Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Exemplar, in the sense developed by philosopher of science Thomas Kuhn, is a well known usage of a scientific theory. According to Kuhn, scientific practice alternates between periods of normal science and extraordinary/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. Normal science presents a series of "puzzles" that are solved as scientists explore their field. The solution to some of these puzzles become well known and are the exemplars of the field.