  This is a chapter for an upcoming book, "Space, Time, and the Limits of Human
Understanding", edited by Shyam Wuppuluri and Giancarlo Ghirardi. I first
present the classical picture of flat space and ever-flowing time, as generally
accepted until about 1900, discuss the challenges posed by modern physics and
the adjustments required by special relativity, general relativity and quantum
mechanics, and finally consider (and reject) a recent hypothesis that
space-time is essentially discrete. Ultimately, none of the simple answers to
"what is space-time?" stand up to reality. We have more work ahead of us!
