171 ���Send requests for reprints to the author, Department of History and Philosophy of Science, Goodbody Hall 130, Indiana University, 1011 East Third Street, Bloomington, IN 47405���7005 e-mail: mlfriedm@indiana.edu. *This paper first appeared in Michael Heidelberger and Friedrich Stadler (eds.) History of Philosophy of Science: New Trends and Perspectives (Dordrecht: Kluwer, 2002) Kluwer Academic Publishers. It appears here with the permission of the editors and of Kluwer Academic Publishers. It also reproduces some passages from my Dynamics of Reason: The 1999 Kant Lectures at Stanford University (Stanford: CSLI Publications, 2001). Philosophy of Science, 69 (June 2002) pp. 171���190. 0031-8248/2002/6902/0001$10.00 Copyright 2002 by the Philosophy of Science Association. All rights reserved. Kant, Kuhn, and the Rationality of Science* Michael Friedman��� Indiana University This paper considers the evolution of the problem of scientific rationality from Kant through Carnap to Kuhn. I argue for a relativized and historicized version of the origi- nal Kantian conception of scientific a priori principles and examine the way in which these principles change and develop across revolutionary paradigm shifts. The distinc- tively philosophical enterprise of reflecting upon and contextualizing such principles is then seen to play a key role in making possible rational intersubjective communication between otherwise incommensurable paradigms. In the Introduction to the Critique of Pure Reason Kant formulates what he calls ���the general problem of pure reason,��� namely, ���How are synthetic a priori judgements possible?��� Kant explains that this general problem involves two more specific questions about particular a priori sciences: ���How is pure mathematics possible?��� and ���How is pure natural science possible?������where the first concerns, above all, the possibility of Euclidean geometry, and the second concerns the possibility of funda- mental laws of Newtonian mechanics such as conservation of mass, in- ertia, and the equality of action and reaction. In answering these questions Kant develops what he calls a ���transcendental��� philosophical theory of our human cognitive faculties���in terms of ���forms of sensible intuition��� and ���pure concepts��� or ���categories��� of rational thought. These cognitive

��������������������� ������������������������ 172 1. The ���general problem of pure reason,��� along with its two more specific sub-prob- lems, is formulated in �� VI of the Introduction to the Critique of Pure Reason at B19��� 24. Sections V and VI, which culminate in the three questions ���How is pure mathematics possible?���, ���How is pure natural science possible?���, and ���How is metaphysics as a science possible?���, are added to the second (1787) edition of the Critique and clearly follow the structure of the 1783 Prolegomena to Any Future Metaphysics, which was intended to clarify the first (1781) edition. This way of framing the general problem of pure reason also clearly reflects the increasing emphasis on the question of pure natural science found in the Metaphysical Foundations of Natural Science (1786). For an ex- tended discussion of Kant���s theory of pure natural science and its relation to Newtonian physics see Friedman, Kant and the Exact Sciences (Cambridge, MA: Harvard Uni- versity Press, 1992), especially chapters 3 and 4. structures are taken to describe a fixed and absolutely universal rational- ity���common to all human beings at all times and in all places���and thereby to explain the sense in which mathematical natural science (the mathematical physics of Newton) represents a model or exemplar of such rationality.1 In the current state of the sciences, however, we no longer believe that Kant���s specific examples of synthetic a priori knowledge are even true, much less that they are a priori and necessarily true. For the Einsteinian revolution in physics has resulted in both an essentially non-Newtonian conception of space, time, and motion, in which the Newtonian laws of mechanics are no longer universally valid, and the application to nature of a non-Euclidean geometry of variable curvature, wherein bodies af- fected only by gravitation follow straightest possible paths or geodesics. And this has led to a situation, in turn, in which we are no longer con- vinced that there are any real examples of scientific a priori knowledge at all. If Euclidean geometry, at one time the very model of rational or a priori knowledge of nature, can be empirically revised, so the argument goes, then everything is in principle empirically revisable. Our reasons for adopting one or another system of geometry or mechanics (or, indeed, of mathematics more generally or of logic) are at bottom of the very same kind as the purely empirical considerations that support any other part of our total theory of nature. We are left with a strongly holistic form of empiricism or naturalism in which the very distinction between rational and empirical components of our total system of scientific knowledge must itself be given up. This kind of strongly holistic picture of knowledge is most closely iden- tified with the work of W. V. Quine. Our system of knowledge, in Quine���s well-known figure, should be viewed as a vast web of interconnected beliefs on which experience or sensory input impinges only along the periphery. When faced with a ���recalcitrant experience��� standing in conflict with our system of beliefs we then have a choice of where to make revisions. These can be made relatively close to the periphery of the system (in which case

������������, ������������, ��������� ��������� ��������������������������������� ������ ��������������������� 173 2. From the first two paragraphs of �� 6, entitled ���Empiricism without the Dogmas,��� of ���Two Dogmas of Empiricism,��� Philosophical Review 60 (1951): 20���43 reprinted in From a Logical Point of View (New York: Harper, 1953), pp. 42���43. we make a change in a relatively low-level part of natural science), but they can also���when the conflict is particularly acute and persistent, for example���affect the most abstract and general parts of science, including even the truths of logic and mathematics, lying at the center of our system of beliefs. To be sure, such high-level beliefs at the center of our system are relatively entrenched, in that we are relatively reluctant to revise them or to give them up (as we once were in the case of Euclidean geometry, for example). Nevertheless, and this is the crucial point, absolutely none of our beliefs is forever ���immune to revision��� in light of experience: The totality of our so-called knowledge or beliefs, from the most ca- sual matters of geography and history to the profoundest laws of atomic physics or even of pure mathematics and logic, is a man-made fabric which impinges on experience only along the edges. Or, to change the figure, total science is like a field of force whose boundary conditions are experience. A conflict with experience at the periphery occasions readjustments in the interior of the field. . . . But the total field is so underdetermined by its boundary conditions, experience, that there is much latitude of choice as to what statements to ree��val- uate in the light of any single contrary experience. . . . If this view is right . . . it becomes folly to seek a boundary between synthetic statements, which hold contingently on experience, and an- alytic statements, which hold come what may. Any statement can be held true come what may, if we make drastic enough adjustments elsewhere in the system. Even a statement very close to the periphery can be held true in the face of recalcitrant experience by pleading hallucination or by amending certain statements of the kind called logical laws. Conversely, by the same token, no statement is immune to revision. Revision even of the logical law of the excluded middle has been proposed as a means of simplifying quantum mechanics and what difference is there in principle between such a shift and the shift whereby Kepler superseded Ptolemy, or Einstein Newton, or Darwin Aristotle?2 As the last sentence makes clear, examples of revolutionary transitions in our scientific knowledge, and, in particular, that of the Einsteinian revo- lution in geometry and mechanics, constitute a very important part of the motivations for this view. Yet it is important to see that such a strongly anti-apriorist conception of scientific knowledge was by no means prevalent during the late nine-