one–many problem also called one-and-many problem, the question whether all things are one or many. According to both Plato and Aristotle this was the central question for pre-Socratic philosophers. Those who answered ‘one,’ the monists, ascribed to all things a single nature such as water, air, or oneness itself. They appear not to have been troubled by the notion that numerically many things would have this one nature. The pluralists, on the other hand, distinguished many principles or many types of principles, though they also maintained the unity of each principle. Some monists understood the unity of all things as a denial of motion, and some pluralists advanced their view as a way of refuting this denial. To judge from our sources, early Greek metaphysics revolved around the problem of the one and the many. In the modern period the dispute between monists and pluralists centered on the question whether mind and matter constitute one or two substances and, if one, what its nature is. See also PRE-SOCRATICS, SPINOZ. E.C.H.