Historians write about scientific revolutions as automatically as of political, economic or social revolutions: the ‘French revolution’ in chemistry led by Lavoisier is almost as familiar as the political revolution which cut off his head. Indeed, the idea that science advances by revolutionary leaps has long been with us, ever since the eighteenth century in fact. For, as Bernard Cohen has shown, it was Enlightenment propagandists for science from Fontenelle and the Encyclopédistes to Condorcet who first began to depict the transformations in astronomy and physics wrought by Copernicus, Newton and others as revolutionary breaks with the past, creating new eras in thought.

And significantly it was through being applied in this way to epochs in science that the term ‘revolution’ itself took on its present meaning. Traditionally, when used to describe political fortunes, ‘revolution’ had, of course, denoted change (the fall of one prince, the rise of a rival); but it was change within an essentially cyclical system in which all dynasties and empires had their rise and fall, their waxings, wanings and eclipses, for human affairs were governed by the endless ‘revolutions’ of Fortune's Wheel. In the traditional metaphor, in other words, it was the orbits of the planets, so gravid with astrological influence, which had defined and governed revolutions in sublunary affairs. But, from the early eighteenth century, the old equation of revolution with cycles began to yield to a secular, directional myth of human destiny.