Human beings come in all types: Some are smarter than others, some are stronger than others, some are more attractive, some have talents that others do not, some are more massive than others, some darker-skinned, and some are so mentally and physically handicapped that they are not able to take care of themselves. All human beings (I'll use the classic 'men' for now on) start their lives naked and helpless, they rely almost completely on their parents for almost a quarter of their lives, struggle in the world for about half of their lives, and finally end up frail and helpless again. But because all men possess different traits from birth and are given different advantages or disadvantages throughout life, most men cannot relate to each other in many things.
By the time we reach adulthood, it is obvious to us that we cannot always "get along," probably because men cannot relate to each other and the less we can relate, the less we tend to get along. Today there is a fierce moral imperative sweeping the world which demands that all men be treated equally. To anyone not indoctrinated from birth with this idea, it is not very obvious that all people should be treated equally. After all, if nobody is equal, why should they be treated as equal? To say "all men should be treated as equal" is about as easy as saying "all cows should be treated as equal." We eat some cows, milk some, put ribbons on some, and wear some on our backs. If a meat cow is born a runt, he will likely be killed off. If a milk cow is infertile, she will probably be killed off. Going further with the comparison, even the color of a cow's skin will determine what it is good for.
But people should not be treated according to what they are good for. They should be treated as equal, even though they do not seem equal at all. Any argument about mutual self-interest or social contracts or the golden rule as being all-important for a stable society fall short before most of human history: Many societies more stable than our own have used caste systems or slavery. Social conventions do not hide plain facts: If people are not equal, we should not pretend they are equal. But it is a plain fact that all men are equal; not physically, but in essence.
Whatever it is that makes someone a human being, it is not smarts or brains or looks or property. If there is something that makes someone a human being, then we all have it, period. This thing is not at all physical, which is why one cannot have more or less of it: One is either human or not human. This "thing" we all have is called a "soul." In fact, it is more accurate to say that it is what we are, or specifically, a 'soul' is 'what makes me human'. Of course, one could not take a cow's body, insert a human soul, and voila! have a human. Even though a soul is a defining factor of a human being, our biology is still very much a part of who we are: We are, by virtue of our biology, part of the same "human family" and are related to each other in that sense. My parents are in this sense a part of me and their parents in them, and so on. My identity as a human being relies partly on being related to all other humans, so I cannot truly say that I am just a soul trapped in this body. I am my body and my soul. My body and soul are one, and that one thing is me.
If this soul stuff sounds too weird or ethereal to make any sense, consider how nonsensical "humanity" and "equality" become when you leave the soul out of the picture. If it is enough to be related to other men to be called "human," then we run into problems when considering things like genetic disease or evolution or -- God forbid -- if a man is grown in a test-tube or has more than two parents or has DNA that does not come from another human. We leave room for eugenics or ideas of master-races if some genetic groups of men can be shown to be "more evolved" or can trace their lineage through more "desirable" families in history or to origins which might suggest that their ancestors were of a more intelligent progenitor species (in which case polygenism could be shown to be very problematic indeed). Consider simply the fact that some families are smarter than others, that most white people cannot get sickle-cell anemia, or that it is much less of a nuisance to have a penis than to have a vagina. It is not enough to be "one of the family" to be considered equal to all other members of that family. Anyone who grew up with his family can attest to that.
Think about the traditional theme of the Totentanz, the "Dance of Death," in which members from all levels society are depicted as being forced by Death to partake in a grim daisy-chain onward into the grave. It was understood, even in cultures well-accustomed to a tradition of hierarchy, that all men were equal when the worldly trappings of status were stripped away. Peasant beside merchant, king beside bishop, queen beside pauper, pope beside knight; all were brought to the same end. To see the opposite side of this is the key to seeing the source and justification of equality: To say that this transient body has a soul is to say that it is alive. Each man's life is equal to the other, because being alive -- i.e. a living human being -- is the same as being alive, regardless of any external embellishment. And this is important, because if we do not see this, we have no reason to be equal.
