Incomplete summary of Thomas S. Kuhn's _The Structure of Scientific Revolutions_ Notes are for my purposes only; use at your own risk December 2004 ***** Preface Kuhn started out in physics, then shifted to history of science and then philosophy, trying to understand what the nature of science is and to explain how/if science is successful in ways that social science isn't. The key puzzle piece for Kuhn is paradigms: "universally recognized scientific achievements that for a time provide model problems and solutions to a community of practitioners". SSR is a short essay-form sketch of the history of science (physics) and Kuhn's conclusions about science and scientific change in light of actual history. ***** Chapter 1: Introduction: A Role for History The history of science as presented in textbooks is ahistorical and inaccurate. In contrast, (new) historians of science attempt to present the history of science in its own time rather than in relation to modern science. Science/theories can not be understood independent of their context; "an apparently arbitrary element, compounded of personal and historical accident, is always a formative ingredient of the beliefs espoused by a given scientific community at a given time." When science is studied in this historical way, historians find that it is difficult to pinpoint instances in scientific progress and difficult to distinguish scientific theories from pseudoscientific ones; this leads to skepticism about scientific progress (theories replacement is not progress, but rather a loss shift between incommensurable world views) and the scientific-ness of current theories ("that those once current views of nature were, on the whole, neither less scientific nor more the product of human idiosyncrasy than those current today"). In the early stages of sciences, there is competition between multiple distinct views. Next comes the main stage of normal science in which scientists work within an accepted paradigm. Sometimes crisis arises (intractable problems), some scientists advance a new paradigm, and eventually a scientific revolution (community-wide rejection of the formerly-accepted theory in favor of an incompatible theory) occurs. Such competition and revolutions are necessary to science. ***** Chapter 2, The Route to Normal Science Normal science is research grounded in past scientific achievements which are accepted by a scientific community as the foundation for further science. The achievements are paradigms; they are those achievements which have met with unprecedented success and which leave open-ended problems to be solved within the traditions they suggest. Before a period of normal science, a science/theory begins with a pre-paradigm stage in which there is no generally accepted view/method; examples include the study of electricity pre-Franklin, motion pre-Aristotle, heat pre-Black, chemistry pre-Boyle/Boerhaave, and historical geology pre-Hutton. The results of this early stage are not scientific, and the emergence of consensus (an initial paradigm) is slow and difficult. Once the stage of normal science is reached, a scientific community emerges, and scientists write for their colleagues upon a shared foundation. ***** Chapter 3, The Nature of Normal Science Normal science involves the actualization of the promise of a paradigm "by extending the knowledge of those facts that the paradigm displays as particularly revealing, by increasing the extent of the match between those facts and the paradigm's predictions, and by further articulation of the paradigm itself." Articulating the paradigm theory involves determining universal constants (Avogadro's number), discovering quantitative laws Boyle's law), and discovering which/how additional phenomena can be encompassed by the theory. ***** Chapter 4, Normal Science as Puzzle Solving Normal science is full of puzzles, and scientists of normal science are puzzle solvers. Normal science includes criteria for choosing problems/puzzles that can be solved within the accepted paradigm; normal science consists of solving these puzzles and tying up loose ends, not at unexpected novelty. ***** Chapter 5, The Priority of Paradigms Paradigms, not rules, can and do determine normal science. In support of this claim Kuhn offers that rules are difficult to discover in practice, that scientists learn through paradigms rather than rules, that rules become important to normal science when (and only when) paradigms are insecure, and that paradigms rather than rules can explain the diversity of scientific specialties. ***** Chapter 6, Anomaly and the Emergence of Scientific Discoveries When normal science is at its most successful, it does not find unexpected novelties; instead, observed phenomena are in conformance with and easily explained by the paradigm theory. However, although normal science does not aim at finding novelties/anomalies, such anomalies are sometimes found. Normal science is an endeavor which leads to the discovery of anomalies. Initially, anomalies (or observation/perception) can be easily explained or even ignored, but not after repeated exposure to less tractable anomalies. Science, or at least some scientists, then further study the area of anomaly, adjusting (often radically) the paradigm theory so the anomalous becomes expected. There is no single moment of discovery/recognition of an anomaly which leads to a paradigm change. Examples from history include Priestley/Lavoisier's discovery of oxygen in the 1770s, Roentgen's discover of X-rays in late 1895, and the discovery of the Leyden jar. The common characteristics to these examples include: "the previous awareness of anomaly, the gradual and simultaneous emergence of both observational and conceptual recognition, and the consequent change of paradigm categories and procedures often accompanied by resistance." ***** Chapter 7, Crisis and the Emergence of Scientific Theories History shows that scientific revolutions (Ptolemaic -> Copernican, phlogiston -> oxygen, Newtonian -> relativity) are preceded by a growing crisis, the failure of the previous paradigm to solve puzzles and deal with anomalies. The well-supported development of a novel theory is a direct response to crisis. In times of crisis, scientists