The phrase “one person’s modus ponens is another’s modus tollens” is a popular philosophical motto that reminds us that a chain of logical reasoning works in both directions: if the truth of some premises entail the truth of a conclusion, then the falsity of the conclusion entails the falsity of at least one of the premises.
Here are some early appearances:
From such a perspective, the explanatory rivalry issue for Eddington and the materialists is precisely the same as for the anti-mechanist except that it has a different denouement (one man’s modus ponens is another man’s modus tollens, as Wesley Salmon has astutely remarked).
Larry Wright (1973). ‘Rival Explanations’. Mind 82:328, p. 510.
Since, as Salmon has put it, “one man’s modus ponens is another man’s modus tollens,” while some argued that [the Platonic Principle] established conclusively the existence of non-experiential sources of knowledge, others no less emphatically inferred from it the impossibility of (synthetic) necessary knowledge.
Alan Ross Anderson (1975). Entailment: the logic of relevance and necessity, p. 244. Princeton University Press.
Did Wesley C. Salmon originate this phrase? If so, where? The closest match I can find in any of Salmon’s books or papers is this, which which is a couple of decades too late to be the origin:
But I have often noticed that, in philosophy as well as other human endeavors, one person’s counterexample is another’s modus ponens.
Wesley C. Salmon (1989). Four Decades of Scientific Explanation, p. 31. University of Minnesota Press.