Men have cast doubt on the human character of certain people whenever they sought arguments to exploit or exterminate their fellow human beings.
In antiquity, slaves were considered as objects and barbarians, and as second class men.
In the sixteenth century, some conquerors considered the Indians as "beasts in human appearance".
The Nazis looked upon some men as "non-human", or Unmenschen.
Such arbitrary classifications dictated by the rulers amounted to real discrimination and thus, in turn, "legitimized" exploitation and/or extermination.