We must never forget that human motives are generally far more complicated than we are apt to suppose, and that we can very rarely accurately describe the motives of another.

Fyodor Dostoyevsky (1821-1881) Russian novelist
The Idiot, Part 3, ch. 3 (1869) [tr. Martin (1915)]
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Alternate translation: "Don't let us forget that the motives of human actions are usually infinitely more complex and varied than we are apt to explain them afterwards, and can rarely be defined with certainty." [tr. Magarshack (1955)]