principium individuationis the cause (or basis) of individuality in individuals; what makes something individual as opposed to universal, e.g., what makes the cat Minina individual and thus different from the universal, cat. Questions regarding the principle of individuation were first raised explicitly in the early Middle Ages. Classical authors largely ignored individuation; their ontological focus was on the problem of universals. The key texts that originated the discussion of the principle of individuation are found in Boethius. Between Boethius and 1150, individuation was always discussed in the context of more pressing issues, particularly the problem of universals. After 1150, individuation slowly emerged as a focus of attention, so that by the end of the thirteenth century it had become an independent subject of discussion, especially in Aquinas and Duns Scotus.
Most early modern philosophers conceived the problem of individuation epistemically rather than metaphysically; they focused on the discernibility of individuals rather than the cause of individuation (Descartes). With few exceptions (Karl Popper), the twentieth century has followed this epistemic approach (P. F. Strawson).
See also INDIVIDUATION, METAPHYSIC. J.J.E.G.