Other critics have regarded the final conclusion of the Parmenides either
as sceptical or as Heracleitean. In the first case, they assume that Plato
means to show the impossibility of any truth. But this is not the spirit
of Plato, and could not with propriety be put into the mouth of Parmenides,
who, in this very dialogue, is urging Socrates, not to doubt everything,
but to discipline his mind with a view to the more precise attainment of
truth. The same remark applies to the second of the two theories. Plato
everywhere ridicules (perhaps unfairly) his Heracleitean contemporaries:
and if he had intended to support an Heracleitean thesis, would hardly have
chosen Parmenides, the condemner of the 'undiscerning tribe who say that
things both are and are not,' to be the speaker. Nor, thirdly, can we
easily persuade ourselves with Zeller that by the 'one' he means the Idea;
and that he is seeking to prove indirectly the unity of the Idea in the
multiplicity of phenomena.

We may now endeavour to thread the mazes of the labyrinth which Parmenides
knew so well, and trembled at the thought of them.

The argument has two divisions: There is the hypothesis that

1. One is.
2. One is not.
If one is, it is nothing.
If one is not, it is everything.

But is and is not may be taken in two senses:
Either one is one,
Or, one has being,