Yesterday upon the stair
I met a man who wasn’t there.
He wasn’t there again today
Oh how I wish he’d go away.

-- William Hughes Mearns

This bit of doggerel - or a variation of it - was the basis for “The Yehudi Principle”, a short story by Fredrick Brown. It’s a cautionary tale, a sort of warning that tells you: You may think you know what’s going on, and you may even think you can use that knowledge to shape the world. But the real world is stranger than you realize, and sooner or later, you’re goi...