In attempting to prove that the soul has three separate faculties, Plato
takes occasion to discuss what makes difference of faculties. And the
criterion which he proposes is difference in the working of the faculties.
The same faculty cannot produce contradictory effects. But the path of
early reasoners is beset by thorny entanglements, and he will not proceed a
step without first clearing the ground. This leads him into a tiresome
digression, which is intended to explain the nature of contradiction.
First, the contradiction must be at the same time and in the same relation.
Secondly, no extraneous word must be introduced into either of the terms in
which the contradictory proposition is expressed: for example, thirst is
of drink, not of warm drink. He implies, what he does not say, that if, by
the advice of reason, or by the impulse of anger, a man is restrained from
drinking, this proves that thirst, or desire under which thirst is
included, is distinct from anger and reason. But suppose that we allow the
term 'thirst' or 'desire' to be modified, and say an 'angry thirst,' or a
'revengeful desire,' then the two spheres of desire and anger overlap and
become confused. This case therefore has to be excluded. And still there
remains an exception to the rule in the use of the term 'good,' which is
always implied in the object of desire. These are the discussions of an
age before logic; and any one who is wearied by them should remember that
they are necessary to the clearing up of ideas in the first development of
the human faculties.

The psychology of Plato extends no further than the division of the soul
into the rational, irascible, and concupiscent elements, which, as far as
we know, was first made by him, and has been retained by Aristotle and
succeeding ethical writers. The chief difficulty in this early analysis of
the mind is to define exactly the place of the irascible faculty (Greek),
which may be variously described under the terms righteous indignation,
spirit, passion. It is the foundation of courage, which includes in Plato