"Peirce was the first [in history] to say straightforwardly that to deduction and induction, we must add a third (which he variously named abduction, hypothesis, retroduction) if we are to categorize properly what it is that makes science."
Ernan McMullin, The inference that makes science, The Aquinas lecture 1992 (Milwaulkee, WI: Marquette University Press, 1992), 85.
Nonetheless, McMullin goes on to stress that what "makes science" is "the process as a whole": "The process as a whole", of which abduction/hypothesis/retroduction as defined eventually by Peirce was for centuries the missing or at least unacknowledged part, "is the inference by means of which we transcend the limits of the observed, even the instrumentally observed" (92). And that extremely complex "process as a whole" McMullin calls retroduction, "The product of [which] is [not law (itself an explanandum), but] theory or causal explanation" (93; cf. 2-3, 24, ). Retroduction "is properly inference, since it enables one to move in thought from the observation of an effect to the affirmation, with [(contra Aristotle)] greater or lesser degree of confidence, of the action of a cause of a (partially) expressed sort", and it makes "an existence claim of [some] kind for [unobservable] theoretical entities. . . ." In this way, realism (94, italics mine) becomes a characteristic of "the inference that makes science," becomes even a sine qua non of modern science. (And that contra the scepticism of Hume (73-76).)
"It is a far cry from the demonstrations of Aristotle to the retroductions of modern theoretical science. Where they differ is, first, that retroduction makes no claim of necessity, and settles for less, much less, than definitive truth." (According to McMullin (passim), it was the Aristotelian quest for "definitive truth" that held science up for centuries.) "It can, under favorable conditions, when theories are well-established, yield practical certainty. Recent discussions of scientific realism show, however, how hedged this assertion must be. . . . Second, the inductions that retroduction relies on are systematic and protracted, not simply a noticing of regularity. Third, the observations from which retroduction begins are, for the most part, performed by sophisticated instruments; the limited scope and lack of precision of the human senses would never permit the range of retroduction that is necessary if the 'invisible realm', as Newton called it, is to be opened up. Fourth, abduction [in this larger sense] often requires the introduction of new concepts and the testing of new language. . . . Finally, though retroduction is, indeed, an act of the intellect, as the epagoge underlying [Aristotelian] demonstration was asserted to be, it is exceedingly complex. . . . And it is open-ended. . . . It does not terminate in an act of intutitive [Aristotelian] insight wherein one sees that nature must be so" (96-97).