MCMULLIN ACCOUNT OF RETRODUCTIVE INFERENCE
It is in fact not easy to disentangle the points that allow us to recognize a process of retroductive inference from the claims McMullin makes concerning this sort of inference. We must concentrate on the definitive account that McMullin provides in the last 5 pages (in the reprint that follows) of The Inference That Makes Science, but it may help to look first at a formulation McMullin provided in a later publication, as a short summary:
Retroduction, argument from observed data to an explanatory causal structure which may itself be unobserved though not necessarily unobservable is of its essence tentative. It terminates in likelihood (in the everyday sense of that term, not the sense given it in probability theory). It allows for the gradual mounting of evidence of all sorts: increasingly troublesome anomalies eliminated, ambiguities resolved, new evidence successfully incorporated, and the rest. Above all, under certain circumstances it encourages more and more persistent questioning of the assumption that the paradigm in possession is beyond challenge or that a potential rival is, on the face of it, absurd. There is a lot of room here between strict reason and credo quia absurdum, the room afforded by an ever-increasing likelihood that may begin from a very low level indeed. (McMullin 2007, 176)
To what extent is this a description, such as a neutral observer of scientific practices might give, and to what extent does it involve claims about the adequacy or rationality or truth-conduciveness of this form of inference?
First: that in such an inference we are “led backwards” from effect to cause, for example, we can read as merely describing the form (from premises about what happens to conclusions about what causes them). But we can also read it as a claim that what happens is always in fact an
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effect, that is, an event that has causes, and in addition that these causes are discovered by retroductive inference. That this composite claim is in fact part of what McMullin maintains becomes quickly evident toward the end of his Aquinas lecture.
That McMullin is making a strong claim on behalf of this form appears also earlier in his critique of Newton, whom he describes as having been misled by the “quasi-demonstrative” form of his own writings, and as having had a distorting influence on eighteenth century methodological reflection, which
was to have negative repercussions for decades to come, until the atoms and ether- vibrations of the early nineteenth century once and for all showed causal inference to underlying structure to be indispensable to the work of the physical scientist. (180)
Second: one feature McMullin lists, which clearly distinguishes this retroductive inference, is the creation of new concepts. We can imagine a situation in which all attempts at explanation fail, within the conceptual framework that has been actualized so far. In that case—and surely there are famous historical cases of this sort—a smaller or larger conceptual revolution is the only way forward. As a distinguishing mark of retroductive inference, though, it has its limits; for this feature is one that may be present, and certainly is not always involved.
Once again, Newton furnishes the bad example of a misdirected empiricism. The need for new concepts and new language appears to be ruled out by Newton’s Third Rule of Reasoning which postulated that the relevant properties of all bodies would be those accessible to the human senses. And this was not incidental, Newton “needed this restriction . . . in order that induction might be, as he claimed, the all-sufficient method of natural science” (185).
Third: that the product of retroduction is a theory which presents a causal explanation, distinct from the sort of empirical law that registers a regularity, is crucial. We can perhaps typically see the feature of causal “explanatoriness” at a glance, and if so it can serve as a hallmark to recognize retroduction. But even here a claim of adequacy or efficacy, not just something offered as description, is entangled with the description:
The language here is, of course, that of scientific realism. It is because the cause is, in some sense however qualified, affirmed as real cause, that retroduction functions as a distinct form of inference. (184)
Here, after all, Newton appears as on the side of the angels. For this phrasing echoes Newton’s First Rule of Reasoning, the “vera causa” principle. What I will suggest though is that inductivism in the naı̈ve form that Newton may have preached, if not practiced, is in any case not the most important rival to McMullin’s view of science.
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