'One', Aristotle says, can be said in many ways and each being is one. Through a comprehensive analysis of the passages in which Aristotle makes sense of these claims, the book provides a detailed account of how the different ways of being one permeate the domain of being and explores Aristotle's approach to the notion of unity from the ontological, cosmological and dialectical point of view. In rejecting what he regards as an 'archaic' conception of being, Aristotle rejects a corresponding 'archaic' conception of the one and shows how, within the framework of his philosophy of being and substance, different problems and different paradigmatic forms of unity coexist and make of unity a privileged ground for the exercise of philosophical thought.