The thought that only what is permanent can change has affected Philosophy in different ways at different periods of its history. At the very dawn of Greek Philosophy it was the guiding principle of the Ionian physicists who sought to comprehend the apparent variety of successive phenomena as the transformations of a single bodily reality. As the difficulties inherent in such a materialistic Monism became more apparent, the felt necessity of ascribing unity of some kind to existence led Parmenides and his Eleatic successors to the extreme view that change, being impossible in a permanent homogeneous bodily reality, must be a mere illusion of our deceptive senses. While yet again the later Ionian physicists, and their Sicilian counterpart Empedocles, sought to reconcile the apparent mutability of things with the criticism of Parmenides by the theory that what appears to the senses as qualitative change is in reality the mere regrouping in space of qualitatively unalterable “elements” or “atoms”—μεῖξις διάλλαξίς τε μιγέντων. At a more developed stage of Hellenic thought, the necessity of taking some account of the mutability as well as of the permanence of existence impelled Plato to draw the momentous distinction between two worlds or orders of being—the real, with its eternal unvarying self-identity, and the merely apparent, where all is change, confusion, and instability. In spite of Plato’s manifest failure to make it intelligible how these two orders, the eternal and the temporal, are ultimately connected, this distinction in one form or another has continued ever since to haunt all subsequent metaphysical construction. Even our modern scientific Materialism, with its loudly avowed scorn for all merely metaphysical questions, shows by its constant endeavour to reduce all material existence to a succession of changes in a homogeneous medium, both the persistence with which the intellect demands a permanent background for change, and the difficulty of finding logical satisfaction for the demand.