One day a girl college student in Canada asked me to define reality for her, for a paper she was writing for her philosophy class. She wanted a one-sentence answer. I thought about it and finally said, “Reality is that which, when you stop believing in it, doesn’t go away.” That’s all I could come up with. That was back in 1972. Since then I haven’t been able to define reality any more lucidly.

How to Build a Universe That Doesn’t Fall Apart Two Days Later