The NY Times just published a great essay (actually, four-part series of essays) by filmmaker Errol Morris on the strange, self-justifying logic employed by one of the architects of the Iraq war. You might dismiss Donald Rumsfeld as a blustering idiot (as I long have) but he is one who managed to convince a lot of people he was right, with serious and tragic consequences.
This quote really distills it:
“Absence of direct evidence, may not be proof of absence, but often absence of evidence is evidence of absence. If a weapons inspector looks for W.M.D. in a building and finds nothing, is that absence of evidence? Or evidence of absence? I would argue that it is evidence of absence. Rumsfeld took a principle used in one context — is there intelligent life elsewhere in the universe — and applied it to Iraq. Imagine someone tells you that there is an elephant in the room. You search the room, opening drawers, checking closets, looking under the bed. No elephant. Absence of evidence or evidence of absence?”
Morris is putting a microscope on the kind of false logic that should raise our bullshit alarms the next time we hear it.