Men are disturbed, not by things, but by the principles and notions which they form concerning things.

Epictetus (c. 55-c. 135 AD) Greek (Phrygian) Stoic philosopher [Ἐπίκτητος, Epíktētos]

The Enchiridion (c. 135)

Alt. trans.: "We suffer not from the events in our lives, but from our judgment about them."