A comprehensible model is proposed aimed at an analysis of the reasons for theory change in science. According to model the origins of scientific revolutions lie not in a clash of fundamental theories with facts, but of “old” fundamental theories with each other, leading to contradictions that can only be eliminated in a more general theory. The model is illustrated with reference to physics in the early 20th century, the three “old” theories in this case being Maxwellian electrodynamics, statistical mechanics (...) and thermodynamics. Modern example, referring to general relativity and quantum field theory, is considered. Key words: Popper, Kuhn, Stepin, Einstein . (shrink)