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|Historical Context=Much of Lakatos's work was intially a response to the problems disciple of [[Karl Popper]]’s , and much of his work was a response to the problems of Popper’s falsificationism, and was expressed in a series of publications works he 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 [[Pierre Duhem|Duhem]]-Quine thesis,[[CiteRef::Stanford (2016)]][[CiteRef::Duhem (1962)]][[CiteRef::Quine (1951)]] 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 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, as parts of what he called a '''"research program'''". Within a research program, not all theoretical assumptions are treated equally. The indispensable central theoretical assumptions of a research program are its '''"hard core'''". Any modification of the "hard core " constitutes the abandonment of the research program and the creation of a new one. 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)]]

Auxiliary propositions Lakatos held that are relevant to the hard core, but are not part of it form a '''protective belt'''research programme should be chosen for both its “explanatory power” and its “heuristic power”. Adherents of That is to say that a research program attempt theory is accepted for its ability to both explain an increasingly wide range of relevant natural past and present phenomena in terms of the core. In so doing, they add to the protective belt of auxiliary propositions. This expansion of the range of applicability of the program constitutes as well as its '''positive heuristic'''. Scientists committed ability to a research program defend the hard core against change by using their ingenuity as needed to make alterations be applied to and posit the protective belt existence of auxiliary propositions to explain new phenomena and avoid falsification of the coreanomalies. This protection of the hard core If any evidence is found against a research program's '''negative heuristic'''. For exampletheory, and if the hard core of the Newtonian physics research program would consist of Newton's three laws of motion theory possesses both greater heuristic and Law of Universal Gravitation. The explanatory powers than known alternatives, its protective belt would include propositions such as "should be modified and falsification. These modifications should be “progressive” and intended to save the Earth is an oblate spheroid" or "Neptune is 17 times more massive than Earth"research programme from degenerating. In the nineteenth centuryThis, for Lakatos, astronomers could not explain represents the movements of the planet Uranus using Newton's theory difference between falsification and known gravitational influencesrejection. Rather than modifying the theory itself, which would have obviated the Newtonian A research programprogramme is considered “progressive” if it can make predictions later confirmed by experiment, they modified the protective belt by positing much in line with the existence Popperian notion of a new planet, whose Newtonian gravitational influence was affecting Uranus''novel predictions''. The prediction was a stunning success, as On the new planetother hand, if a theory fails to be named Neptuneoffer such predictions and merely attempts to “save” itself from a disproving instance, was discovered in 1846.[[CiteRef::Chalmers (2013)]][[CiteRef::Lakatos (1978a)|pit is considered “degenerative”. 31-48]]

Lakatos held stipulates that a modification is '''progressive''' if ''all'' of the following conditions are met: * The modification has some excess empirical content, i.e. it increases the overall empirical content of a research program should be evaluated programme (by making novel predictions or increasing their precision and accuracy).* Some of this excess empirical content has been corroborated in terms experiments and observations.* The modification is in organic unity with the rest of the programme. Thus, according to Lakatos, there are three types of both its regressive (''ad hoc'explanatory power') modifications. A modification is ''; its ability to explain known phenomena, and its 'regressive''heuristic power'if at least one of the following obtains:* It does not increase the empirical content of the programme, i.e. it doesn''; its ability to successfully explain newly discovered phenomena t make novel predictions or to predict increase their existenceprecision/accuracy (''ad hoc₁''). If * It introduces excess empirical content, but fails to corroborate any evidence of this excess content empirically (''ad hoc₂'').* it is found against a theory, and if not in organic unity with the rest of the theory possesses both greater heuristic and explanatory powers than known alternatives, Lakatos supposed that its protective belt should be modified and falsification thereby averted. There thus can be no programme (''ad hoc₃'crucial experiments'; a research program cannot be instantly overthrown by a single experimental finding taken in isolation).

A modification to a research program is deemed '''progressive''The term ' if it increases the research program's overall empirical content; for example, by making novel predictions or increasing the precision and accuracy of prediction, if some of this new empirical content has been corroborated by experiments and observations, and if the modification is in organic unity with the rest of the program. A modification is deemed '''regressive'''if it does not increase the empirical content of a program by making novel predictions or improving its accuracy, or if it introduces excess empirical content, but fails to corroborate any of this excess content empirically, or if it is not in organic unity with the rest of the program. The term 'organic unity' is intended to mean that modifications should be contiguous with the rest of the programprogramme. For instance, if the research program programme is Darwin's theory of natural selection“natural selection”, a modification which adds the proposition “extra-terrestrial beings intervened with human evolution” would not be contiguous – not in organic unity – with the rest of the research programme and is therefore ''ad hoc₃''.