possibility that a theory has been falsified if an alternative theory is available that they judge credible. If there is not, scientists will continue to adhere to the established conceptual framework. If a paradigm shift has occurred, the textbooks will be rewritten to state that the previous theory has been falsified. Kuhn further developed his ideas regarding incommensurability in the 1980s and 1990s. In his unpublished manuscript The Plurality of Worlds, Kuhn introduces the theory of kind concepts: sets of interrelated concepts that are characteristic of a time period in a science and differ in structure from the modern analogous kind concepts. These different structures imply different “taxonomies” of things and processes, and this difference in taxonomies constitutes incommensurability. This theory is strongly naturalistic and draws on developmental psychology to “found a quasi-transcendental theory of experience and of reality.” Exemplar Kuhn introduced the concept of an exemplar in a postscript to the second edition of The Structure of Scientific Revolutions (1970). He noted that he was substituting the term 'exemplars' for 'paradigm', meaning the problems and solutions that students of a subject learn from the beginning of their education. For example, physicists might have as exemplars the inclined plane, Kepler's laws of planetary motion, or instruments like the calorimeter. According to Kuhn, scientific practice alternates between periods of normal science and revolutionary science. During periods of normalcy, scientists tend to subscribe to a large body of interconnecting knowledge, methods, and assumptions which make up the reigning paradigm (see paradigm shift). Normal science presents a series of problems that are solved as scientists explore their field. The solutions to some of these problems become well known and are the exemplars of the field. Those who study a scientific discipline are expected to know its exemplars. There is no fixed set of exemplars, but for a physicist today it would probably include the harmonic oscillator from mechanics and the hydrogen atom from quantum mechanics. Kuhn on scientific progress The first edition of The Structure of Scientific Revolutions ended with a chapter titled "Progress through Revolutions", in which Kuhn spelled out his views on the nature of scientific progress. Since he considered problem solving to be a central element of science, Kuhn saw that for a new candidate paradigm to be accepted by a scientific community, In the second edition, Kuhn added a postscript in which he elaborated his ideas on the nature