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|Historical Context=Much of Lakatos 's work was intially a disciple response to the problems of [[Karl Popper]], and much of his work was a response to the problems of Popper’s ’s falsificationism, and was expressed in a series of works he published publications 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)]]

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

Lakatos stipulates held that a modification is '''progressive''' if ''all'' research program should be evaluated in terms of the following conditions are met:[[CiteRef::Lakatos (1970)|pp. 31-34]] * The modification has some excess empirical contentboth its explanatory power; its ability to explain known phenomena, i.e. it increases the overall empirical content of a research programme (by making novel predictions and its heuristic power; its ability to successfully explain newly discovered phenomena or increasing to predict their precision and accuracy)existence.* Some of this excess empirical content has been corroborated in experiments and observations.* The modification If any evidence is in organic unity with the rest of the programme. Thusfound against a theory, according to Lakatos, there are three types of regressive (''ad hoc'') modifications. A modification is '''regressive''' and if at least one of the following obtains:[[CiteRef::theory otherwise possesses both greater heuristic and explanatory powers than known alternatives, Lakatos (1971a)|pp. 112, n. 2]]* It does not increase supposed that falsification should be averted by modifying the empirical content of the programme, i.e. it doesnresearch program't make novel predictions or increase their precision/accuracy (''ad hoc₁'')s protective belt.* It introduces excess empirical content, but fails to corroborate any of this excess content empirically (There thus can be no 'crucial experiments'ad hoc₂'').* it is not ; a research program cannot be instantly overthrown by a single experimental finding taken in organic unity with the rest of the programme (''ad hoc₃'')isolation.

A modification to a research program is deemed '''progressive''' 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 programmeprogram. For instance, if the research programme program is “natural selection”Darwin's theory of 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''.