... and months after this question was posted, no mention of Sidney Harris?
Start with his "... and then a miracle occurs" and his "You want proof? I'll give you proof!" . Continue from there.
Gerhard "That's Enough Subconscious For Today" Paseman, 2010.07.06
The first one available for sale as prints, notecards, etc. http://www.newyorkerstore.com/All-Industry-Cartoons/I-think-you-should-be-more-explicit-here-in-step-two/invt/118181