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Lakatos held that a research programme should be chosen for both its “explanatory power” and its “heuristic power”. That is to say that a theory is accepted for its ability to both explain past and present phenomena, as well as its ability to be applied to and posit the existence of future phenomena and anomalies. Given any evidence against a theory, if the theory possesses both greater heuristic and explanatory powers than its counterparts, its protective belt should be allowed to undergo modifications and avoid falsification. These modifications should be “progressive” and intended to save the research programme from degenerating. This, for Lakatos, represents the difference between falsification and rejection.

A research programme is considered “progressive” if it can make predictions later confirmed by experiment, much in line with the Popperian notion of “novel predictions”. On the other hand, if a theory fails to offer such predictions and merely attempts to “save” itself from a disproving instance, it is considered “degenerative”. Lakatos established the following criteria for appraising modifications:

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