Quote Without A Source I'm writing a paper about Fernando Pessoa and his heteronyms and want to use a quote from Pessoa as written on the Wikipedia article:  "the theatre of being" or "drama in people."

Wikipedia does not cite the source of these quotes, and I assume from the context of the sentence they're in that it is a direct quote from Pessoa's work, but I can't read through everything he's written to try to find it. Is it possible to cite quote as "source unknown" or something similar? I'm fairly certain I've heard the quote before from a professor (she also did not know where to look for the quote's source) so I don't believe it's fake.
I would recommend against it. I've been bitten by this in the past: A famous quote attributed to a certain person, that after some investigation seems to be mistaken. 

Specifically: Many sources claim that Newton referred to "The Great Principle of Similitude" in regards to dimensional analysis, but having searched all through his Principia and other writings and not finding it, I'm now pretty sure the phrase comes from Rayleigh in a letter to Nature (No. 2368, Vol. 95; March 18, 1915). There's even a class at Stanford by that name, which recently changed its description to attribute Rayleigh instead (link). 

In summary: If you can't source it, then you probably shouldn't reference it. I feel pretty bad about saying something to several years of math students that turned out to be incorrect. Make a commitment to not distribute misinformation.