My understanding is that being alive isn't enough to be able to sense. Scientifically speaking, there's a clear line between being alive & being conscious. Id est, the bacterium is alive but is not conscious; in spite of having the ability to produce such biological processes as digestion and reproduction, it doesn't have the ability to experience; the bacterium is unconscious. 'Life' has a slightly different meaning to the biologist in comparison to the philosopher. Looking upon it from such a position, the bacterium and the embryo have something in common – they both are alive yet unconscious.
Sure. There is certainly a distinction between being alive, and being conscious. But the foetal human and the comatose human are (a) both alive, and (b) both not conscious. Thus far, there is nothing to distinguish between them.
The patient is currently in an unconscious state, but the patient was conscious earlier in time—last week, perhaps; the unborn fetus, on the other hand, was not. Think about it this way – the patient has lived until this moment, enjoying life, making contributions to society, having desires and attachments; the unborn has done none of these things.
OK, so one difference is that the comatose human was
conscious at some point in the past, whereas the foetus will
be conscious at some point in the future (if events take their normal course).
It’s still not clear to me, though, why that should be a material difference when it comes to the morality of killing either of them. If anything, that consideration should work the other way. After all, the comatose patient has
been conscious; he has had the experiences and benefits that consciousness brings. This is an unalterable historical fact which my killing him now cannot change. Whereas my killing the foetus will prevent it ever attaining consciousness. It will therefore affect consciousness much more profoundly than my killing the comatose patient. If we have an ethical obligation to respect consciousness, then my killing the foetus would seem to be more ethically troubling than my killing the comatose patient.
Look, should the patient die to-morrow or next Friday, there are likely to be people who shall mourn over the patient—hir kin, hir spouse, friends, associates, even the neighbours—and likewise, there could be people who will gloat over hir death. In contrast, the unborn has no one to neither mourn nor gloat over hir death. Why? It is simple – the unborn didn't get the chance to become an individual, to experience this world, to make changes in it; the unborn has no good nor evil deeds. Hence no one knows what kind of a person would've the unborn turn out to become had it been granted the chance.
But now you are talking about the feelings and awareness of others, not of the subject who is to be killed. The logic of your position is that, if no-one would mourn the comatose patient, or have any feelings one way or the other about his death, killing him is not ethically problematic. That’s a fairly brutal conclusion; most people will hesitate before affirming it.
And what you’re really saying, I think, is the only value the comatose patient has (or the foetus has, I assume) lies in how other people feel about him, or in the fact that other people feel about him at all; that he has no inherent moral dignity or value. If that’s correct then his own past consciousness is irrelevant to this question, isn’t it?