The Platonic unity of differences or opposites is the beginning of the
modern view that all knowledge is of relations; it also anticipates the
doctrine of Spinoza that all determination is negation. Plato takes or
gives so much of either of these theories as was necessary or possible in
the age in which he lived. In the Sophist, as in the Cratylus, he is
opposed to the Heracleitean flux and equally to the Megarian and Cynic
denial of predication, because he regards both of them as making knowledge
impossible. He does not assert that everything is and is not, or that the
same thing can be affected in the same and in opposite ways at the same
time and in respect of the same part of itself. The law of contradiction
is as clearly laid down by him in the Republic, as by Aristotle in his
Organon. Yet he is aware that in the negative there is also a positive
element, and that oppositions may be only differences. And in the
Parmenides he deduces the many from the one and Not-being from Being, and
yet shows that the many are included in the one, and that Not-being returns
to Being.

In several of the later dialogues Plato is occupied with the connexion of
the sciences, which in the Philebus he divides into two classes of pure and
applied, adding to them there as elsewhere (Phaedr., Crat., Republic,
States.) a superintending science of dialectic. This is the origin of
Aristotle's Architectonic, which seems, however, to have passed into an
imaginary science of essence, and no longer to retain any relation to other
branches of knowledge. Of such a science, whether described as
'philosophia prima,' the science of ousia, logic or metaphysics,
philosophers have often dreamed. But even now the time has not arrived
when the anticipation of Plato can be realized. Though many a thinker has
framed a 'hierarchy of the sciences,' no one has as yet found the higher
science which arrays them in harmonious order, giving to the organic and
inorganic, to the physical and moral, their respective limits, and showing