Aristotle here makes a point against Plato and his school, inasmuch as,
starting from the assumption that of the world of sense there could be no
knowledge, no apprehension fixed or certain, and setting over against this a
world of general forms which were fixed and certain, they had nothing with
which to fill this second supposed world except the data of sense as found
in individuals. Plato’s mistake was in confusing the mere ‘this,’ which is
the conceived starting-point of any sensation, but which, like a
mathematical point, has nothing which can be said about it, with individual
objects as they exist and are known in all the manifold and, in fact,
infinite relations of reality. The bare subject ‘this’ presents at the one
extreme the same emptiness, the same mere possibility of knowledge, which is
presented at the other by the bare predicate ‘is.’ But Plato, having an
objection to the former, as representing to him the merely physical and
therefore the passing and unreal, clothes it for the nonce in the various
attributes which are ordinarily associated with it when we say, ‘this man,’
‘this horse,’ only to strip them off successively as data of sensation, and
so at last get, by an illusory process of abstraction and generalisation, to
the ultimate generality of being, which is the mere ‘is’ of bare predication
converted into a supposed eternal substance.