are motivated to look for alternatives. Further, it is only in times of crisis that scientists (as a community) can afford to give alternatives a chance. "[The] invention of alternatives is just what scientists seldom undertake except during the pre-paradigm stage of their science's development and at very special occasions during its subsequent evolution. So long as the tools a paradigm supplies continue to prove capable of solving the problems it defines, science moves fastest and penetrates most deeply through confident employment of these tools.... retooling is an extravagance to be reserved for the occasion that demands it. The significance of crises is the indication they provide that an occasion for retooling has arrived." ***** Chapter 8, The Response to Crisis Scientists do not reject accepted paradigms whenever they encounter anomalies; on the contrary, it is only when there is another promising paradigm to accept as a replacement that a paradigm is abandoned. (To abandon a paradigm without accepting another is to abandon science.) Normal science is full of puzzles (anomalies); any puzzle can be seen as the source of crisis, but few are. Often scientists will stick with a paradigm, in the face of anomalies, for as long as there are many other problems which can be solved within the paradigm. It takes more than just an anomaly to evoke crisis. "All crises begin with the blurring of a paradigm and the consequent loosening of the rules for normal research." And, typically as a result of drawing attention to the area of an anomaly, a "crisis may end with the emergence of a new candidate for paradigm and with the ensuing battle over its acceptance." The shift from one paradigm to another is not a cumulative process, but a reconstruction. "The resulting transition to a new paradigm is scientific revolution". ***** Chapter 9, The Nature and Necessity of Scientific Revolutions Scientific revolutions, like political revolutions, are inaugurated by the sense that the established paradigm no longer functions adequately. Revolutions make changes that could not be made within the established paradigm. Paradigm choice cannot be settled by logic and experiment alone. New theories are logically incompatible with the old ones; they are necessarily and irreconcilably different. Paradigms differ not only in their theories, but also in their methods and standards. Paradigms provide both a map and directions for map-making. Adopting a new paradigm may necessitate redefine (an area of) science. "The normal-science tradition that emerges from a scientific revolution is not only incompatible but often actually incommensurable with that which has gone before." There are no external standards to judge between theories. Standards are neither raised nor lowered but simply changed with the adoption of new paradigms. (Thus it is not unscientific to speak of the occult gravity in the Newtonian paradigm, although it may have been pre-Newton.) When scientists of different paradigms debate one another, they are in some sense not speaking the same language. ***** Chapter 10, Revolutions as Changes of World View Paradigm changes cause scientists to see (perceive, observe, interpret) the world differently, and so it seems that "after a revolution scientists are responding to a different world." The world itself does not change, but at least in some areas the way scientists work in and relate to the world does. Scientific observations are made from within a paradigm. Where a scientist of one paradigm sees a star or constrained fall or phlogiston or a chemical compound, a scientist of another paradigm sees a planet or a pendulum or oxygen or a physical mixture. ***** Chapter 11, The Invisibility of Revolutions Science is presented in textbooks, and textbooks present science in an ahistorical fashion which makes revolutions appear nearly invisible. The goal of science textbooks is to teach contemporary science and the current set of paradigms, not past paradigms nor the history of transition between paradigms. To do so, textbooks may rewrite history by presenting episodes of history ahistorically (backwards), applying current standards to past scientists, tracing the history of only that science which led to the accepted paradigm, and by inaccurately portraying past scientists as aiming at the current paradigm. "The result is a persistent tendency to make the history of science look linear or cumulative"; science history and revolutions do not fit in textbook science. ***** Chapter 12, The Resolution of Revolutions Adopting a new paradigm does not merely consist in comparing it to nature (as puzzle-solving in normal science does), but in comparing rival paradigms with each other. "It makes a great deal of sense to ask which of two actual and competing theories fit the facts better. Though neither Priestley's nor Lavoisier's theory, for example, agreed precisely with existing observations, few contemporaries hesitated more than a decade in concluding that Lavoisier's theory provided the better fit of the two." There is no easy way to decide between paradigms. Scientists may convert one another, but they cannot prove their paradigms. Competing paradigms are incommensurable; scientists of different paradigms talk past one another and cannot communicate fully without conversion. Proponents of differing paradigms have different standards, disagree about what problems must be resolved, use the same terms differently (motion, mass, etc), and see the world differently so are practicing science in different worlds. In times of crisis, first a few (typically younger) scientists who have been focused on the are of anomaly begin to work in a new paradigm. Eventually, and not all at once, all of the (living) scientists in their sub-field have been converted. This is a slow process, as normal science depends upon scientists being very committed to the existing paradigm and not abandoning it too easily. In it's early stages, a new paradigm is not advanced enough (has not solved enough problems) to merit replacing the older paradigm. A few scientists are converted by appeal to simplicity and other aesthetic reasons. The decision between paradigms "must be based less on past achievement than on future promise.... A