scientific theories. This time, he is more interested in the social, political and philosophical implications of these theories. Pinker thinks that it is important to address these non-scientific issues because they often get in the way of a rational and clearheaded evaluation of the science.

He points out that many people tend to base their views about the truth or falsity of certain scientific theories not on the evidence for and against them, but on their supposed moral, political and philosophical consequences.

Such backwards reasoning is especially apparent when it comes to scientific theories about human nature. Take, for example, the idea that boys and girls are born with different mental aptitudes. For some feminists, this idea seems so politically dangerous that they are prepared to argue against it, whatever the evidence may be. Instead of starting out with an open mind and judging the idea according to the evidence, they proceed in the opposite direction. They want it to be false, and so conclude that it must be.

Pinker sets out to remedy the depressing tendency of many social scientists to engage in this particularly dishonest kind of wishful thinking. He does so by showing how many of the views that are commonly taken to have politically conservative consequences are, in fact, quite compatible with a politically correct, liberal democratic agenda. This whole argument, however, seems decidedly at odds with Pinker's contention that scientific theories must be judged purely on the factual evidence and not on their supposed political consequences. If the political and ethical consequences of a scientific theory are irrelevant to its truth-value, as Pinker claims at the start, there is simply no point in showing how you can be a politically correct evolutionary psychologist.

It would have been much more refreshing if, instead of trying to sweeten the pill of scientific truth by coating it with a smear of liberalism, Pinker had simply challenged his readers to swallow the pill of truth no matter how bitter it is.

Still, this is a minor quibble that should not detract from what is, by any measure, a remarkable and powerful book. Pinker's prose sweeps the reader along effortlessly, despite the complexity and sheer size of the intellectual territory he covers.

The question is: will those who have clung so obstinately to the defunct idea of the blank slate now be persuaded by the evidence against it, or will they continue to subscribe to ideas merely because they want them to be true?