Full Idea
Just as you can't know if a portrait of Socrates is good without seeing the man, so when the intellect gazes on sensations but not the external objects it cannot know whether they are similar.
Gist of Idea
How can the intellect know if sensation is reliable if it doesn't directly see external objects?
Source
Sextus Empiricus (Outlines of Pyrrhonism [c.180], II.75)
Book Reference
Sextus Empiricus: 'Outlines of Pyrrhonism', ed/tr. Bury,R.G. [Prometheus 1990], p.120