Some claim that the collapse of the Twin Towers on September 11, 2001 was actually caused by a controlled demolition orchestrated by the U.S. government. Dramatic conspiracy theories of this kind are all over the place, but they are often dismissed as crazy. Sometimes, however, they turn out to be true: the NSA, as we have learned, conducted secret surveillance of millions of people for more than ten years. Does this show that we shouldn’t be so dismissive of conspiracy theories after all, or that we simply refuse to accept the existence of coincidence? What is a conspiracy theory, anyway, and how is it different from other kinds of theories? John and Ken form a cabal with Brian Keeley from Pitzer College, author of “On Conspiracy Theories.”