Full Idea
Epicurus says illusions are not false perceptions, because the senses accurately report the pattern of atoms; for instance, the edges are worn off the pattern produced by a square tower, so its perception as a round tower is true.
Gist of Idea
Illusions are not false perceptions, as we accurately perceive the pattern of atoms
Source
report of Epicurus (Letter to Herodotus [c.293 BCE], 47-53) by Deborah K.W. Modrak - Classical theories of Mind
Book Reference
'Encyclopedia of Classical Philosophy', ed/tr. Zeyl,Donald J. [Fitzroy Dearborn 1997], p.347
A Reaction
As so often, Epicurus got it right, because Democritus got it right, thus demonstrating that good philosophy must be preceded by good physics. However, good physics must be preceded and followed by good philosophy.