Man is the only animal who does not feel at home in nature, who can feel evicted from paradise, the only animal for whom his own existence is a problem that he has to solve and from which he cannot escape.

Erich Fromm (1900-1980) American psychoanalyst and social philosopher

The Anatomy of Human Destructiveness, ch. 10 (1973)
    (Source)

Sometimes elided, "Man is the only animal for whom his own existence is a problem he has to solve."