I think if nothing else humanity is a either a rationalisation or intuit of yourself in the other, its why so many people are fooled so easily by psychopaths or other predators, because it resembles and acts like a human being so well they dont realise that it considers them prey. Like the cobras who mesmerise their prey.
How is it that their humanity alone qualifies them as equal? I would question the origin of an alternative conception, a lot of the objectification of human individuals and capacities I consider unconscious "eminations", cultural "transference" or social constructs from the moral under pinnings and principles of the economy.
I'm not sure if I'm using the correct concepts or words, I frequently get out of my depth with discussions like this one but tend to err on the side of older Marxist ideas about the unconscious or unacknowledged economic base of society and its influence upon the superstructure of ideology, culture, religion etc.