decision of that kind can only be made on faith." As more and more scientists convert, more work is done to advance the theory to a state where it is attractive to those still in the older paradigm. Crucial experiments, improved quantitative prediction, and the success of surprising predictions finally convert the remaining scientists. ***** Chapter 13, Progress through Revolutions Science and progress are tied together. We call science those fields in which there is progress. When we ask whether a field (psychology) is a science, the question may be whether that field is progressive. "Does a field make progress because it is a science, or is it a science because it makes progress?" Finally, why is there progress in science in ways that there is no progress in art, political theory, etc? What makes such progress possible? Within any paradigm (scientific or not), there is progress, but we fail to see progress in non-sciences because there are so many competing schools. Normal science is somewhat unique in that the scientific community is mostly working from a single paradigm; solving the problems defined by the paradigm is necessarily progress. Other differences between science and other endeavors include that scientists are isolated from larger society in that they work for their colleagues, can choose to work on those puzzles which are likely be resolvable, and learn from ahistorical textbook literature rather than the classics which would make science look less cumulative. Paradigm shifts do not seem to be entirely progressive, although there is progress in some areas. A new paradigm solves some problem which the older paradigm had not, while often preserving a large part of the advances of the older paradigm. A new paradigm may grow in depth or breadth, but might not grow in both. Science may progress by solving more problems and increasing precision, but we may have to give up the idea that "changes of paradigm carry scientists and those who learn from them closer and closer to the truth." The development process of science that has been described is that of evolution from beginnings, not of evolution toward anything. A Darwinian account can explain the success of science without the need for teleological goals. ***** Postscript -- 1969 The postscript (published 7 years after SSR) is an attempt to clear up some misunderstandings, including: the multiple senses of the word paradigm, the reading that science is a subjective/irrational/relativistic enterprise, incommensurability and translation, and the application of SSR to non-sciences. 1. Paradigms and Community Structure: "A paradigm is what the members of a scientific community share, and, conversely, a scientific community consists of men who share a paradigm." Members of a scientific community (more so than of non-scientific communities) share a common education and methodology. Paradigms govern not subject matter, but a group of scientists. Revolutions need not be large or seem revolutionary to those outside a small group. Revolutions need not be preceded by crises, although they usually are. 2. Paradigms as the Constellation of a Group of Commitments: Scientists in a community share a "disciplinary matrix" which includes symbolic generalizations, metaphysical parts, values, and exemplars of problem-solutions. Common values concern predictions and accuracy, the ability to form puzzles, simplicity, etc. Although scientists share these values, they may apply them and get different results. This does not imply subjectivity or irrationality. "[I]ndividual variability in the application of shared values may serve functions essential to science." 3. Paradigms as Shared Examples: Scientific knowledge is not embedded in rules and theory alone; exemplars play an important role. Science students learn to solve new problems by recognizing them as being similar to exemplar problems which they know how to solve. "Tacit knowledge" is learned not by acquiring rules for doing science, but by doing science. 4. Tacit Knowledge and Intuition: Science does not rest on unanalyzable individual intuitions. The intuitions it rests on are shared, not individual, intuitions, and they are not unanalyzable. Member of a (scientific) community learn (through textbooks, exemplars, etc) to see in the same way. Further, they learn to see in a useful (successful) way. 5. Exemplars, Incommensurability, and Revolutions: Scientists of incommensurable theories are unable to communicate in a few areas, but not all. Theory-choice debates cannot be formulated as logical/mathematical proofs, but there are good (non-subjective) reasons for theory-choice. There is no neutral algorithm which leads each individual to the same decision, but there are shared values which ensure that the majority of the community will be persuaded to accept a common theory. Persuasion is difficult because scientists of different theories spake from incommensurable viewpoints and use the same words differently. To communicate, scientists must become translators. To accept a new theory is to become not a translator, but a native speaker. 6. Revolution and Relativism: Scientists of different theories belong to different "language-culture" communities, but this need not imply relativism (that multiple groups are right). There is progress in science; given two theories from the history of science, we can determine which is the later (more successful) theory based on accuracy of prediction, the number of problems solved, simplicity, scope, etc. "One often hears that successive theories grow ever closer to ... the truth." Kuhn does not think there is, or need be, a match between ontology of a theory and a "real" ontology of nature. 7. The Nature of Science: What scientists do and what scientists ought do are linked; studying what scientists do/have done is useful. "Though scientific development may resemble that in other fields more closely than has often been supposed, it is also strikingly different": there are few competing schools, the scientific community judges its own work, scientists learn from textbooks, have puzzle-solving as a goal, have a shared value system, etc. There is need to study not just the community structure of science, but also of non-scientific fields.