“One Man’s Modus Ponens”, 2012-05-01 (; backlinks; similar):
One man’s modus ponens is another man’s modus tollens is a saying in Western philosophy encapsulating a common response to a logical proof which generalizes the reductio ad absurdum and consists of rejecting a premise based on an implied conclusion. I explain it in more detail, provide examples, and a Bayesian gloss.
A logically-valid argument which takes the form of a modus ponens may be interpreted in several ways; a major one is to interpret it as a kind of reductio ad absurdum, where by ‘proving’ a conclusion believed to be false, one might instead take it as a modus tollens which proves that one of the premises is false. This “Moorean shift” is aphorized as the snowclone, “One man’s modus ponens is another man’s modus tollens”.
The Moorean shift is a powerful counter-argument which has been deployed against many skeptical & metaphysical claims in philosophy, where often the conclusion is extremely unlikely and little evidence can be provided for the premises used in the proofs; and it is relevant to many other debates, particularly methodological ones.