Full Idea
Everything exists which anyone perceives.
Gist of Idea
Everything exists which anyone perceives
Source
Metrodorus (Chi) (Natural Science (lost) [c.340 BCE], B2), quoted by (who?) - where?
Book Reference
'Ancilla to the Pre-Socratic Philosophers', ed/tr. Freeman,Kathleen [Harvard 1957], p.121
A Reaction
cf Berkeley and Epicurus. This misses out the problem of perceptual error, such as a square tower looking round from a distance, or one person in a group thinking they have seen something. It is still a good criterion, though!