The Atomists – Leucippus (if he existed) and Democritus – had this idea that there were a load of atoms zooming around a void, and sometimes they’d bump into each other, and as a result – occasionally – form compound substances.
Aristotle complained that the Atomists hadn’t explained the source of all this motion; basically, he didn’t much like that the idea that motion and the continuation of motion might not need an explanation.
So here’s an amusing thing (if you’re amused by things that aren’t amusing, that is). This is how Frederick Copleston handles this issue in Volume 1 of his (remarkable, actually) history of philosophy (pp. 74-5)
To us, indeed, it may well seem strange to deny chance and yet to posit an eternal unexplained motion…but we ought not to conclude that Leucippus meant to ascribe the motion of the atoms to chance: to him eternal motion and the continuation of motion required no explanation. In our opinion, the mind boggles at such a theory and cannot rest content with Leucippus’ ultimate; but it is an interesting historical fact, that he himself was content with this ultimate and sought no “First Unmoved Mover”.
Okay, well that’s pretty clear. Bad Leucippus. This is how Bertrand Russell handles the same issue in his History (pp. 66-7).
Aristotle and others reproached him [Leucippus] and Democritus for not accounting for the original motion of atoms, but in this the atomists were more scientific than their critics. Causation must start from something, and wherever it starts no cause can be assigned for the initial datum. The world may be attributed to a Creator, but even then the Creator Himself is unaccounted for. The theory of the atomists, in fact, was more nearly that of modern science than any other theory propounded in antiquity […] All causal explanations…must have an arbitrary beginning. That is why it is no defect in the theory of the atomists to have left the original movements of the atoms unaccounted for.
Copleston and Russell were both writing at roughly the same time, but they have a very different take on this issue. The explanation? Copleston was a Jesuit priest; Bertrand Russell, wasn’t.