great modern master of metaphysics there are several points which it will
be useful to consider, such as the unity of opposites, the conception of
the ideas as causes, and the relation of the Platonic and Hegelian
dialectic.

The unity of opposites was the crux of ancient thinkers in the age of
Plato: How could one thing be or become another? That substances have
attributes was implied in common language; that heat and cold, day and
night, pass into one another was a matter of experience 'on a level with
the cobbler's understanding' (Theat.). But how could philosophy explain
the connexion of ideas, how justify the passing of them into one another?
The abstractions of one, other, being, not-being, rest, motion, individual,
universal, which successive generations of philosophers had recently
discovered, seemed to be beyond the reach of human thought, like stars
shining in a distant heaven. They were the symbols of different schools of
philosophy: but in what relation did they stand to one another and to the
world of sense? It was hardly conceivable that one could be other, or the
same different. Yet without some reconciliation of these elementary ideas
thought was impossible. There was no distinction between truth and
falsehood, between the Sophist and the philosopher. Everything could be
predicated of everything, or nothing of anything. To these difficulties
Plato finds what to us appears to be the answer of common sense--that Not-
being is the relative or other of Being, the defining and distinguishing
principle, and that some ideas combine with others, but not all with all.
It is remarkable however that he offers this obvious reply only as the
result of a long and tedious enquiry; by a great effort he is able to look
down as 'from a height' on the 'friends of the ideas' as well as on the
pre-Socratic philosophies. Yet he is merely asserting principles which no
one who could be made to understand them would deny.