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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.

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Wesley Salmon did write this, but in a book published twenty years after people were attributing it to him:

Some would say the received view is correct, so there are no legitimate functional explanations. But one person's modus ponens is another person's modus tollens. Others would say there are legitimate functional explanations in science, so the received view is not correct.

-- Wesley C. Salmon, Causality and Explanation, Oxford University Press, New York, 1998 (emphasis mine)

He doesn't attribute the saying to anyone else, so it might be something he came up with himself, as Wright and Anderson suggested. I was also unable to find any version of this adage in earlier works of his (I tried many different versions: searching for "modus ponens" together with either "another's modus tollens" or "another man's modus tollens" or "another person's modus tollens", plus all the same searches with ponens and tollens switched).

My hypothesis, which will be pretty much impossible to verify, is that Wright and Anderson were ascribing this idea to a more personal communication from Salmon, something they heard from him rather than something he'd published. They were all academic philosophers working in top US universities at around the same time, so it's reasonable to guess that they might have known each other, or at least met each other at conferences or other events. Anderson died in 1973 (actually before the book that you cited from him was published in 1975), and he was a Professor of Philosophy at the University of Pittsburgh from 1965 to 1973, during which time Salmon was president of the Philosophy of Science Association (1971 to 1972) and a faculty member at Indiana University Bloomington (1963 to 1973). Later, Salmon even took Anderson's old job at the University of Pittsburgh. I was unable to find much information about Larry Wright (the philosopher, not the basketball player or street drummer), but he's also a professor of philosophy, so it seems a reasonable working hypothesis that he might have met, or attended a talk by, Salmon in the 1970s.

It also makes sense that Salmon might have said "one man’s modus ponens is another man’s modus tollens" (the exact phrase that both Wright and Anderson quoted) back in the 1970s when "man" was more commonly used to refer to humanity in general, and then later improved his own phrasing (or an editor improved it) to the more gender-neutral "one person’s modus ponens is another person’s modus tollens" which we find in his 1998 book. Apart from the "man" to "person" change, the quotes are identical.

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  • "Improved" is a matter of opinion.
    – user14111
    Oct 28 at 23:51
  • @user14111 True, but I'm suggesting that it was Salmon's opinion: explaining why he slightly altered the quote from one decade to another, because he saw the new version as an improvement.
    – Rand al'Thor
    Oct 30 at 18:05

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