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."