Phil has a good post up on the topic, diametrically opposed to my recent one. Read it. He discusses the distinction between soul and sentience:
This is an interesting turn. He is not interested in discussion of the soul as "is not testable. It is a matter of faith." Ah, but this is too simplistic. After all, the reliance on "sentience" is many ways merely a hark back to faith. The difference, however, is this - discussion on the soul has a long history, over a thousand years, with much thinking involved. One could at least give it credit for thought. "Sentience," on the other hand, is much more fuzzy. Indeed, "sentience" seems only to mean our own subjective consideration of who we consider "truly" human (whoeverthehell "we" are). One can't have it both ways - either keep the soul out (including such thin secularized versions thereof), or keep it in (allowing for the secular and theological versions).
This is an interesting point, but I don't think the two are as conflatable or the idea of sentience as mysterious as he makes it out to be. An easy test is this: I know I am sentient; I do not know if I have a soul. The reason we can't know for sure if someone else is sentient is the same reason we can't know for sure they exist at all. However, unless we're going to succumb to the narcissistic fallacy, it's reasonable to assume that if "cogito ergo sum" then "cogitat ergo est" for anyone exhibiting behavior consistent with self-awareness. That's probably not philosophically sound, but it passes the test of common sense. Despite the fact that people have been writing about what a soul is for thousands of years, we're still no closer to knowing what it is or if it exists. However, in the past hundred years we've made rapid scientific advances in neuroscience, neurobiology, psychology, and all the other fields that relate to consciousness, awareness, and sentience. I feel confident we will eventually be able to give a scientific account for what comprises sentience and who has it. One caveat: I'm not attempting to be terribly exact with my language here; I'm using "sentience" and "self-awareness" more or less interchangeably, though they're not. Even so, I think everyone knows what I'm talking about. If not, we can digress in the comments.
He goes on to write something I just don't understand:
What is "human"? Well, that is a trick, isn't it? After all, we are dealing with a pluralistic society, one with many comprehensive notions of the good, what humanity is, and the like. And, as so many folks have made so very clear, as any invocation of God would be a step towards theocracy, the Inquisition, and the like, we can't necessarily rely on any of them. So, let's rely on the basics, the "relatively reliable methods of inquiry" (mentioned, and used, before, here). "Soul" can't work, as it's not "testable." But, along the same lines, "sentience" also fails the test, since what counts as "sentience" (first self-willed act, ability to cognitively know desires, etc.) will also be tied up in comprehensive notions of the good.
How is "sentience" "tied up in comprehensive notions of the good"? This may be obvious to Phil or other more philosphically-versed readers, but to me this is a large jump that I cannot follow.
He concludes thusly:
Life must be the foundation for the legal protection of human beings. If for no other reason, than for one's own self-interest, or else one might one day find him/herself in the category of non-human, available for killing without legal protection, all for the crime of existence.
I would certainly agree except for the fact that abortion has not been the first step in slippery slope towards the general devaluation of human life. I would imagine that part of the reason for this is that there is an unmistakeable difference between a fetus and a human: birth. If you want to look at biology as Phil does, you can't get a clearer line of demarcation than that. Of course, the obvious problem with this is that we blur the lines between newborns and third-trimester fetuses (rightly so), leading to these discussions. But back to my point about the lack of a slippery slope. Although I shamefully haven't checked these facts myself (so if they're wrong, please provide reputable data and argument), but nations where abortion has been strictly criminalized have seen a marked decrease in compassion towards and concern for children. This would seem to be the very outcome which Phil sees abortion leading towards. If it did, I think I would lean strongly against keeping abortions legal, but I have yet to see any evidence to support that.