Behavior is a mirror in which everyone displays his own image.

[Das Betragen ist ein Spiegel in welchem jeder sein Bild zeigt.]

Johann Wolfgang von Goethe (1749-1832) German poet, statesman, scientist

Elective Affinities [Die Wahlverwandtschaften], Part 2, ch. 5, “From Ottilie’s Journal [Aus Ottiliens Tagebuche]” (1809) [tr. Hollingdale (1971)]
    (Source)

(Source (German)). Alternate translation:

Behavior is a mirror in which every one shows his image.
[Niles ed. (1872)]