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|Historical Context=Much of Lakatos’ work was a response to the problems of Popper’s falsificationism, which was expressed in a series of works published between 1935 and the early 1970's. Lakatos rejected the idea that a false prediction was alone grounds for rejecting a theory. Most theories, he pointed out, are born in an “ocean of anomalies” and are therefore falsified from the moment of their inception. For example, Copernican heliocentric astronomy predicts that the stars should change in apparent position as the Earth revolves around the sun, but for three centuries after Copernicus proposed his theory, all attempts to detect this stellar parallax failed. Astronomers nevertheless accepted the theory on other grounds. The failure of Newtonian mechanics to account for the motions of the planet Mercury was known for many decades, during which the theory also wasn't rejected.[[CiteRef::Musgrave and Pigden (2016)]] A well known criticism of falsificationism, the Quine-[[Pierre Duhem|Duhem]]-Quine thesis ,[[CiteRef::Stanford (2016)]][[CiteRef::Duhem (1962)]][[CiteRef::Quine (1951b1951)]], and which Lakatos championed, was that the failure of a prediction could be due to a problem anywhere in the network of theories and auxiliary assumptions responsible for that prediction. Lakatos thus argued that Popper's theory was overly restrictive and inconsistent with much of scientific practice. In scientific practice, Lakatos observed that if a theory is the best available of its kind, it is typically allowed to undergo modifications to account for all data and not rejected.

Following the Duhem-Quine-Duhem thesis, Lakatos recognized that scientific theories could not be appraised individually. Rather, all of the theoretical assumptions bearing on an experimental finding had to be assessed holistically, in terms of what Lakatos called a "research program". Within a research program, not all theoretical assumptions are treated equally. It contains a "hard core" of theoretical assumptions, which are its indispensable constituents. Any modification of the "hard core" results in the creation of a new research program. Adherents of a research program attempt to explain an increasingly wide range of relevant natural phenomena in terms of the core. In so doing, they create a "protective belt" of auxiliary propositions. This expansion of the range of applicability of the program constitutes its "positive heuristic". Scientists committed to a research program defend the "hard core" against change by using their ingenuity as needed to make alterations to the "protective belt" of auxiliary propositions to explain phenomena and avoid falsification of the core. This protection of the hard core is a research program's "negative heuristic". For example, the "hard core" of the Newtonian physics research program would consist of Newton's three laws of motion and Law of Universal Gravitation. The protective belt would include propositions such as "the Earth is an oblate spheroid" or "Neptune is 17 times more massive than Earth". In the nineteenth century, astronomers could not explain the movements of the planet Uranus using Newton's theory and known gravitational influences. Rather than modifying the theory itself, which would have obviated the Newtonian research program, they modified the protective belt by positing the existence of a new planet, whose Newtonian gravitational influence was affecting Uranus. The prediction was a stunning success, as the new planet, to be named Neptune, was discovered in 1846.[[CiteRef::Chalmers (2013)]][[CiteRef::Lakatos (1978a)]]