There has been plenty published on the history of the 20th century, but little attention paid to the period's intellectual legacy. But that was before Peter Watson decided to take up the challenge.

When I tell people that my next book, A Terrible Beauty, is an intellectual history of the 20th century, there is invariably a sharp intake of breath. Then, if the conversation has not collapsed completely, when they have recovered from what often appears to be shock, the polite ones usually say something like: "That is an ambitious project." The more candid among them will pass the bottle and commiserate: "How can anyone claim to be an expert on everything?" Either way, they have a point.

I decided to write the book for the traditional reason: it was the kind of work I wanted to read and there is nothing like it on the shelves. There are any number of excellent histories of the 20th century but, for perfectly understandable reasons, they concentrate on military matters, the two world wars, on politics, decolonisation, the great depression and the cold war. It is an awful catalogue of catastrophes.

But one consequence of this set of priorities is that many of the more interesting things that happened in the past 100 years are overlooked entirely. For example, in the first volume of Sir Martin Gilbert's A History of the 20th Century , there is no mention of relativity, of Matisse or Rutherford, of Joyce or Proust, no W. E. B. Du Bois or Margaret Mead, no Oswald Spengler or Virginia Woolf, no Sinclair Lewis and, therefore, no Babbitt . Gilbert was taken to task for this by, among others, Richard Overy in The THES , for in leaving out such names one gets a skewed picture of the century.

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The century might have been catastrophic in many respects, but in intellectual affairs it was a triumph. But any attempt to cover the intellectual landscape of an entire century is, as those kinder souls put it, "ambitious". How does anyone dare go about it?

I cannot, and do not, claim any special insight or privileged vantage point, but having been a newspaperman for most of my life ( New Society, The Sunday Times, The Times, The Observer ) and armed with an adequate (if not exactly munificent) publisher's advance, one method suggested itself: I spent several months reading and then went on the road, visiting universities, research institutes and think-tanks, mainly in Western Europe and North America, with other visits to Russia, Israel and Latin America.

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This turned into a real pleasure, meeting writers, scientists, philosophers, sociologists, historians, economists, musicians, architects, film-makers and many other types of academic, who either feature in my book or are world authorities on this or that aspect of scholarship. During the course of these conversations, which on occasion lasted for three hours or more and sometimes led to protracted correspondence, I would ask my interlocutor what in their opinion were the three most important ideas in their field in the 20th century.

Some people provided five ideas while others plumped for just one, and if that happened I did not quibble. All seemed to enjoy the exercise and made a stab at answering the question seriously. But what fascinated me most was the measure of agreement I found across many disciplines. For example, in economics, three experts - two of them Nobel prizewinners - overlapped to the point where they suggested just four ideas between them, when they could have given nine. Much the same level of consensus was evident in chemistry, physics, biology, historiography, archaeology, anthropology, philosophy, music, painting - and yes, even in fiction (about trends though, rather than individual authors).

As my time on the road lengthened and it became clearer that the measure of agreement was greater than I had anticipated, I began to throw this observation into the discussion as well. Few scientists were surprised. The early part of the 20th century was characterised by the discovery of a number of fundamental entities - the gene, the quantum and the chemical bond. Once you think about it, if fundamental really does mean fundamental, a direct consequence of this, sooner or later, should be for the sciences to come together, to be seen as telling essentially the same story from different angles. And so agreement about what is important should coalesce naturally. This form of consensus is itself a measure of progress.

Through the discoveries of Niels Bohr, Linus Pauling and others, physics was joined to chemistry, later to astronomy, later still (through various dating techniques) to archaeology. Mathematics and physics have always been close, but in the latest work by Ian Stewart and his colleagues, even the physical structure of living objects and mathematical functions have been directly related in ways never envisaged before the 20th century. Meanwhile, genetics was linked to linguistics, to anthropology and all three (again) to archaeology. Chemistry and geology have come together, so have sociology and economics. Evolutionary psychology is the latest conjoining, of biology and psychology. This is the process E. O. Wilson calls "consilience", and it is an entirely natural evolution of the discovery of fundamental entities.

I found less agreement among the arts, and perhaps this was inevitable. Even so, the agreement was more marked than I had expected. In literature, postcolonial writers vie with American novelists and dramatists for the most virile use of English. The decline of British fiction was mentioned by most of those specialists I interviewed, in whatever country they happened to be. Latin American magical realism was on everyone's lips. I found many people uneasy with the collapse of the canon. Yes, there are those who rejoice in the "carnival" of postmodern multiculturalism, and there is no question that many cultures that were once lost have been recovered. But has this led to a new narrative history of how we came to be where we are? Has it introduced any new ideas by which we can live? Not yet.

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In philosophy, everyone - everyone I interviewed, that is - mentioned the divide between the analytic philosophy favoured in North America and Britain on the one hand and the more rhetorical, polemical continental school on the other.

This measure of agreement encouraged me. It meant that the book was do-able, that it ought to prove possible to produce a synthesis that would render the main strands of 20th-century thought coherent without doing too much violence to the facts. And I was not shouldering alone all the decision-making about what to include and what to ignore. The final decision was mine, but I was "borrowing" the authority of many experts.

The drawing together of the sciences is, I believe, the most important intellectual achievement of the 20th century. The one story the sciences tell - of the origin of the universe, of particles, of the elements, the solar system, the earth and the life forms and civilisations spread across the globe - is the greatest single achievement of the human brain. And the most exciting.

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There are those, such as the astronomer John Barrow, who argue that science may be approaching the limits of what it can explain and that consequently we may be entering a postscientific age. Perhaps. But that involves a complete change in the rules by which we have learned to think and is also for the future.

So far as the 20th century is concerned - the subject of A Terrible Beauty - the most important change in our way of thinking is best summed up by a story I heard about Max Planck. Planck came from a very well-heeled and cultured Prussian family, and he played the piano well enough that, had he wanted, he could have been a professional.

However, when he was asked as a young man (this was in the 1870s) what he wanted to do, he replied that he hoped to become a physicist. The family thought it was a joke. His cousin, the historian Max Lenz would punningly refer to scientists ( Naturforscher ) as foresters ( Naturforster ) - that is, hicks. No one would make that joke today, not with the same self-satisfaction anyway, and that fact is a measure of how radically our thinking has changed in a mere 100 years.

Peter Watson is research associate in the Illicit Antiquities Research Centre, at the McDonald Institute for Archaeological Research, Cambridge. A Terrible Beauty: The People and Ideas that Shaped the Modern Mind is published next month by Weidenfeld & Nicolson, £25.00.

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