There's this hypothetical situation which gets thrown around quite a bit in discussions which goes like this: You take a human being, and you replace their hand with a synthetic but otherwise completely functional robotic hand. Everyone agrees that that person is most definitely still a human being, albeit with a robotic hand. You then take both his arms and replace them with robotic equivalents, you could still say he's a human being. You then escalate it to the point where you slowly replace everything in the entire body with a synthetic equivalent that is functionally exactly the same as it originally was. For example when you get to the brain you could hypothetically replace very small chunks of cells with manufactured ones. The question is, at what point does that person stop being human being? Because by the end of the procedure they are completely artificial.