It was Freud who observed that after Copernicus, belief in a God who had created the world and kept it in his care was no longer tenable. Although, four-and-a-half centuries after the great astronomer's death, there are many millions who have still not heard the bad news, one sees what the Viennese magus meant. It is hard for us in our sceptical and hard-boiled age to appreciate what a shock to the belief-system of the 16th century was the announcement that the Earth does not sit supreme and immobile at the core of the universe, but is a satellite among other satellites of the sun. Suddenly, half-way through the second millennium, Man, the magnum miraculum, found himself unceremoniously pushed from the centre of creation.
Copernicus himself, a devout and cautious churchman, and most unrevolutionary of revolutionists, suppressed his masterwork, De Revolutionibus, for decades out of fear of what effect it would have on the minds, and souls, of men. Yet he was by no means the first watcher of the heavens to postulate a heliocentric universe; the idea had been around since the pre-Socratics. Even in his own time the hypothesis was not unique to him - great discoveries rarely are the work of individuals operating alone. Copernicus was, above all, a synthesiser, drawing together into a single work the free-floating ideas and speculations of many of his contemporaries.
Nor was he entirely accurate. In fact, the Copernican system does not work; it was not required to. Sufficient was it that the theory should, in the jargon of the day, "save the phenomena", that is, that the proposed model of planetary motion should match the observable progress of heavenly bodies, but not necessarily explain it. Thus, a measure of the novelty of Newton's approach is his ringing declaration, Hypotheses non fingo, by which he meant that he would not propose hypotheses, but keep strictly to the facts. By Newton's time, of course, what since the Greeks had been known as "natural philosophy" had - regrettably, some would say - turned into "physics".
There are a number of reasons why Copernicus's system would not work in the real, as distinct from hypothetical, world, one of the main ones being that he, like every other cosmologist before him, held rigidly to the belief that the planets move in perfect circles. How could they do otherwise, since they had been set in their orbits by a perfect God, who, by definition, must have worked by perfect geometry, and the only perfect geometrical shape for a planet to move in is a circle?
Here there swings into science's ken a new star. Johannes Kepler was one of the supreme minds of the Renaissance, despite being physically stunted, sickly, scrofulous and suffering from double vision - this last not such a drawback as it might seem for an astronomer of Kepler's day: Copernicus, for instance, in the course of his lifetime took no more than a handful of star-readings, while Kepler himself depended for his data on the work of Tycho Brahe, whom he worked with, contentiously, for a time, and whose precious star tables he was forced to pilfer after the great Dane had died. It was Kepler, the true revolutionary, who, faced with unavoidable evidence, recognised that the planets move not in circles, but in ellipses. Once that acknowledgment was made, the way was open for him to formulate his three laws of planetary motion, which were, in turn, the mainstay of the scaffolding around which Newton built his great system of the world.
Yet even Kepler was a man of his time. Profoundly devout, he believed that God had built the universe upon a grid made of the five perfect geometric solids - sphere, pyramid, cube, icosahedron, dodecahedron - which determined the orbits between the six planets that were all that were then known of the solar system. At the end of his life, when he had laid down the laws of planetary motion and opened up a new age of speculative science, he was still struggling to prove the theory of the five perfect solids, even stooping to massage the numbers so as to make them accommodate God's irreproachable planning. Galileo, the first technologist, recognised the significance of Kepler's work - without seeing the need to reply, even once, to the many enthusiastic, generous, and flattering letters that poor lonely Kepler wrote to him over the years.
Newton, too, acknowledged his scientific forebears, observing that he had been allowed to see so far only because he had stood on the shoulders of giants - though a recent biographer speculates, convincingly, that Newton intended that famous piece of faux-humility as a cruel gibe against his most dangerous rival, the physically stunted Robert Hooke - but Newton really was unique, possessing what was perhaps the greatest mathematical brain the world has ever known. His finest work was done before he was 30. It is not an exaggeration to say that he invented the modern world: from his work came the Industrial Revolution; indeed, the entire technology of the 18th and 19th centuries, and of the first third of our own century, was a direct result of his findings in the laws of motion and mechanics, while his theory of gravity was the foundation on which Einstein worked - the only picture on the wall of Einstein's study at Princeton was a reproduction of Kneller's portrait of Isaac Newton.
But Newton, too, was a believer. The Supreme Watchmaker that he postulated as the inventor and guardian of the universe was unmistakably the Christian God. Recognising that his system of the world required that there should be absolute space and absolute time, and knowing that there are not such things, he airily declared that they had their necessary existence in the "mind of God"; if ever there was a case of the Lord being bent to accomplish man's work, this was it.
Like Kepler before him, Newton was deeply interested in the occult underside of science. The last years of his long life he devoted to arcane explications of obscure passages of the Bible, and, more fascinating still, to work in alchemy - specifically, efforts to transform lead into gold. We may exclaim at such anachronisms, we may even chuckle over them, but we should also recall that much of 20th-century physics bears an uncanny resemblance to the search for the philosopher's stone. The world, as Lewis Wolpert never tires of telling us, does not operate according to what we call common sense; quantum physics flatly contradicts reality as we experience it, yet quantum mechanics works, and works spectacularly, as is evidenced in everything from microchips to atomic weapons. As the millennium limps to a close, and physicists come ever closer to formulating TOE - the Theory of Everything, simultaneously some of the most astute scientific minds are turning again to speculations on the existence of God, or at least of a God, that is, a controlling being of some supreme kind. Whether they are just indulging in self-deluding chiliasm, or turning to the Beyond out of despair of the Here, it is evidence that even if God is dead, we shall continue to insist on re-inventing Him.
John Banville is Chief Literary Critic and Associate Literary Editor of The Irish Times; his new novel, Eclipse, will be published next autumn