Kuhn states 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. This phase is characterized by several incompatible and incomplete theories. One theory eventually becomes sufficiently accepted that scientists begin to successfully use it methodically. Other knowledge, such as common terminology, common experimental methods and equipment and, to a greater or lesser degree, a common interpretation of scientific phenomena, develops into a paradigm.
After this occurs, 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, but 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 one's knowledge and assumptions to a certain problem.
It is important to note that there are advantages and disadvantages 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, they may be led astray for a very long time before an anomaly 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.
As anomalies -- the failures of the current paradigm to take into account observed phenomenon -- accumulate, their signficance is judged by the practioners of the discipline. Some may be dismissed as errors in observation, others as only requiring small fixes in the current paradigm. Eventually, Kuhn claims, the anomalies become too great for many of the practicing scientists, leading to a loss of faith in the dominant paradigm and ushering in a crisis period whereby revolutionary science is practiced, whereby new paradigms are explored (most of which are fruitless), old axioms are re-examined, and a general instability permeates. After some time, a new paradigm is devised, and thus begins the period where the adherents of the old paradigm and the new paradigm make appeals for their belief systems. In time, the new paradigm replaces the old, and a paradigm shift has occurred.
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) revolves around the Earth. The Ptolemaic theory used 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. 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 of Suess and Wegener's plate tectonics in the 1960s by geologists.
The transition period between paradigms was neither quick nor calm. Because of the subjective nature of deciding which evidence was "significant" and which anomalies could be disregarded, pure logic, Kuhn argued, was never the determiner for the new paradigm. Rather, the usefulness of the new paradigm -- whether it gave further work to normal science -- and whether it seemed, in the eyes of the practitioners, to be a likely solution, played high. And sometimes, as Max Planck observed, and Kuhn quoted (SSR, p. 151):
According to Kuhn, the scientific paradigms before and after a paradigm shift are so 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. Kuhn went so far as to say that they were incommensurable — literally, lacking comparison, untranslateable. New theories were not, as they had thought of before, simply extensions of old theories, but radically new worldviews whose basic axioms and first-principles were different from the theories they replaced.
- "a new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generations grows up that is familiary with it."
This incommensurability applies not just before and after a paradigm shift, but between conflicting paradigms. It is simply not possible, according to Kuhn, to construct an impartial language that can be used to perform a neutral comparison between conflicting paradigms, because the very terms used belong within the paradigm and are therefore different in different paradigms.
This has important implications for other attempts to explain scientific progress. Kuhn (SSR, section XII) points out that the probabilistic tools used by verificationistsists are in themselves inadequate to the task of deciding between conflicting theories, since they are a component of the very paradigms they seek to compare. Similarly, observations intended to falsify a statement will be part of one of the paradigms they seek to compare, and so inadequate to the task. Advocates of such paradigms are in an insidious position: "Though each may hope to convert the other to his way of seeing science and its problems, neither may hope to prove his case. The competition between paradigms is not the sort of battle that can be resolved by proof." (SSR, p. 148).
Kuhn attributes the success of science to the way in which scientists are able to work within a paradigm, removing the need to repeatedly work from first principles. For Kuhn, it is that scientists work within a particular kind of community that explains the astonishing success of science: "The scientific community is a supremely efficient instrument for maximising the number and precision of the problem solved through paradigm change." (SSR, p. 169).
Relevance of SSR
SSR is interpreted by postmodern and post-structuralist thinkers as having undermined the enterprise of science by showing that scientific knowledge is dependent on the culture of groups of scientists rather than on adherence to a specific, definable method. In this regard Kuhn is considered a precursor to the more radical thinking of Paul Feyerabend. Kuhn's work has also been interpreted as blurring the demarcation between scientific and non-scientific enterprises because it describes scientific progress without reference to an idealised scientific method that can be used to distinguish science from non-science. In the years after the publication of The Structure of Scientific Revolutions, debate raged with adherents of Popper's falsificationism such as Imre Lakatos.
On the one hand, logical positivists and many scientists criticize Kuhn's "humanizing" of the scientific process going too far, while the postmodernists in line with Feyerabend have criticized Kuhn for not going far enough. SSR was also embraced by those wishing to discredit or attack the authority of science, such as Creationists and radical environmentalists, and the changing national attitudes about science which occured at the same time of the book's publication (Rachel Carson's Silent Spring was released in the same year), and modern scholars have wondered whether Kuhn himself would have made more explicit that he meant not to create a tool with which to undermine science had he seen what was coming down the pipe.
Outside of the history and philosophy of science, the book's basic tenets have been adopted and co-opted by a variety of fields and disciplines. Changes in politics, society, and business are often expressed in Kuhnian terms, however poor their analog to science may seem to scientists and historians. The terms paradigm and "paradigm shift" have become such notorious buzzwords that they are in many circles considered hollow and empty, and rarely have any strong connection to Kuhn's original text.
In 1987, Kuhn's work was reported as the most heavily cited book of the 20th century, and the Times Literary Supplement labeled it as one of the "The Hundred Most Influential Books Since the Second World War."
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