We've got soul in our flashes of inspiration

A panorama of mental invention leads Mary Warnock to question if any step forward can be considered without a foot in the past

In a newspaper article published the day before his monumental Ideas appeared, Peter Watson complained that there have been no new ideas since the beginning of the 20th century. Contrary to common opinion, he argued, the world is changing only very slowly; today's grandparents and even great-grandparents already knew about most of the things that dominate modern life, at least in a primitive form: cars, aeroplanes, television, DNA, the subconscious, the theory of relativity, all were more or less familiar to them in their youth or soon thereafter. Watson concluded: "We need some new ideas badly."

As one of those great-grandparents, I would be inclined to disagree. When I reflect on the new possibilities in genetic medicine, to say nothing of the habits and the vocabulary of the text-message age, I frequently feel myself as lacking in conceptual grasp as an antique member of the genus Testudinidae. And this disagreement illustrates one of the questions raised by the grand plan of Ideas. What counts as a new idea? How do we recognise one when we see it? Watson is not interested in just any new idea. He aims to identify those that have changed the world.

At the beginning of human history, it is not difficult to identify seminal new ideas. No one could dispute the importance of the emergence of language, controversial though theories of its places and dates of origin may be. Equally clearly, the invention of alphabets and writing must be counted as a transforming new idea. The invention of the wheel, and of alloyed metal harder than any natural metal and therefore capable of being sharpened to a far greater degree, transformed farming and warfare. Even if such world-changing innovations took place in different places and at different times - though it is amazing how many of them have been plausibly ascribed to the Sumerians, inhabiting the southernmost parts of what is now Iraq, from about 3,400BC - it is easy to think of them as gradually developing ideas, not coming to any individual as a blinding revelation, yet possessing an internal coherence such that it makes sense to contrast the world before and after their coming into existence and being generally accepted.

The history of ideas at this distance looks like a steady progress, an inevitable development of man's competence and his understanding. But the further the history advances towards ourselves, when we are asked to measure time in decades rather than millennia or centuries, the more difficult it becomes to identify ideas. Are they the conscious product of one thoughtful person, philosopher or scientist, or are they to be ascribed to something such as the spirit of the age, where influences, unquestioned assumptions, deliberate revolutions, chosen policies and even lingering superstitions all may have their different parts to play in the production of a new style of thinking, which can be, more or less helpfully, labelled by future historians as, say, scholasticism, empiricism, romanticism or imperialism? Anyone concerned with the history of ideas (and in my view there is no subject so absorbing or so necessary for our understanding of the present and the future) must face such problems.

Watson is well aware of the difficulties, and he is especially conscious of the fact that civilisations and individuals may have a foot in two worlds, the forward and the backward-looking. Indeed, he begins his introductory chapter with an apt and illuminating quotation from John Maynard Keynes who, in a lecture delivered in 1942 after the study of some of Sir Isaac Newton's neglected papers, presented a hitherto wholly unknown side of Newton - not "the first and greatest of the modern age of scientists" but "the last of the magicians". This was the Newton who was passionately involved in the search for the philosopher's stone, and who believed that a plan of the layout of Solomon's temple would be the best guide to the topography of heaven. Thus even Newton's revolution in the physical sciences, often thought of as the beginning of the age of enlightenment, turns out to have been, after all, embedded in much older and more irrational ideas. Progress is not always steady, nor is it always easy to distinguish the permanently influential from the ephemeral or dotty.

Watson has not always avoided the inconsistencies of scale that these problems engender. The devil, as they say, is in the detail. A huge sweeping panorama of all invention and all thought must be backed up and rendered credible by evidence, geological, archaeological or literary. The difficulty is to decide how much of the evidence to include in the vast narrative, especially where it is open to conflicting interpretations.

Watson claims that most of the controversies are to be found, or at least referred to, in his endnotes, which need not be read by those who merely want to be swept along by the story. In fact, however, the endnotes are very short and, to be illuminating, would require the reader to look up the secondary sources quoted. Moreover, they are exceedingly difficult to consult, there being no referential page or even chapter numbers at the top of each page of notes. Thus, unless the reader is going through the book from cover to cover consulting every note as he goes with the help of a bookmark, he can get hopelessly lost. This is a grave fault in a book of this size that is perhaps more likely to be consulted topic by topic than it is to be read as a single narrative. But it is a fault perhaps better laid at the publisher's door than the author's.

Meanwhile, in the body of the text, readers, according to their own interests or expertise, may be irritated by the paucity of the evidence cited. In my own case, I have no quarrel with the general judgment that science and philosophy as free-thinking speculation had their origins in the sixth-century Ionian pre-Socratics, starting with Thales of Miletus (he of whom Aristotle said that no one expected a philosopher to be good about practical matters, and who was said always to be falling into wells); but the brisk summary of their main ideas seemed too superficial to be illuminating. Similarly, I agreed with the importance in the story of Plato and Aristotle, but wanted a fuller consideration of the degree to which the works of both philosophers developed over time, and the manner in which they, each in his different way, struggled with the vocabulary in which abstract thought could be expressed, the concepts of logic, grammar and metaphysics gradually emerging out of the down-to-earth and concrete contemporary Greek language. But then this is my private passion, and every reader will have his own, finding superficiality where another, less informed or less fanatical, will find illumination. This is the price that has to be paid for a work on such an ambitious scale, and the benefits of this scale are undeniable.

Whatever one's particular interests, it is enlightening to be made to think of the history of all religions, not only those of the West; to be reminded of the prodigious achievements, both theoretical and practical, of China in the 1st and 2nd centuries AD - including the introduction of written public examinations - and, of course, of the genius of those far more ancient Sumerians. (Many readers, however, may be disconcerted by the ascription to Coleridge, rather than Shelley, of the proposition that poets are the unacknowledged legislators of the world and may experience a momentary twinge of doubt. What other less easily detectable inaccuracies may lurk in the text?) Because of the devil in the details, it is perhaps inevitable that the most stimulating part of the book is the combination of its introduction and conclusion, where the power of the grand design can be allowed to have full sway, and can be seen to give coherence to the chapters in between.

In the introduction, after noticing how often historians of ideas have nominated trinities of ideas as central - including Francis Bacon, who, in 1620, argued that there were just three inventions "unknown to the ancients" that were central to the new scientific age, namely printing, gunpowder and the magnet - Watson proposes his own trinity: the experiment, Europe and the soul.

The conclusion is devoted to justifying what may at first seem a surprising choice in the light of what has gone before. Here all the threads are gathered together, and we learn not merely what Watson judges to be historically important but also what he values. He illustrates the central importance of the experiment in the history of ideas by tracing the development of concepts in physics through the work of the Cavendish Laboratory in Cambridge from its foundation in the late 19th century. He argues that the idea of experiment is linked to that of Europe (rather broadly interpreted to include America and Australia) since experiment is essentially democratic; it replaces authority, or rather it has its own authority, but an authority that is intelligible to and within the grasp of everyone equally.

Out of this democratic idea - exemplified above all in the great European universities - comes the idea of the individual, the soul of each person, an idea he believes to be both more valuable and wider than the idea of God, or religion. Indeed, because he holds that free intellectual development is fragile, and always under threat, he is inclined to fear any religion that may turn authoritarian or fundamentalist.

In the end, he expresses disappointment that despite the work of Freud, we have got so little way towards understanding the individual, the soul or self. Here, I suspect, he may be looking in the wrong direction, hoping for more from science than it could ever give. But that is another story.

Baroness Warnock taught philosophy at Oxford University until 1985 and was mistress of Girton College, Cambridge from 1985 to 1991.