Plato in the earlier period of his thought, deceived by the beauty of mathematics intelligible in unchanging perfection, conceived of a super-world of ideas, forever perfect and forever interwoven. In his latest phase he sometimes repudiates the notion, though he never consistently banishes it from his thought. His later Dialogues circle round seven notions, namely-The Ideas, The Physical Elements, The Psyche, The Eros, The Harmony, The Mathematical Relations, The Receptacle. I mention them because I hold that all philosophy is in fact an endeavour to obtain a coherent system out of some modification of these notions. They largely explain themselves as to their general meanings, apart from any precise coordination. The Psyche is, of course, the Soul; the Eros is the urge towards the realization of ideal perfection. The Receptacle is expressly stated by Plato to be a difficult notion; so that we may safely put aside easy explanations of it. I explain it to myself as the conception of the essential unity of the Universe conceived as an actuality, and yet in abstraction from the 'life and motion' in which all actualities must partake. If we omit the Psyche and the Eros, we should obtain a static world. The 'life and motion', which are essentials in Plato's later thought, are derived from the operation of these two factors. But Plato left no system of metaphysics.(AI 275)

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