According to Kuhn the development of a science is not uniform but has alternating ‘normal’ and ‘revolutionary’ (or ‘extraordinary’) phases. The revolutionary phases are not merely periods of accelerated progress, but differ qualitatively from normal science. Normal science does resemble the standard cumulative picture of scientific progress, on the surface at least. Kuhn describes normal science as ‘puzzle-solving’ (1962/1970a, 35–42). While this term suggests that normal science is not dramatic, its main purpose is to convey the idea that like someone doing a crossword puzzle or a chess problem or a jigsaw, the puzzle-solver expects to have a reasonable chance of solving the puzzle, that his doing so will depend mainly on his own ability, and that the puzzle itself and its methods of solution will have a high degree of familiarity. A puzzle-solver is not entering completely uncharted territory. Because its puzzles and their solutions are familiar and relatively straightforward, normal science can expect to accumulate a growing stock of puzzle-solutions. Revolutionary science, however, is not cumulative in that, according to Kuhn, scientific revolutions involve a revision to existing scientific belief or practice (1962/1970a, 92). Not all the achievements of the preceding period of normal science are preserved in a revolution, and indeed a later period of science may find itself without an explanation for a phenomenon that in an earlier period was held to be successfully explained. This feature of scientific revolutions has become known as ‘Kuhn-loss’ (1962/1970a, 99–100).
If, as in the standard picture, scientific revolutions are like normal science but better, then revolutionary science will at all times be regarded as something positive, to be sought, promoted, and welcomed. Revolutions are to be sought on Popper's view also, but not because they add to positive knowledge of the truth of theories but because they add to the negative knowledge that the relevant theories are false. Kuhn rejected both the traditional and Popperian views in this regard. He claims that normal science can succeed in making progress only if there is a strong commitment by the relevant scientific community to their shared theoretical beliefs, values, instruments and techniques, and even metaphysics. This constellation of shared commitments Kuhn at one point calls a ‘disciplinary matrix’ (1970a, 182) although elsewhere he often uses the term ‘paradigm’. Because commitment to the disciplinary matrix is a pre-requisite for successful normal science, an inculcation of that commitment is a key element in scientific training and in the formation of the mind-set of a successful scientist. This tension between the desire for innovation and the necessary conservativeness of most scientists was the subject of one of Kuhn's first essays in the theory of science, “The Essential Tension” (1959). The unusual emphasis on a conservative attitude distinguishes Kuhn not only from the heroic element of the standard picture but also from Popper and his depiction of the scientist forever attempting to refute her most important theories.
This conservative resistance to the attempted refutation of key theories means that revolutions are not sought except under extreme circumstances. Popper's philosophy requires that a single reproducible, anomalous phenomenon be enough to result in the rejection of a theory (Popper 1959, 86–7). Kuhn's view is that during normal science scientists neither test nor seek to confirm the guiding theories of their disciplinary matrix. Nor do they regard anomalous results as falsifying those theories. (It is only speculative puzzle-solutions that can be falsified in a Popperian fashion during normal science (1970b, 19).) Rather, anomalies are ignored or explained away if at all possible. It is only the accumulation of particularly troublesome anomalies that poses a serious problem for the existing disciplinary matrix. A particularly troublesome anomaly is one that undermines the practice of normal science. For example, an anomaly might reveal inadequacies in some commonly used piece of equipment, perhaps by casting doubt on the underlying theory. If much of normal science relies upon this piece of equipment, normal science will find it difficult to continue with confidence until this anomaly is addressed. A widespread failure in such confidence Kuhn calls a ‘crisis’ (1962/1970a, 66–76).
Scientific Change
In The Structure of Scientific Revolutions periods of normal science and revolutionary science are clearly distinguished. In particular paradigms and their theories are not questioned and not changed in normal science whereas they are questioned and are changed in revolutionary science. Thus a revolution is, by definition revisionary, and normal science is not (as regards paradigms). Furthermore, normal science does not suffer from the conceptual discontinuities that lead to incommensurability whereas revolutions do. This gives the impression, confirmed by Kuhn's examples, that revolutions are particularly significant and reasonably rare episodes in the history of science.