“The
definition of insanity is doing the same thing over and over and expecting
different results.”
Benjamin Franklin
“Sanity
is a madness put to good uses.”
George Santayana
“No
excellent soul is exempt from a mixture of madness.”
Aristotle
“Everyone
is more or less mad on one point.” 
Rudyard Kipling
“I
became insane, with long intervals of horrible sanity.”
Edgar Allan Poe
“I
can calculate the motion of heavenly bodies but not the madness of people.”
Isaac Newton
“All
of us are crazy in one way or another.”
Yiddish Proverb