I doubt that any man knows enough to be a pessimist. Neither am I a reckless optimist. I do not think that things necessarily come out all right in the end if left to themselves.

Norman Cousins (1915–1990) American editor

Who Speaks for Man? (1953)

Cousins used the phrase on numerous occasions.  Variants:

• "No one really knows enough to be a pessimist."
• "No one is smart enough to be a pessimist."