The first edition of The Structure of Scientific Revolutions ended with a chapter titled "Progress through Revolutions", in which Kuhn spelled out his views on the nature of scientific progress. Since he considered problem solving (or "puzzle solving")[1] to be a central element of science, Kuhn saw that for a new candidate paradigm to be accepted by a scientific community,
"First, the new candidate must seem to resolve some outstanding and generally recognized problem that can be met in no other way. Second, the new paradigm must promise to preserve a relatively large part of the concrete problem-solving ability that has accrued to science through its predecessors. Novelty for its own sake is not a desideratum in the sciences as it is in so many other creative fields. As a result, though new paradigms seldom or never possess all the capabilities of their predecessors, they usually preserve a great deal of the most concrete parts of past achievement and they always permit additional concrete problem-solutions besides."
— Kuhn (1962, p. 169)
In the second edition, Kuhn added a postscript in which he elaborated his ideas on the nature of scientific progress. He described a thought experiment involving an observer who has the opportunity to inspect an assortment of theories, each corresponding to a single stage in a succession of theories. What if the observer is presented with these theories without any explicit indication of their chronological order? Kuhn anticipates that it will be possible to reconstruct their chronology on the basis of the theories' scope and content, because the more recent a theory is, the better it will be as an instrument for solving the kinds of puzzle that scientists aim to solve. Kuhn remarked: "That is not a relativist's position, and it displays the sense in which I am a convinced believer in scientific progress."[27][28]
