The moral theory Aristotle used to justify slavery is an ancient species of virtue ethics, which says natural slaves are slaves because their souls are incomplete, lacking certain qualities, such as the ability to think properly, and so they needed to have masters to tell them what to do. Clearly, thus, by looking into history and seeing Aristotle’s words (and the similar sentiments of his teacher Plato in Gorgias and his disciple St. Thomas Aquinas in Summa Contra Gentiles), one sees slavery can easily be justified by moral arguments. However, what is at issue is whether any of these arguments are good, and whether they actually reveal something wrong with the underlying theory used to justify slavery; this seems to be the case with a number of modern normative theories, including but not limited to standard versions of utilitarianism, deontology, virtue ethics, and natural rights theory.
Utilitarianism, a theory of morality and economics pioneered by the intellectuals J.S. Mill and Jeremy Bentham in the 19th century, adheres to a principle stating simply, “the greatest good for the greatest number”. As the theory is described in academia, according to this greatest pleasure principle, there are two ways in which the theory could be used to justify institutional slavery. First, if the moral theorist is simply looking at what is best for the most people, a micro-majority of 51% could justify the enslavement of the other group, the macro-minority, of 49% on the basis that it is maximizing the good of the greatest number of people. Additionally, utilitarianism does not recognize, except indirectly, “how the sum of satisfaction is distributed among individuals” (Rawls, 1971, p. 26). As a result, individual differences are not taken into account, and the theory gives no practical means of measuring what is, by some definition,