Sigmund Freud

"We will therefore turn to the less ambitious question of what men themselves show by their behavior to be the purpose and intention of their lives. What do they demand of life and wish to achieve in it? The answer to this can hardly be in doubt. They strive for happiness; they want to become happy and to remain so. This endeavor has two sides, a positive and a negative aim. It aims, on the one hand, at an absence of pain and unpleasure, and, on the other, at the experiencing of strong feelings of pleasure. In its narrower sense the word 'happiness' only relates to the last. In conformity with this dichotomy in his aims, man's activity develops in two directions, according as it seeks to realize — in the main, or even exclusively — the one or the other of these aims."