Full Idea
If a cone is cut parallel to the base are the two new surfaces equal or unequal? If they are unequal, the cone must have gone up in steps. If they are equal then the cone must have been a cylinder, which is absurd.
Clarification
Seems to be a proof of atomism
Gist of Idea
If a cone is horizontally sliced the surfaces can't be equal, so it goes up in steps
Source
Democritus (fragments/reports [c.431 BCE], B155), quoted by Plutarch - 72: Against Stoics on common Conceptions 1079e1
Book Reference
'The First Philosophers', ed/tr. Waterfield,Robin [OUP 2000], p.188