What Leucippus and Democritus had understood was that the world can be comprehended using reason. They had become convinced that the variety of natural phenomena must be attributable to something simple, and had tried to understand what this something might be. They had conceived of a kind of elementary substance from which everything was made. Anaximenes of Miletus had imagined this substance could compress and rarefy, thus transforming from one to another of the elements from which the world is constituted. It was a first germ of physics, rough and elementary, but in the right direction. An idea was needed, a great idea, a grand vision, to grasp the hidden order of the world. Leucippus and Democritus came up with this idea. ... The idea of Democritus’s system is extremely simple: the entire universe is made up of a boundless space in which innumerable atoms run. Space is without limits; it has neither an above nor a below; it is without a centre or a boundary. Atoms have no qualities at all, apart from their shape. They have no weight, no colour, no taste. ... Atoms are indivisible; they are the elementary grains of reality, which cannot be further subdivided, and everything is made of them. They move freely in space, colliding with one another; they hook on to and push and pull one another. Similar atoms attract one another and join. ... We know of his thought only through the quotations and references made by other ancient authors, and by their summaries of his ideas.