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 these theories "wins" and this ushers in the second of Kuhn's phases: Normal Science. A scientist working within this phase has an overriding theory (or set of theories) which Kuhn calls a paradigm. Within normal science, the scientist's job is to elaborate, expand, and further justify the paradigm. Eventually however, problems arise, and the theory is modified in an ad hoc way to accommodate experimental evidence which might seem to contradict the original theory. Eventually, the explanatory theory fails to explain some large and important phenomenon, and someone proposes a radical redefinition of the whole set of theories. This is what Khun 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 other way round. Prior to Copernicus there was an elaborate set of mathematical formulas which were used to predict the movements of the 'heavenly bodies' but those formulas were becoming increasingly complex as the movement of those bodies was know with more detail. However, Copernicus offered a radical new explanatory theory, which decreased the complexity of the current theory. Of course, once Copernicus' theory was accepted by other astronomers, it ushered in a new period of normal science.
Another much more recent example is the acceptance of plate tectonics the 1960's by geologists and special relativity in the 1910's by physicists.
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.
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