Aristotle (384–322 BCE) was one of the greatest of the ancient Greek philosophers. In his Politics (350 BCE), he speaks of equality and inequality in Athenian democracy.
Some forms of class inequality that existed before modern times have since almost entirely disappeared. Here, Aristotle justifies slavery:
[M]an is by nature a political animal. And therefore, men, even when they do not require one another’s help, desire to live together; not but that they are also brought together by their common interests in proportion as they severally attain to any measure of well-being. This is certainly the chief end, both of individuals and of states. And also for the sake of mere life (in which there is possibly some noble element so long as the evils of existence do not greatly overbalance the good) mankind meet together and maintain the political community. And we all see that men cling to life even at the cost of enduring great misfortune, seeming to find in life a natural sweetness and happiness.
… The rule of a master, although the slave by nature and the master by nature have in reality the same interests, is nevertheless exercised primarily with a view to the interest of the master, but accidentally considers the slave, since, if the slave perish, the rule of the master perishes with him. On the other hand, the government of a wife and children and of a household, which we have called household management, is exercised in the first instance for the good of the governed or for the common good of both parties, but essentially for the good of the governed … The conclusion is evident: that governments which have a regard to the common interest are constituted in accordance with strict principles of justice, and are therefore true forms; but those which regard only the interest of the rulers are all defective and perverted forms, for they are despotic, whereas a state is a community of freemen …
[T]hat 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.
… [T]he 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 … 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 …
Of household management we have seen that there are three parts—one is the rule of a master over slaves … another of a father, and the third of a husband. A husband and father … rules over wife and children, both free, but the rule differs, the rule over his children being a royal, over his wife a constitutional rule. For although there may be exceptions to the order of nature, the male is by nature fitter for command than the female, just as the elder and full-grown is superior to the younger and more immature. But in most constitutional states the citizens rule and are ruled by turns, for the idea of a constitutional state implies that the natures of the citizens are equal, and do not differ at all. Nevertheless, when one rules and the other is ruled we endeavour to create a difference of outward forms and names and titles of respect … The relation of the male to the female is of this kind, but there the inequality is permanent. The rule of a father over his children is royal, for he rules by virtue both of love and of the respect due to age, exercising a kind of royal power … For a king is the natural superior of his subjects, but he should be of the same kin or kind with them, and such is the relation of elder and younger, of father and son.
… [T]he freeman rules over the slave after another manner from that in which the male rules over the female, or the man over the child … [T]he slave has no deliberative faculty at all; the woman has, but it is without authority, and the child has, but it is immature. So it must necessarily be supposed to be with the moral virtues also; all should partake of them, but only in such manner and degree as is required by each for the fulfilment of his duty … [T]he courage of a man is shown in commanding, of a woman in obeying. And this holds of all other virtues, as will be more clearly seen if we look at them in detail, for those who say generally that virtue consists in a good disposition of the soul, or in doing rightly, or the like, only deceive themselves … All classes must be deemed to have their special attributes; as the poet says of women, ‘Silence is a woman’s glory’, but this is not equally the glory of man.
Aristotle. 350 BCE. Politics. The Internet Classics Archive, MIT. Bk3 Pt 6, Bk1 Pt 5, Bk 1 Pt 12, Bk1 Pt 13.