Here's my old teacher Peter Berkowitz, in his review of Ramesh Ponnuru's Party of Death:
Invisible to the naked eye, lacking body or brain, feeling neither pleasure nor pain, radically dependent for life support, the early embryo, though surely part of the human family, is distant and different enough from a flesh-and-blood newborn that when the early embryo's life comes into conflict with other precious human goods or claims, the embryo's life may need to give way.
The beginning describes four differences between newborns and early embryos. The first one -- invisibility to the naked eye -- isn't especially relevant to the moral difference Berkowitz is trying to draw. If you suddenly shrank and became invisibly tiny, it would still be wrong to kill you. The next two, though, are genuinely important. Brains are necessary for humans to have minds, and as I've argued before, minds are necessary for any sort of moral status. We ought to prevent creatures from being in pain and promote their pleasure, so the inability of the fetus to feel these sensations is a big deal. I'm less certain about the life support issue, but maybe there's some way to use it in an argument.
