Full Idea
It is impossible to assent to an apparent falsehood, or to deny an apparent truth.
Gist of Idea
We can't believe apparent falsehoods, or deny apparent truths
Source
Epictetus (The Discourses [c.56], 3.07.15)
Book Reference
Epictetus: 'The Discourses, The Handbook, Fragments', ed/tr. Gill,C [Everyman 1995], p.165
A Reaction
The way some philosophers write you would think that most beliefs just result from private whims or social fashion. That happens, of course, but most beliefs result from direct contact with reality.