The Complete Aristotle (eng.)

And a possession may be defined as an instrument of
action, separable from the possessor.
V
    But is there any one thus intended by nature to be a slave, and
for whom such a condition is expedient and right, or rather is not
all slavery a violation of nature?
    There is no difficulty in answering this question, on grounds
both of reason and of fact. For that some should rule and others be
ruled is a thing not only necessary, but expedient; from the hour
of their birth, some are marked out for subjection, others for
rule.
    And there are many kinds both of rulers and subjects (and that
rule is the better which is exercised over better subjects—for
example, to rule over men is better than to rule over wild beasts;
for the work is better which is executed by better workmen, and
where one man rules and another is ruled, they may be said to have
a work); for in all things which form a composite whole and which
are made up of parts, whether continuous or discrete, a distinction
between the ruling and the subject element comes to fight. Such a
duality exists in living creatures, but not in them only; it
originates in the constitution of the universe; even in things
which have no life there is a ruling principle, as in a musical
mode. But we are wandering from the subject. We will therefore
restrict ourselves to the living creature, which, in the first
place, consists of soul and body: and of these two, the one is by
nature the ruler, and the other the subject. But then we must look
for the intentions of nature in things which retain their nature,
and not in things which are corrupted. And therefore we must study
the man who is in the most perfect state both of body and soul, for
in him we shall see the true relation of the two; although in bad
or corrupted natures the body will often appear to rule over the
soul, because they are in an evil and unnatural condition. At all
events we may firstly observe in living creatures both a despotical
and a constitutional rule; for the soul rules the body with a
despotical rule, whereas the intellect rules the appetites with a
constitutional and royal rule. And it is clear that the rule of the
soul over the body, and of the mind and the rational element over
the passionate, is natural and expedient; whereas the equality of
the two or the rule of the inferior is always hurtful. The same
holds good of animals in relation to men; for tame animals have a
better nature than wild, and all tame animals are better off when
they are ruled by man; for then they are preserved. Again, the male
is by nature superior, and the female inferior; and the one rules,
and the other is ruled; this principle, of necessity, extends to
all mankind.
    Where then there is such a difference as that between soul and
body, or between men and animals (as in the case of those whose
business is to use their body, and who can do nothing better), the
lower sort are by nature slaves, and it is better for them as for
all inferiors that they should be under the rule of a master. For
he who can be, and therefore is, another’s and he who participates
in rational principle enough to apprehend, but not to have, such a
principle, is a slave by nature. Whereas the lower animals cannot
even apprehend a principle; they obey their instincts. And indeed
the use made of slaves and of tame animals is not very different;
for both with their bodies minister to the needs of life. Nature
would like to distinguish between the bodies of freemen and slaves,
making the one strong for servile labor, the other upright, and
although useless for such services, useful for political life in
the arts both of war and peace. But the opposite often happens—that
some have the souls and others have the bodies of freemen. And
doubtless if men differed from one another in the mere forms of
their bodies as much as the statues of the Gods do from men, all
would acknowledge that the inferior class should be slaves of the
superior. And if this is true of the body, how much more just that
a similar distinction should exist in the soul? but the beauty of
the body is seen, whereas the beauty of the soul is not seen. It is
clear, then, that some men are by nature free, and others slaves,
and that for these latter slavery is both expedient and right.
VI
    But that those who take the opposite view have in a certain way
right on their side, may be easily seen. For the words slavery and
slave are used in two senses. There is a slave or slavery by law as
well as by