“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

There is always some madness in love. But there is also always some reason in madness.
Friedrich Nietzsche

There is no great genius without some touch of madness.
Aristotle

You're only given a little spark of madness. You mustn't lose it.
Robin Williams

Everybody's youth is a dream, a form of chemical madness.
F. Scott Fitzgerald

A person needs a little madness, or else they never dare cut the rope and be free.
Nikos Kazantzakis

It takes madness to find out madness.
Lady Gregory

In a completely sane world, madness is the only freedom.
J. G. Ballard