Fyodor Dostoevsky:

Man is a mystery: if you spend your entire life trying to puzzle it out, then do not say that you have wasted your time. I occupy myself with this mystery, because I want to be a man.

From an 1839 letter to his brother. [Source: Konstantin Mochulsky’s essay, “Dostoevsky and The Brothers Karamazov,” found in the Bantam Classic edition of The Brothers Karamazov]