f 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 how they all work together in the world
and in man.

Plato arranges in order the stages of knowledge and of existence. They
are the steps or grades by which he rises from sense and the shadows of
sense to the idea of beauty and good. Mind is in motion as well as
at rest (Soph.); and may be described as a dialectical progress which
passes from one limit or determination of thought to another and back
again to the first. This is the account of dialectic given by Plato in
the Sixth Book of the Republic, which regarded under another aspect
is the mysticism of the Symposium. He does not deny the existence of
objects of sense, but according to him they only receive their true