People will observe you to see
How well you have observed.
The man who only observes himself however never gains
Knowledge of men. He is too anxious
To hide himself from himself. And nobody is
Cleverer than he himself is.
Source, in full
"Speech to Danish working-class actors on the art of observation" [Rede an dänische Arbeiterschauspieler über die Kunst der Beobachtung] (1934), from The Messingkauf Poems, published in Versuche 14 (1955); trans. John Willett in Poems, 1913-1956, pp. 235-236