There is a problem that catches the light occasionally, and is pushed off into political correctitude, but one day will have to be met. Humans are all as good as one another. If they are stupid or disabled or anything this doesn’t detract from their worth as people. This is fine – I’m not disagreeing. Animals are worth less than humans. Dead humans are worth less than humans. This is also fine, and I’m not disagreeing. However there’s an inconsistency.
These views can only work as long as the gaps between these things and humans are not filled. Humanity isn’t binary. There is, at least potentially, a sliding scale between characteristic humanness and, say, characteristic antness, involving variations in many characteristics. Similarly for living and dead. At what point as you travel away from normal human characteristics do you suddenly draw a line and value a creature/person a little less?
In practice as soon as you stop relating to them, but this is hardly the basis for a moral distinction. Wherever you draw a line, it must be admitted that it is arbitrary. So while we might take pride in our fair treatment of all mankind, regardless of their characteristics, we must agree that we could just as legitimately draw the line elsewhere and treat our celebratedly cared for lowest-capability people as animals.
Aside from where to draw the line is the question of why to have one. Why does a characteristic (such as intelligence or ‘level of consciousness’) varying among animals vary their moral worth, while the same characteristic varying among humans doesn’t? Their differences are judged using different rules, but not because of relevant inherent differences.
This problem hasn’t fully emerged with animals yet (perhaps more with dead people, and very little with robots), but that does little to the argument: our ethics are inconsistent.