Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe.
Variant: Only two things are infinite, the universe and human stupidity, and I'm not sure about the former.
Earliest attribution found in: Frederick S. Perls In and Out the Garbage Pail (1969), p. 33. This quote is listed under "Disputed" on Wikiquote website. Probably apocryphal.
