You've heard plenty of stories about schizophrenia
But never had evidence of one who thought himself mad
You're going crazy, and desperately reachin' for reason
But the strain to stay sane's your only demon; best believe it

from Eyedea (Ft. Dj Abilities) – The Dive (a) Lyrics on Rap Genius

Meaning

You’ve heard plenty of stories of schizophrenia so you consider this as a possibility; but then again, you have no evidence of anyone who thought themselves crazy, which is somewhat comical, since you are searching for evidence and feeding this dive into madness and questioning of everything you know. You feel as if all these thoughts are driving you insane, and you’re desperately trying to find an answer, but the desire to stay sane is your only demon, you better believe it. (This strain to stay sane is your only demon can be interpreted in a few ways:
1) The strain to keep sane is described as a demon, which is normally thought of as an evil creature, so perhaps Eyedea thinks that sanity is the demon keeping you from the truth, and madness is the ultimate truth/reality.
2) The strain to stay sane is the only thing that keeps you from letting yourself become completely insane/not even attempting to rationalize all these new questions in your head. Eyedea may have just used the word demon as a word to rhyme, but I lean more towards #1.)

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