For years now, some guy has been patrolling downtown San Francisco with a sign that expresses his theory of the Kennedy assassination in a nutshell: Nixon Did It.

Recently, I notice, he's added and Clinton.

In other words, he's claiming that in 1963, 50-year-old Nixon conspired with then-teenager Bill Clinton to kill John F. Kennedy.

Yeah, right.

Hey, I'm not saying conspiracies don't happen. I have been involved in a few myself. For instance, I conspired with my wife for years to make our kids believe in Santa Claus. What's more, every parent in the neighborhood was in on it!

Okay, okay, I know: Serious conspiracy buffs are talking about dangerous conspiracies, political ones. Do those actually happen?

Sure, they do; all the time. In 44 BC, for example, a group of Roman nobles conspired to murder Julius Caesar, the most popular, powerful, and wealthy politician in the world's richest, strongest city. This one was even bigger than the JFK assassination, folks.

They did it to stop Caesar from dismantling Rome's republican form of government and making himself into a king.

What happened as a result? A civil war broke out, leading to the collapse of Rome's republican form of government and the rise of a new imperial system headed by an emperor more powerful than any king.

Killing Caesar was easy. Controlling history was another matter.

That's what divides Conspiracy Theory, as I'll call it, from the theory that any particular event may be the product of a conspiracy: the notion that history is controllable.

Conspiracy Theory proposes that some small group is producing the events we call "the news" as part of a grand design to rule the world. People may differ on who is doing the controlling, but the underlying framework looks pretty similar from one "theorist" to the next. Nearly all Conspiracy Theorists say that:

• Members of a powerful, secret cabal are shaping history to suit their own needs.
• The media doesn't report on them, because they control the media.
• The government doesn't stop them, because they control the government.
• The agents of this cabal are everywhere, but they go unrecognized because they blend in so uncannily.
• These agents do not themselves know much about the master plan, because they are acting in blind obedience to their superiors. Only the core members of the inner circle of the tip-top echelon of the pyramid truly know what is going on.

Don't ask who believes this stuff. Ask instead: Who on Earth could have dreamed it up?