Suite Mentale by Randall Garrett

Insanity is a retreat from reality, an escape within the mind from the reality outside the mind. But what if there is no detectable reality outside the mind? What is there to escape from? Suicide -- death in any form -- is an escape from life. But if death does not come, and can not be self-inflicted, what then?

And when the pressure of nothingness becomes too great to bear, it becomes necessary to escape; a man under great enough pressure will take the easy way out. But if there is no easy way? Why, then a man must take the hard way.