McMullin did not much like the use of “true” and “truth” in this context. In his well-known “A Case for Scientific Realism,” he ends by apparently eschewing claims to truth altogether:

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I do not think that acceptance of a scientific theory involves the belief that it is true. Science aims at fruitful metaphor and at ever more detailed structure. To suppose that a theory is literally true would imply, among other things, that no further anomaly could, in principle, arise from any quarter in regard to it. [ . . . ] Scientists are very uncomfortable at this use of the word “true,” because it suggests that the theory is definitive in its formulation . . . .

The realist would not use the term “true” to describe a good theory. He would suppose that the structures of the theory give some insight into the structures of the world. But he could not, in general, say how good the insight is. He has no independent access to the world, as the antirealist constantly reminds him. His assurance that there is a fit, however rough, between the structures of the theory and the structures of the world comes not from a comparison between them but from the sort of argument I sketched above, which concludes that only this sort of reasoning would explain certain contingent features of the history of recent science. (McMullin 1984, 35)

These passages make it clear, however, that the discomfort with “true” signals only an implicature: use of “true,” in ordinary contexts, will convey a possibly quite unwarranted certainty. That contextual import is not typical in philosophical debate. Indeed, it was the scientific realists of the 1950s and 1960s who convinced the philosophical community to sever all logical connection between certainty or verification (on the side of the subject or speaker) and truth or reference. Nor does McMullin shrink back from language that we are entirely used to connecting with knowledge, and thereby to truth and reference. To have some insight into X is to know something true about X, and implies the reality of X; to have assurance of a fit between model and world is to have assurance that there is in fact a fit between the two. These points are not affected by the degree of caution with which any of this should be asserted.

But McMullin’s caution here is also alerting us to significant differences between his scientific realism and that of other realists he mentions, such as Hilary Putnam (circa. 1980) or Richard Boyd. For McMullin’s historically and philosophically informed view of science is nuanced and sophisticated in ways that much writing of the time was not. A remarkable feature of his view is its emphasis on the metaphorical power of theory, with its implications for intellectual fertility and the adventure of scientific exploration, both experimental and theoretical. Discussion of this aspect of his scientific realism—grounded in a conception of science quite different from that found in naı̈ve realism—would warrant a separate article, and I will leave it aside here. Instead we can look closely at how McMullin characterizes our current topic, the character of retroductive inference in science, in the above-mentioned article that makes his distinctive case for scientific realism. For there too we see a remarkable twist away from the more traditional epistemology’s conception of rules of right reason that one might have suspected of McMullin’s allegiance.

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Here McMullin introduces the claim that retroduction is the inference that makes science as, first of all, a historical conclusion about the sciences, and does immediately enmesh that conclusion in claims about its epistemic efficacy, but only to expose the radical contingency of any such claim.

First, the historical conclusion:

A third consequence one might draw from the history of the structural sciences is that there is a single form of retroductive inference involved throughout. (McMullin 1984, 29)

Then the claim of a warrant for truth:

As C. S. Peirce stressed in his discussion of retroduction, it is the degree of success of the retroductive hypothesis that warrants the degree of its acceptance as truth. (1984, 29)

which is classified as a logical point:

What the history of recent science has taught us is not that retroductive inference yields a plausible knowledge of causes. We already knew this on logical grounds. (1984, 29)

But then comes the twist: if that is not what we learned from the history, what precisely was learned is an actual, contingent, empirical fact about retroductive inference:

What we have learned is that retroductive inference works in the world we have and with the senses we have for investigating that world. This is a contingent fact, as far as I can see. [ . . . ] There could well be a universe in which observable regularities would not be explainable in terms of hidden structures, that is, a world in which retroduction would not work. Indeed, until the eighteenth century, there was no strong empirical case to be made against that being our universe. (1984, 29–30)

The sentence I omitted in this last quote is: “This is why realism as I have defined it is in part an empirical thesis.” And McMullin emphasizes this in several ways: “The realist seeks an explanation for the regularities he finds in science, just as the scientist seeks an explanation for regularities he finds in the world” (1984, 34), the realist’s “assurance that there is a fit, however rough, between the structures of the theory and the structures of the world comes not from a comparison between them but from the sort of argument I sketched above, which concludes that only this sort of reasoning would explain certain contingent features of the history of recent science” (1984, 35).

But at this point, it seems to me, McMullin has severely undermined his own argument. For this means that the argument for the efficacy of retroductive inference, and indeed for the claim that it is the inference that makes science, is an argument that is itself an instance of retroductive inference.

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McMullin was quite right to dismiss naı̈ve forms of inference to the best explanation in the current literature as easily criticized (McMullin 2007, 175). But at this point his form of defense of retroduction certainly recalls the more familiar “that the practice of science as inference to the best explanation accounts for the success of science is true, for it is the best explanation of that success.”

Not every circle is vicious. To show that one may refer to the soundness and completeness proofs for classical deductive logic, which are indeed rigorous proofs in the sense that they themselves follow the rules of classical logic. We gain real understanding of logic by going through those arguments, and this might be offered as a parallel for the use of a form of inference in the study of that form. But there is a great disanalogy. In the case of logic we do not add, as McMullin did for retroduction, that there could well be a universe in which logical inference would fail to preserve truth.

If the warrant for the claim that retroductive inference accounts for scientific success is itself the conclusion of a retroductive inference, what is the warrant for that? For its warrant, it would have to return to the purported logical connection between such inference and the warrant for belief, indicated by McMullin’s rather offhand claim that “retroductive inference yields a plausible knowledge of causes” is something we “already knew . . . on logical grounds.” And whatever those logical grounds are meant to be, they would have to be something that is very reassuring after all, offsetting the admission of pure contingency, about how assured we can be of having true insight into the causes behind the phenomena. But that is where realist and empiricist part ways.

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