With measured tread, Johan Cornets de Groot climbed the many steps to the first tier of the tower. He did so solemnly, as seemed proper for his rank as burgomaster of Delft in southern Holland. De Groot had been invited to witness a physics experiment proposed by Stevin, the Flemish engineer, polymath and private physics instructor to Maurits, Prince of Orange.

This is the way it happened in my imagination. What these two men actually did has not been recorded, except for the setup and outcome of their experiment. The year was 1585, in an era when the scientific acceptance of observations and experimental evidence was beginning to grow in the minds of the intelligentsia (the illiterate stonemason and the shipwright had always respected facts, of course). In our 21st century, surrounded at all times and in all places by the products of science, it is difficult to appreciate how radically new it was to conduct an experiment that brushed aside nineteen centuries of philosophical opinion and, indeed, to devise such a test in the first place.

High above the ground, the experimental apparatus was held ready: two leaden balls, one ten times heavier than the other, prepared by Simon Stevin of Brugghe. He was a scientist in the best modern sense of the word: his brain held a vast amount of knowledge; he was familiar with all the classical works on physics and mathematics known in his time; his own work advanced science and engineering; and he informed non-scientists about the wonders of the world – among them Maurits, Prince of Orange, for whom he composed a fat compendium of theoretical and practical physics and mathematics entitled Wisconstige gedachtenissen (Flemish for something between ‘mathematical musings’ and ‘mathematical inventions’).

Stevin knew a lot, but he also understood that science is not so much about knowing as it is about searching. Of course it is necessary to be aware of the state of knowledge, but mostly to determine one's point of departure on a voyage into terra incognita, and possibly to get some idea about what direction to take in that immeasurable land. Like Galileo, Huygens, Newton and others, Stevin was one of the founding fathers of science, known as ‘natural philosophy’ at the time.