It is apparently to this passage of Plato that Aristotle refers in describing the ambiguity of the word οὐσία Metaph. vi. § 2.:  “Now Οὐσία seems to belong most manifestly to bodies:  wherefore animals and plants and their parts we say are οὐσίαι, also natural bodies as fire and water and earth and all such things, and all either parts of these, or products either of parts or the whole, as the heaven and its parts, stars, moon, and sun.  But whether these are the only οὐσίαι or there are others also, or none of these but others of a different kind, is a matter for inquiry.  Some think that the boundaries of bodies, as a surface, and a line and a point and a unit (μονάς), are οὐσίαι, even more so than body and solid.  Further, one class of persons thinks that besides things sensible there is no οὐσία, and another that there are many things, and these more enduring (ἀΐδια), as Plato thinks that the ideas (εἴδη) and the mathematical elements are two kinds of οὐσία, and that the οὐσία of sensible bodies is a third.”