The Complete Aristotle (eng.)

does not involve vice and injustice in oneself. In
itself, then, being unjustly treated is less bad, but there is
nothing to prevent its being incidentally a greater evil. But
theory cares nothing for this; it calls pleurisy a more serious
mischief than a stumble; yet the latter may become incidentally the
more serious, if the fall due to it leads to your being taken
prisoner or put to death the enemy.)
    Metaphorically and in virtue of a certain resemblance there is a
justice, not indeed between a man and himself, but between certain
parts of him; yet not every kind of justice but that of master and
servant or that of husband and wife. For these are the ratios in
which the part of the soul that has a rational principle stands to
the irrational part; and it is with a view to these parts that
people also think a man can be unjust to himself, viz. because
these parts are liable to suffer something contrary to their
respective desires; there is therefore thought to be a mutual
justice between them as between ruler and ruled.
    Let this be taken as our account of justice and the other, i.e.
the other moral, virtues.

Nicomachean Ethics, Book VI
    Translated by W. D. Ross
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1
    Since we have previously said that one ought to choose that
which is intermediate, not the excess nor the defect, and that the
intermediate is determined by the dictates of the right rule, let
us discuss the nature of these dictates. In all the states of
character we have mentioned, as in all other matters, there is a
mark to which the man who has the rule looks, and heightens or
relaxes his activity accordingly, and there is a standard which
determines the mean states which we say are intermediate between
excess and defect, being in accordance with the right rule. But
such a statement, though true, is by no means clear; for not only
here but in all other pursuits which are objects of knowledge it is
indeed true to say that we must not exert ourselves nor relax our
efforts too much nor too little, but to an intermediate extent and
as the right rule dictates; but if a man had only this knowledge he
would be none the wiser e.g. we should not know what sort of
medicines to apply to our body if some one were to say ‘all those
which the medical art prescribes, and which agree with the practice
of one who possesses the art’. Hence it is necessary with regard to
the states of the soul also not only that this true statement
should be made, but also that it should be determined what is the
right rule and what is the standard that fixes it.
    We divided the virtues of the soul and a said that some are
virtues of character and others of intellect. Now we have discussed
in detail the moral virtues; with regard to the others let us
express our view as follows, beginning with some remarks about the
soul. We said before that there are two parts of the soul-that
which grasps a rule or rational principle, and the irrational; let
us now draw a similar distinction within the part which grasps a
rational principle. And let it be assumed that there are two parts
which grasp a rational principle-one by which we contemplate the
kind of things whose originative causes are invariable, and one by
which we contemplate variable things; for where objects differ in
kind the part of the soul answering to each of the two is different
in kind, since it is in virtue of a certain likeness and kinship
with their objects that they have the knowledge they have. Let one
of these parts be called the scientific and the other the
calculative; for to deliberate and to calculate are the same thing,
but no one deliberates about the invariable. Therefore the
calculative is one part of the faculty which grasps a rational
principle. We must, then, learn what is the best state of each of
these two parts; for this is the virtue of each.
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2
    The virtue of a thing is relative to its proper work. Now there
are three things in the soul which control action and
truth-sensation, reason, desire.
    Of these sensation originates no action; this is plain from the
fact that the lower animals have sensation but no share in
action.
    What affirmation and negation are in thinking, pursuit and
avoidance are in desire; so that since moral virtue is a state of
character concerned with choice, and choice is deliberate desire,
therefore both the reasoning must be true and the desire right, if
the choice is to be good, and the latter must pursue just what