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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.

Auxiliary propositions that are relevant to the hard core, but are not part of it form a "'''protective belt'''" . 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 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 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)|p. 31-48]]

Lakatos held that a research programme program should be chosen for evaluated in terms of both its “explanatory power” and its “heuristic power”. That is to say that a theory is accepted for '''explanatory power'''; its ability to both explain past and present known phenomena, as well as and its '''heuristic power'''; its ability to be applied successfully explain newly discovered phenomena or to and posit the predict their existence of new phenomena and anomalies. If any evidence is found against a theory, and if the theory possesses both greater heuristic and explanatory powers than known alternatives, Lakatos supposed that its protective belt should be modified and falsificationthereby averted. There thus can be no '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''' 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   These modifications should be “progressive” and intended to save the research programme from degenerating. This, for Lakatos, represents the difference between falsification and rejection.