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Thomas Samuel Kuhn (July 18, 1922 - June 17, 1996) wrote extensively on the history of science, and developed several important notions in the philosophy of science.

He is most famous for his book The Structure of Scientific Revolutions[?] in which he presented the idea that science does not "evolve gradually toward truth", but instead undergoes periodic revolutions which he calls paradigm shifts.

Kuhn's analysis of the history of science suggests to him that the practice of science comes in three phases. The first phase, which is undergone only once, is the pre-scientific phase, in which there is no consensus on any theory of explanation. This phase is generally characterized by several incompatible and incomplete theories.

Eventually one of the theories becomes so generally accepted that practicioners of science begin to successfully use it in methodical ways. Other knowledge such as common terminoligy, common experimental methods and equipment, and to a greater or lesser degree a common interpretation of scientific phenomina all develop into a paradigm. After this ocurrs, normal science begins. Kuhn explains that normal science is what scientists spend most of their careers doing. It can only be performed under a specific paradigm, and its goal is to explain and expand the paradigm. Kuhn explained normal science as a process of puzzle solving. Armed with knowledge provided by a paradigm, scientists can begin to make well founded and trusted assumptions about what they are studying. This may seem to violate long held ideals about objectivity in science, however it is extremely difficult to study anything without making at least a few basic assumptions (see 'Naive Empiricism). The challenge of normal science is to see how well one can apply all his/her knowledge and assumptions to a certain problem.

It is important to note that there are two main advantages and disadvanteges to using a paradigm to make assumptions about a particular topic. The advantage is that if all scientists are using similar assumptions, then their methods, terminology, and analyses will all be very homogenous and easily compared; it allows for greater communication and cooperation between people. However, if many scientists use similar assumptions, which are not entirely correct, it may lead scienctists astray for a very long time before an anomoly occurs which brings attention to the problem. When this happens there is usually a period of disagreement between scientists, and the theory is modified in an ad hoc way to accommodate experimental evidence which might seem to contradict the original theory.

Eventually, the current explanatory theory fails to explain some phenomenon or group thereof, and someone proposes a replacement or redefinition of the theory. This is what Kuhn calls a paradigm shift, ushers in a new period of revolutionary science. Kuhn believes that all scientific fields go through these paradigm shifts multiple times, as new theories supplant the old.

One well known Kuhnian example involves Copernicus' suggestion that the earth revolves around the sun, rather than the Ptolemaic suggestion that the Sun (and the other planets and stars) revolved around the Earth. Prior to Copernicus there was an elaborate set of epicycles (circles on top of circles) which were used to predict the movements of the 'heavenly bodies'. Ptolemy's original epicyclic combinations were, by the Middle Ages, becoming noticeably less adequate, and 'fixes' by later astronomers were more and more elaborate. Copernicus offered a return to an alternative view (suggested by many in Antiquity) but with rather better data to support it; this new account decreased the complexity of theory necessary to account for the available observations. Of course, once Copernicus' theory was accepted by other astronomers, it ushered in a new period of 'normal science'. Refinements added by Kepler and Newton adhered to the new paradigm.

Other more recent examples are the acceptance of Einstein's general relativity to replace Newton's account of gravity in the 1920s and 1930s and Suess and Wegener's plate tectonics the 1960s by geologists.

According to Kuhn, the science before and after a paradigm shift are so much different that their theories are incomparable - the paradigm shift does not just change a single theory, it changes the way that words are defined, the way that the scientists look at their subject, and perhaps most importantly the questions that are considered valid, and the rules used to determine the truth of a particular theory.

It is important to understand that Kuhn's contribution to the philosophy of science is more precisely an observation about the sociology of science as practiced by humans. We can imagine a practice of science by Little Green Men (LGM) somewhere whose characteristic way of handling science, changes in theory, new data, etc. do not work the way Kuhn suggested in Structure. Thus we can see that a philosophy of science which applies to both human practice and to LGM practice could not be, as it is said by many, what Kuhn claimed. A reading of Structure makes clear that he, himself, was not making such a claim about the nature of science. There is an old observation by Max Planck:

"An important scientific innovation rarely makes its way by gradually winning over and converting its opponents.... What does happen is that its opponents gradually die out, and that the growing generation is familiarised with the idea from the beginning."
Plank's comment predates Kuhn. However, it is not germane to fundamental questions about the nature of science and scientific method. Popper's falsifiability is a much better candidate for a characteristic of science which will be invariant across all those doing science (i.e., both humans and the LGM and everyone else will have to do it the same way).

Kuhn is very often misunderstood to have said something 'post-modern' about the nature of science. That is -- approximately -- that he showed that scientific truth changes with the scientist or the group of scientists doing it. This is further extended, in further misunderstanding, to suggest that such things as cultural relativity, prejudice, and so on are at the heart of science. This deconstruction of science has become common among some observers, but cannot be extended very far since science differs in fundamental ways from other intellectual enterprises. It is fundamentally a misunderstanding of the nature of science and, under the best conditions, its practice.

Like all human endeavors, science has shown itself fully susceptible to fad, fashion, delusion, and prejudice. Stephen Jay Gould's The Mismeasure of Man is a book-length exposure of this regrettable fact in regard to claims -- 'scientific' claims -- about the relative intelligence of groups based on ethnicity, gender, etc.

Science, at least in theory, has a method of distinguishing between scientific claims based on fad and fashion and those which can actually survive experimental testing (see Scientific method). This potential is not always promptly applied, but when applied, with consistent and methodical reasoning, distinguishes 'science' from 'non-science.' It should also rescue Kuhn's account of the sociology of human science from its too common misunderstandings, but it too often doesn't.

Selected works : Thomas S. Kuhn, The Structure of Scientific Revolutions, University of Chicago Press, Chicago, 1962 - ISBN 0226458083

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