What if there were creatures that looked just like human beings but were not? Their consciousness was deeply alien and they had no moral value at all. Call them human beings 2. You could think of them as wind-up toys, or lures — predators that are able to pretend to be humans in order to trap us somehow, reptiles or mushrooms. The percentage of human beings 2 could be quite small — just 1/10 of 1/10 of 1% of the general population.
As far as I know nobody believes this. And yet we cannot prove that it is not true. It just is not true, and it would be very evil or insane to act as if it were true.