We all try to make order out of chaos. In the earliest days of the human race our ancestors saw powers beyond their understanding and gave them names. Seasons, thunderstorms, waterfalls, volcanoes - even trees and rocks - all graced primitive pantheons. Perhaps they thought that by giving them names, they might learn how to avoid their wrath and even, by propitiation, influence them in their favour.

Beyond these early fetishisms there evolved the first religions and sciences of humankind - for most of human history there was no distinction between these two things. False though their analogies were, odd and disturbing as their conclusions and beliefs became, particularly in neolithic times, they continued to reflect the same fundamental aim, to engage the gods, the powers of the world, and to persuade them to intercede favourably in their lives.

These were the folk who built Stonehenge and the ancient empires of South America. They did strange, tremendous things, killing their own people in sacrifices, believing even that the shadow of night was a real and physical door to Hell.

Many of these societies were, to our modern mind, not wholly sane at all. Their people, their priests and kings, saw the world very differently to us.

Yet their quest was the same - to understand what was going on, to know and influence the big things that happened in their world.

And then, about twenty-five centuries ago, an odd thing happened. A small, loose federation of city-states in south-eastern Europe found itself relatively secure from outside attack for a century or two. This had happened before, but in this society the leisure of its upper classes was spent in the strange pursuit of exploring thought itself.

The Greeks laid down the bases for logical thought. The eternal syllogistic theorems, as valid today as they were then, were discovered. Fundamental geometrical and algebraic concepts were defined. Empirical experiments were undertaken, such as correctly measuring the diameter of the Earth - using a stick.

Yet it was still only a spark in the darkness, surrounded by great Mysteries. The Pythagorean brotherhood made a religion of mathematics, holding as sacred and secret from non-initiates the existence of the fifth perfect solid, the dodecahedron.

Yet because they had practical applications, even though the Greeks themselves soon fell into obscurity, their ideas did not. Neither did the idea from long, long before, that the mysteries of the world could be read with the right key, the right mental tools, the right names ... the right something.

By the sixteenth century the world was changing. The long digression of the Dark Ages was at last giving way to the Renaissance. In a new way, a German scholar tried again to read God in the stars.

Johannes Kepler was exceptional in many ways. First, he was a brilliant mathematician, one of the greatest who ever lived. Secondly, he was a humble and earnest man, whose faith and belief in God never faltered. Finally, he was dogged and persistent. He kept following his dream where all others would have given up.

He lived at a time when the debate about the Earth's position at the centre of the universe was heating up. He did the maths and it was clear to him that the Sun was at the centre. The motions of the planets made this evident even on a first approximation. Not only this, he decided that he knew why the planets were at their respective distances from the Sun. Assuming their orbits were perfect circles, which had been taken for granted since Ptolemy's day in the second century AD, you could fit one of the Pythagorean perfect solids exactly in between the orbits of each of the successive planets as you moved out from the Sun.

It was a revelation to him, and he spent years on his proof of it. The key word was exactly. He was convinced he had found the hand of God in the world. For real proof he knew he needed better data, and the man who had it was Tycho Brahe. Kepler put up with Brahe's arrogance for years to get it. Finally, after Brahe died, he got it, and set to work on his final calculations.

And here it gets really interesting, because of course he was completely and totally wrong in what he set out to prove. Yet he was such an honest and rigorous scientist, so modern in his respect for observation and facts, that fudging the data never occurred to him. For further years he worked at it. He found he could get everything to fit, except the orbit of Mars, which was out of whack, in observational terms, by eight minutes of arc, just a quarter of the diameter of the full Moon.

Nobody before had ever worried about such a slight discrepancy. Nobody before had even been able to measure such a slight discrepancy. Kepler thought he had found the key to the universe, and had worked for most of his adult life to prove it. A hundred other people would have ignored the troublesome eight minutes of arc and published it anyway - and faded immediately into historical obscurity.

Not Kepler. He knew the data were correct. He knew that Tycho, for all his faults (and there were many of these), worshipped the precision of his observations no less than he did. And he believed in mathematics as the key to solving the ultimate question of the universe. God would not have built the world in perfect harmony, and then moved Mars ever so slightly askew.

The work was awesomely difficult. Calculus had not yet been invented, which meant that Kepler had to plot his planetary curves point by point. Remember that the Earth was moving around the Sun, and spinning on its axis, while all the planets themselves moved around the Sun. All Kepler had was a list - of planetary positions and timings.

Through his own personal, scientific and mathematical integrity, in a world where such things were not considered important even by some learned men, by an amazing exercise in trial and unremitting error, he realised that what he had actually proved was that the planets could not possibly move in circles at all.

With pen and paper, and with a scientific honesty that would frankly shame some of today's eminent scientists, he worked away until he finally, in desperation, tried the idea that planets moved in ellipses, oval-shaped paths, rather than circles.

It worked. The planets do move in ellipses. Kepler's work, though he never got his perfect solids to fit within the planetary orbits, and never proved that God used mathematics as a signpost to humans in designing the solar system, gave us instead the immortal laws of planetary motion, which in turn informed Newton's own genius half a century later in formulating his theory of gravitation.

Kepler lived for the same desire that made our ancestors worship trees and volcanoes, and which makes us in our own turn build ten-metre telescopes. His genius reveals a watershed between the past and the present in human thought. The drive which kept him at his task for all those years is no different to that which keeps us now at our particle accelerators and giant telescopes.

Monkey curiosity? Something genetic? The evolutionary experiment which humanity represents, "do big brains have survival value?" is now entering a very interesting phase. Indeed, science has at last given us such understanding and power over the forces of the world that we may not survive the experiment. Nuclear weapons, global warming, overpopulation ... But for us there is no returning. The risk of racial oblivion is very real, but the prize, if we can prevent the destruction of our world and learn to live together - is equally awesome: the stars. Our fate is in our own hands. With our minds and the tools we have made, both mental and physical, we have now got to continue the journey to its end.