Imre Lakatos, mid-20th Century philosopher of science, argued against the Popperian view of science as the progressive removal of disproven facts. The philosopher Karl Popper had held that no fact could ever be proven through experiments – no matter how many million times Newton may have seen an apple fall from a tree, one can never be certain that the million and first time the apple will not go up instead of down. Therefore, Popper held that the purpose of scientific experimentation is not to prove facts, but to disprove false ideas.
However, Lakatos argued that many scientific theories cannot be falsified through experiment. Scientific Research Programmes have "Hard Cores", ideas absolutely central to the existence of the school. If an experiment seems to disprove a theory which is part of the hard core, the scientists who work within the Research Programme will devise a new theory to explain away the anomaly. Such secondary theories form a protective belt around the hard core, and the creation and destruction of these theories constitutes scientific progress, the day to day work of scientists.
Therefore, if a physicist sees an apple fall up instead of down, he will not conclude that this disproves the law of gravity. Rather, he will bring in a new theory to explain the observation – that a strong wind blew the apple away, or that too much marijuana causes hallucinations.
If a large enough group of scientists, for whatever come to believe that an idea in the hard core is false, then the result will be a fracture within the school, and the disbelievers will form their own Scientific Research Programme, with a different hard core of theories, as well as different negative and positive heuristics.
Lakatos’s Hard Core mirrors Kuhn’s normal science, the principle differences between their theories being that in Lakatos view, different scientific research programmes coexist at the same time, and the process of change is evolutionary, not revolutionary.