The paper revisits the rationality principle from the particular perspective of the unity of social sciences. It has been argued that the principle was the unique law of the social sciences and that accordingly there are no deep differences between them (Popper). It has also been argued that the rationality principle was specific to economics as opposed to the other social sciences, especially sociology (Pareto). The paper rejects these opposite views on the grounds that the rationality principle is (...) strictly metaphysical and does not have the logical force required to deliver interesting deductions. Explanation in the social sciences takes place at a level of specialization that is always higher than that of the principle itself. However, what is peculiar about economics is that it specializes the explanatory rational schemes to a degree unparalleled in history and sociology. As a consequence, there is a backward-and-forward move between specific and general formulations of rationality that takes place in economics and has no analogue in the other social sciences. (shrink)