Asimov (along with the antagonists of the story) continuously change their ideas of what makes someone human throughout the piece. Before the scientists found ways to surgically implant mechanical organs into human beings, it was agreed that having mechanical body parts (or at least a certain percentage of them) made one non-human. Then, humans began prolonging their own lives with mechanical organs and decided that the brain in a body must be organic to be human. I’m not so sure I agree with this though.. It would certainly be odd to see a human-shaped body composed entirely of metal except for his human brain. It would be hard to count that as a human being. However, our brains hold our knowledge, emotions, memories, and self-awareness. If an organism had a human brain, it should be claimed human, since it is capable of thoughts, feelings, emotions, speech, etc. The robotics organization then decided that the organism containing the human brain must be able to die in order to be called a human being. This makes sense to me, since death is the ultimate things that separates organic from inorganic, human from robot.