Twenty-four centuries ago, in a windblown town on the northern coast of Greece, a man who reportedly could not stop laughing made one of the strangest and most consequential claims in the history of thought. Everything you have ever seen or tasted or feared, he said, is built from just two ingredients: tiny indivisible bodies, and the empty space they move through. He called that empty space nothing, and then he did something no one had dared to do before him. He insisted that nothing is every bit as real as something.

This is the story of Democritus of Abdera, the laughing philosopher, and of the simple, scandalous idea at the center of his system: that color, sweetness, warmth, and beauty exist only by agreement, while in truth there are only atoms and the void. It is also the story of how thoroughly the world tried to forget him. His books were lost, his name was avoided by the most famous philosopher who ever lived, and almost everything we know about him survives in the words of the people who set out to prove him wrong.

Yet his questions refused to die. Is empty space truly empty? Where does the riot of the world come from, if its building blocks have no color and no taste? Can there be necessity without purpose, and freedom without a loophole? And how far down does knowledge reach before it runs out? These are not museum pieces. They are the living questions of modern science, which has spent the last century rediscovering, almost against its will, nearly everything the laughing Greek guessed, and overturning the one thing everyone assumes he got right.

Written with wit and wonder for the curious listener, this book follows the oldest idea about nothing all the way to a startling modern address, where empty space turns out to be the fullest thing there is, and where the weight of nothing can finally be given a number.