Full Idea
They thought that truth lay in appearances, which they appreciated are contradictory and infinite, so they made the shapes of atoms infinite. Thus the infinite changes in compounds create the infinitely varies appearances.
Gist of Idea
Because appearance is infinitely varied, atomists assume infinitely many shapes of atom
Source
report of Democritus (fragments/reports [c.431 BCE], A009) by Aristotle - Coming-to-be and Passing-away (Gen/Corr) 315b9
Book Reference
'The First Philosophers', ed/tr. Waterfield,Robin [OUP 2000], p.182