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."