Can you source the quote?

Robert Anton Wilson. Ready to hold forth, but not always sure he was right. 

Here is an oft-cited quote from Robert Anton Wilson: "If you think you know what the hell is going on, you're probably full of shit." It was used in Boing Boing blog post, and you've probably seen it in other places.

While this sounds like a pithy restatement of the Cosmic Schmuck principle, the folks at the RAW Trust  would like the source of the putative RAW "If you think"  quote. Did it come from a book? An essay? What can it be traced to? If you can help, please say something in the comments.