In the 17th century, everything people held true was swept away. Having the rug pulled from under you is a catalyst for shifts in culture, says Anthony Pagden

The American philosopher Richard Rorty once said that philosophy - and, by implication, all intellectual life - was "part of the conversation which we are". As with other conversations, this one involves a number of speakers.

Sometimes they agree with one another, sometimes they disagree, sometimes they all speak at once. Great intellectual movements usually begin when there are a great many discordant voices struggling to come to terms with immense external changes, even if those changes are not always immediately understood as such. As with all conversations, the directions in which they lead are generally more easily seen in retrospect.

The great transformations that have shaped Western intellectual life - the Renaissance, the Enlightenment, Modernism and Post-modernism - were all identified as such by those who lived through them. But it is only with hindsight that they have been viewed as distinct "movements" in the intellectual history of the West. All of these began as a response to external changes and increasing dissatisfaction with the intellectual tools available to explain them.

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Take, for instance, what is loosely called the Scientific Revolution or - the term I prefer because it was used at the time - the New Philosophy of the 17th century. This began when perceptive Europeans came to realise that the two major upheavals of the 16th century had robbed them of certainty.

The Reformation and the wars of religion that racked Europe from the mid-16th century until the 1640s had destroyed the religious authority of the Church, and the discovery of America had undermined traditional accounts of human nature. If man could disagree so wildly over the meaning of God's words, and if powerful societies could develop that diverged so far from what in Europe was held to be natural, then perhaps, as French essayist Michel de Montaigne suggested, there were no universal truths, only local custom. Poet John Donne expressed it like this: "'Tis all in pieces, all coherence gone; / All just supply, and all Relation: / Prince, Subject, Father, Son, are things forgot." And if that were the case, what basis could there be for any kind of knowledge?

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Such despair led to the revival of an ancient philosophical doctrine that had lain dormant for centuries - scepticism. The moderate sceptic believes that he has no grounds for understanding but his own senses. In such a world, the philosopher has to discard prior knowledge, has to begin only with the raw material - the world itself - that lies before him. From this came a belief that the only kind of science had to be based on experiment, that the only kind of philosophy had to be one, as John Locke argued, that began with an account of the human sensations or, as Rene Descartes claimed, with the simple irrefutable fact of thinking itself: cogito ergo sum - I think therefore I am.

By the middle of the 17th century, all the old certainties and all the old sciences had in effect been swept away. In particular, a view of the world that relied heavily on theology - or what was disparagingly referred to as "scholasticism" - had gone for good. People still believed that a divinity had created the universe, but they now thought the world, human and natural, was an independent entity that could be understood only by studying it.

This revolution had come about as a consequence of an extended conversation across the whole of Europe: Thomas Hobbes talked to Descartes, as did Locke; Descartes talked to Galileo, as did Hobbes; and a host of less well-known thinkers sent letters back and forth across the continent. The movement their conversation created, despite its immense complexity, had begun as an attempt to find the answer to a single question: what is knowledge? Although that question had been asked over and over in preceding centuries, it was only when all the old methods of reaching an answer seemed to have been exhausted that it became obvious that a radical new set of philosophical methods would have to be devised. A new intellectual movement had begun.

Like all real conversations, every intellectual movement is a continuous process. The New Philosophy of the 17th century, and the revolution in scientific thinking that accompanied it, would not have been possible without the changes in moral and social thinking associated with the humanism of the 15th century. This had been the period that had witnessed not only the rediscovery of the sceptical writings that had inspired Montaigne and his successors, but also the creation of a more anthropocentric view of the universe, a view of man as the measure of all things.

The New Philosophy also paved the way for the Enlightenment. By the end of the 17th century, the religious wars were at an end and the shock of new worlds had greatly diminished. The scepticism and rationalism that had inspired the New Philosophy began to seem overly reductionist. Jean d'Alembert, along with Denis Diderot, was the creator of the most characteristic Enlightenment project, the ·¡²Ô³¦²â³¦±ô´Ç±è¨¦»å¾±±ð . He said that once the "yoke of scholasticism" had been lifted by Descartes and his followers, the way was open for a world in which all mankind would govern its actions by reason alone.

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In the bleak world of the 17th century, it had made sense to argue that humans stayed together and behaved decently towards one another only out of fear and rational self-interest - what David Hume called "the selfish philosophy". What the Enlightenment added to the rationalism of its predecessors was the sense that humans shared not only reason but also a common identity that reached beyond calculation. Humans were not merely selfish, they were also compassionate and caring. And they were moral, not, as theologians had insisted, because God had ordered them to be so but because a moral disposition was a part of their being.

The great thinkers of the Enlightenment secularised the Christian God. In so doing they demolished the claims of family, king and country, all those hallowed customs, and God-made laws by which humans had governed their lives. As the greatest of the 18th-century philosophers, Kant, put it: "To criticism everything must submit. Religion through its sanctity, and law-giving through its majesty, may seek to exempt themselves from it. But they then awaken just suspicion and cannot claim that sincere respect which reason accords only to that which has been able to sustain the test of free and open examination." The allegiance of all humans could now be only to humanity itself.

Similar moments, with similar interplay between external circumstances and the struggle to make sense of them, are found at the origins of almost every subsequent intellectual movement. Romanticism was a turning away from abstractions and from the cosmopolitanism and universalism of the Enlightenment in response to the nationalism that grew out of the French Revolution and the Napoleonic Wars. Once again, country, kin and custom became the focus of human life. Humanity was not an abstraction. Humans existed only as members of different nations or peoples, and each one of these was different. "Frenchmen, Italians, Russians etc. I know," wrote the embittered French conservative Joseph de Maistre, "I know, too, thanks to Montesquieu, that one can be a Persian. But as for man, I declare I have never met him in my life."

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Similarly, logical positivism and Freudian psychology in the early 20th century can be read as attempts to find deeply buried rational codes of meaning and conduct beneath the apparent disorder created by the First World War. Post-modernism, most of whose founders have lived through the horrors of the Second World War, is in part a rejection of the possibility of rational understanding when faced with anything so inexplicable as the Holocaust. Human history is reducible to little more than our own self-descriptions, or, in Derrida's phrase, "there is nothing beyond the text".

And since nothing can exist in a void, and a wholly new intellectual movement would be a wholly unintelligible one, each of these has drawn heavily on the past. Romanticism drew on Scottish and Germanic folk tales as a way of capturing what it imagined to be the primitive heart of a nation. Twentieth-century rationalism revived some of the claims of the 17th-century New Philosophy. Post-modernism owes conflicting debts to both Kant and Hegel, and every age has unashamedly plundered the classical past.

Now, as we move into a new century with its own share of conflicts, I sense that the fascination with language and the insistence on the unreality of the world that has come to be called Post-modernism is fading.

In its place a new scientism is on the rise. Cognitive science, once linked to sociobiology and stridently condemned as an intolerably inhuman form of reductionism, is slowly coming back. A new conversation, and with it a new intellectual movement, is about to begin - perhaps.

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Anthony Pagden is professor of political science and history, University of California, Los Angeles, US.