In times of war it seems imperative to see your opponent as less than human. The same is true of enslavement. In order to justify the inhuman treatment first of Native Americans and then of Africans who were compelled to serve as slaves, an effort was made to dehumanize them.
In seeking to answer this question of who is human, we turn again to Richard Beck and his book Unclean. Beck writes of what he refers to as infrahumanization, an understanding of people deemed less than human:
The phenomenon of seeing people as less than human is called infrahumanization. Historically, infrahumanization occurs when one group of people comes to believe that another group of people does not possess some vital and defining human quality such as intellect or certain moral sensibilities. These infrahumans might be human from a biological perspective, but they are believed to lack some moral or psychological attribute that makes them fully human, on par with the “superior” group. And as we have seen, sociomoral disgust is critically involved in the process of infrahumanization. For example, in America a classic case of infrahumanization is found in the first U.S. Constitution (Article 1. Section 2) in which slaves were considered, for the purposes of the census, to be three fifths of a person. And, in psychological support of this state-sanctioned infrahumanization, disgust properties were attributed to the slaves: bad smell, filthiness, animal-like features, imbecility. The two processes, disgust and infrahumanization, often go hand in hand. [Richard Beck. Unclean: Meditations on Purity, Hospitality, and Mortality (p. 102). Cascade Books, an imprint of Wipf and Stock Publishers. Kindle Edition.]
Down through history people have justified inhumane acts on the basis that the other was not fully human. Often, as Beck notes, the other is viewed with disgust. Those of us who are Americans have as part of our past this reality, that our Founders, could enshrine in the Constitution, a document designed to protect freedom, a vision of part of the populace that denied them the same freedoms.
But it's not just the Founders who found a way of justifying slavery on the basis of infrahumanization. Consider this statement made by Presbyterian theologian Robert Lewis Dabney:
Here inspiration defines that equality as in full force between master and slave; and as entirely compatible with that relation. Here is the great charter of Bible republicanism. Men have by nature, a general equality in this; not a specific one. Hence, the general equality of nature will by no means produce a literal and universal equality of civil condition; for the simple reason that the different classes of citizens have very different specific rights; and this grows out of their differences of sex, virtue, intelligence, civilization, &c., and the demands of the common welfare. Thus, if the low grade of intelligence, virtue and civilization of the African in America, disqualified him for being his own guardian, and if his own true welfare (taking the "general run" of cases) and that of the community, would be plainly marred by this freedom; then the law decided correctly, that the African here has no natural right to his self-control, as to his own labour and locomotion. Hence, his natural liberty is only that which remains after that privilege is retrenched. Still he has natural rights, (to marriage, to a livelihood from his own labour, to the Sabbath, and to the service of God, and immortality, &c., &c). Freedom to enjoy all these constitute his natural liberty, and if the laws violate any of it causelessly, they are unjust.
Note the text in bold print, according to this Christian theologian, Africans were of low grade in intelligence and virtue -- that is, they could be looked at with sociomoral disgust and warrant enslavement. Because they were not capable of determining their own destiny, they could be made the property of others who were capable. They had certain rights, but like children, they needed supervision.
So, who is really a human and how do we determine what makes one human?