Full Idea
No perceptible object is geometrically straight or curved; after all, a circle does not touch a ruler at a point, as Protagoras used to say, in arguing against the geometers.
Gist of Idea
No perceptible object is truly straight or curved
Source
Protagoras (fragments/reports [c.441 BCE], B07), quoted by Aristotle - Metaphysics 998a1
Book Reference
'The First Philosophers', ed/tr. Waterfield,Robin [OUP 2000], p.220