“Only by contact with evil could I have learned to feel by contrast the beauty of truth and love and goodness.”
Contrast with what she calls evil sharpens Keller’s appreciation of truth, love and goodness, tying gratitude to moral discernment rather than uncomplicated cheerfulness.
Source details
Optimism: An Essay (1903), Project Gutenberg #31622. The sentence belongs to Keller’s personal moral and religious account of optimism.