The Complete Aristotle (eng.)

a rational
principle. Whether these are separated as the parts of the body or
of anything divisible are, or are distinct by definition but by
nature inseparable, like convex and concave in the circumference of
a circle, does not affect the present question.
    Of the irrational element one division seems to be widely
distributed, and vegetative in its nature, I mean that which causes
nutrition and growth; for it is this kind of power of the soul that
one must assign to all nurslings and to embryos, and this same
power to fullgrown creatures; this is more reasonable than to
assign some different power to them. Now the excellence of this
seems to be common to all species and not specifically human; for
this part or faculty seems to function most in sleep, while
goodness and badness are least manifest in sleep (whence comes the
saying that the happy are not better off than the wretched for half
their lives; and this happens naturally enough, since sleep is an
inactivity of the soul in that respect in which it is called good
or bad), unless perhaps to a small extent some of the movements
actually penetrate to the soul, and in this respect the dreams of
good men are better than those of ordinary people. Enough of this
subject, however; let us leave the nutritive faculty alone, since
it has by its nature no share in human excellence.
    There seems to be also another irrational element in the
soul-one which in a sense, however, shares in a rational principle.
For we praise the rational principle of the continent man and of
the incontinent, and the part of their soul that has such a
principle, since it urges them aright and towards the best objects;
but there is found in them also another element naturally opposed
to the rational principle, which fights against and resists that
principle. For exactly as paralysed limbs when we intend to move
them to the right turn on the contrary to the left, so is it with
the soul; the impulses of incontinent people move in contrary
directions. But while in the body we see that which moves astray,
in the soul we do not. No doubt, however, we must none the less
suppose that in the soul too there is something contrary to the
rational principle, resisting and opposing it. In what sense it is
distinct from the other elements does not concern us. Now even this
seems to have a share in a rational principle, as we said; at any
rate in the continent man it obeys the rational principle and
presumably in the temperate and brave man it is still more
obedient; for in him it speaks, on all matters, with the same voice
as the rational principle.
    Therefore the irrational element also appears to be two-fold.
For the vegetative element in no way shares in a rational
principle, but the appetitive and in general the desiring element
in a sense shares in it, in so far as it listens to and obeys it;
this is the sense in which we speak of ‘taking account’ of one’s
father or one’s friends, not that in which we speak of ‘accounting
for a mathematical property. That the irrational element is in some
sense persuaded by a rational principle is indicated also by the
giving of advice and by all reproof and exhortation. And if this
element also must be said to have a rational principle, that which
has a rational principle (as well as that which has not) will be
twofold, one subdivision having it in the strict sense and in
itself, and the other having a tendency to obey as one does one’s
father.
    Virtue too is distinguished into kinds in accordance with this
difference; for we say that some of the virtues are intellectual
and others moral, philosophic wisdom and understanding and
practical wisdom being intellectual, liberality and temperance
moral. For in speaking about a man’s character we do not say that
he is wise or has understanding but that he is good-tempered or
temperate; yet we praise the wise man also with respect to his
state of mind; and of states of mind we call those which merit
praise virtues.

Nicomachean Ethics, Book II
    Translated by W. D. Ross
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    Virtue, then, being of two kinds, intellectual and moral,
intellectual virtue in the main owes both its birth and its growth
to teaching (for which reason it requires experience and time),
while moral virtue comes about as a result of habit, whence also
its name (ethike) is one that is formed by a slight variation from
the word ethos (habit). From this it is also plain that none of the
moral virtues arises in us by