 The introduction of the receptacle is an important innovation in Plato’s metaphysics, and clearly a development that takes him beyond the metaphysics of the middle  dialogues.[17]  Little attention was given in those dialogues to the question of what it is to be a sensible particular (other than being something that participates in forms), or what is required of particulars to be such that forms may be exemplified in them. In order to be effective in their role of exemplifying forms, particulars must possess certain general characteristics qua particulars: they must be spatially extended and spatially situated, and they must be constituted of some indeterminate but determinable “stuff” that is subject to determination by way of participating in forms. Although the receptacle does not appear by name in any other of the later dialogues, it clearly has affinities to the concept of the apeiron (indefinite or indeterminate) of the metaphysical scheme in the Philebus. It is impossible, however, to determine the chronological relation between these two dialogues with certainty (see note 7), and thus impossible to infer which of the two schemes Plato might have thought to be the more definitive.