Two thousand years ago, on a hillside in eastern Gaul, a Roman army under Julius Caesar trapped a Gallic army inside a hill town called Alesia, and then did something stranger than a siege. Caesar built two walls, one facing inward at the men he had caught, to hold them in, the other facing outward at the quarter of a million Gauls marching to break the ring, to keep them out. Miles of timber and ditch, raised in a few weeks, and the object of all that labor was a number: the count of futures still open to the men inside, which the walls drove down to one. The one left them, when the digging was done, was capture.

This show is about the ideas that copy themselves through us, the ones we carry and hand on without inspecting them, and today's idea is a number. My new book reaches the world this week. It is called The Eighteen-Minute Lie, and it is the biography of a statistic that was never true, followed from the hour someone forged it to the moment it convinced a civilization that it had lost its mind. I spent two years on it, in part because I owed the work. I helped carry the lie. You have heard the louder version of my number, the one that beat mine and traveled the world. The human attention span, it says, has fallen to eight seconds, one tick shorter than a goldfish's. You have met it in a TED talk, on a morning show, inside a slide deck built to sell you the software that will rescue the same focus the previous slide told you that you had lost. It is one of the most repeated figures of the age.

So how did the cartoon win? In the summer of 1925, in a hot courtroom in Dayton, Tennessee, William Jennings Bryan agreed to defend a law against the teaching of evolution. Clarence Darrow put him on the stand and took him apart in front of the country. A newspaperman named H. L. Mencken filed dispatches that turned the old orator into a national figure of fun, and the dispatches were funnier than they were fair. Then Bryan died, five days later, and Mencken wrote an obituary that buried the man before the grave was dug. The prose was brilliant. It was also a kind of murder, a reputation killed while the body was still warm. A play came along a generation on, Inherit the Wind, and finished the job. The meme was sealed, and it has been copying itself ever since.