THE INFERENCE THAT MAKES SCIENCE
by Ernan McMullin
Abstract. In his Aquinas Lecture 1992 at Marquette University, Ernan McMullin discusses whether there is a pattern of inference that particularly characterizes the sciences of nature. He pursues this theme both on a historical and a systematic level. There is a continuity of concern across the ages that separate the Greek inquiry into nature from our own vastly more complex scientific enterprise. But there is also discontinuity, the abandonment of earlier ideals as unworkable. The natural sciences involve many types of inference; three of these interlock in a special way to produce “retroductive inference,” the kind of complex inference that supports causal theory.
Keywords: abduction; Thomas Aquinas; Aristotle; causality; demonstration; Galileo Galilei; inference; realism; science; theory
Is there a pattern of inference that particularly characterizes the sciences of nature? Theorists of science, from Aristotle’s day to our own, have on the whole tended to answer in the affirmative, though views have changed as to what that pattern is. It has usually been linked, in one way or another, with explanation. To demonstrate in proper scientific form, Aristotle noted, is also to explain. The credibility of a theoretical inference, it might be said today, is proportionate to its explanatory success.
My aim in this essay is to pursue this theme, the nature of the inference that constitutes a claim as “science,” both on a historical and a systematic level. As historians, we shall find a continuity of concern, a link across the ages that separate the Greek inquiry into nature from our own vastly more complex scientific enterprise. But we shall also discover discontinuity, the
Ernan McMullin (1924–2011) held the John Cardinal O’Hara Chair of Philosophy, and was director of the program in history and philosophy of science at the University of Notre Dame, South Bend, IN, USA. The text is reproduced from Ernan McMullin, The Inference that Makes Science (Milwaukee: Marquette University Press, 1992, 1–112). Beginnings of pages in the original are indicated with the page number between / and /. C© Marquette University Press, 1992. Reprinted with permission.
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abandonment of earlier ideals as unworkable. Indeed, it is arguable that the failure (in its own terms) of Aristotelian natural philosophy may to some degree have been linked with its emphasis on demonstration, on a science of nature that would rest on causal claims held to be evident in their own right. We shall find /2/ a worry among medieval Aristotelians that a demonstrative science of nature might be very difficult of achievement or might even be out of reach. The deep shift that we have come to call the “Scientific Revolution” can be regarded as in large part an attempt to construct an alternative to demonstration, a “New Organon,” as its most influential protagonist dubbed it. We shall discover that the New Organon was fundamentally ambiguous, that it involved two quite different patterns of inference. It took more than two centuries before this was finally recognized. And even in our own century, it was implicitly denied, first by the logical positivists, and more recently by those who, for whatever reason, reject scientific realism. Since our canvas is such a large one, we shall have to be content with broad strokes. Besides Aristotle, there will be a host of other characters: Grosseteste, Zabarella, Bacon, Whewell, Peirce . . . . And Aquinas, needless to say, will not be forgotten.
We shall come to see that the natural sciences involve many types of inference-pattern; three of these interlock in a special way to produce what we shall call retroductive inference, the kind of complex inference that supports causal theory. Since theories are primarily designed to explain, explanatory power obviously plays a major part in their warranting. But there is a good deal /3/ of disagreement about how this warranting role may best be understood. We shall, for example, challenge the thesis often associated with the hypothetico-deductive (H-D) account of scientific knowledge which would limit the warrant of a theory to the sum of the verified consequences deductively derivable from it.
It may be worth noting from the beginning that the attempt to define “the inference that makes science” is not intended to furnish a criterion of demarcation between science and nonscience. The issue of demarcation has been actively debated ever since Popper made it central to his philosophy of science. We shall not address it here. Suffice it to say that retroductive inference makes use of ingredients that are commonplace in human reasoning generally. One finds them in any inquiry into causes, in the work of a detective or a newspaper reporter, for example. What is distinctive about the way in which explanatory theories are constructed and tested in natural science is the precision, as well as the explicitness, with which retroductive inference is deployed. But this alone is not enough to enable a sharp boundary line to be drawn. There will be large areas where a clear cut decision will not be possible, where, for example, the questions: “good science or bad science?” and “science or non-science?” will inevitably overlap. /4/
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The sciences of human behavior pose a further, and equally debatable, question. Is the pattern of inference that constitutes these as “science” the same as (or, at least, very similar to) that employed by the natural sciences? Do they explain in more or less the sense in which, say, chemistry explains? Do they work back from observed effects to underlying structural causes as chemical theories do? Once again, we shall have to set aside an important issue in order to focus on the already-large one at hand. Our concern here is with the natural sciences, and with a single question: in what kind of complex inference do they (ideally) culminate? Of course, this limitation would have been foreign to the intentions of Aristotle, from whose seminal work on the theory of science, the Posterior Analytics, our inquiry takes its start. His aim was to discover what the ideal of knowledge (episteme) should be, while warning against seeking a greater degree of precision in any domain than the nature of the inquiry admits.