This paper offers my current view of a joint research project, with Bernard R. Goldstein, that examines Kepler's unification of physics and astronomy. As an organizing theme, I describe the extent to which the work of Kepler led to the appearance of the form of Copernicanism that we accept today. In the half century before Kepler's career began, the understanding of Copernicus and his work was significantly different from the modern one. In successive sections I consider the modern conception of Kepler's contribution to Copernicanism, the most influential sixteenth century view of Copernicus's work and its sequel, Kepler's work from the viewpoint of this tradition, and finally the historical origins of the modern view of Copernicanism.

This paper continues my application of theories of concepts developed in cognitive psychology to clarify issues in Kuhn's mature account of scientific change. I argue that incommensurability is typically neither global nor total, and that the corresponding form of scientific change occurs incrementally. Incommensurability can now be seen as a local phenomenon restricted to particular points in a conceptual framework represented by a set of nodes. The unaffected parts in the framework constitute the basis for continued communication between the communities supporting alternative structures. The importance of a node is a measure of the severity of incommensurability introduced by replacing it. Such replacements occur incrementally so that changes like that from the conceptual structure of Aristotelian celestial physics to the conceptual structure of Newtonian celestial physics occur in small stages over time, and for each change it is in principle possible to identify the arguments and evidence that led historical actors to make the revisions. Thus the process of scientific change is a rational one, even when its beginning and end points are incommensurable conceptual structures. It is also apparent, from a detailed examination of the conceptual structure of astronomy at the time of Copernicus, that the kind of conceptual difficulty identified as incommensurability may occur within a single scientific tradition as well as between two rival traditions.

We question the claim, common since Duhem, that sixteenth century astronomy, and especially the Wittenberg interpretation of Copernicus, was instrumentalistic rather than realistic. We identify a previously unrecognized Wittenberg astronomer, Edo Hildericus (Hilderich von Varel), who presents a detailed exposition of Copernicus’s cosmology that is incompatible with instrumentalism. Quotations from other sixteenth century astronomers show that knowledge of the real configuration of the heavens was unattainable practically, rather than in principle. Astronomy was limited to quia demonstrations, although demonstration propter quid remained the ideal. We suggest that Oslander’s notorious preface to Copernicus expresses these sixteenth century commonplaces rather than twentieth century instrumentalism, and that neither ‘realism’, nor ‘instrumentalism’. in their modern meanings, apply to sixteenth century astronomy.