The fact that an early foetus is less sentient than a carrot doesn't matter because its visible characteristics are accidents just as the communion wafer, which certainly looks like a cracker, is substantially the body of Christ.
In context, this is from the author's comparison of special creationists' views on species differentiation with Aquinas' views on substantial forms. Special creationists believe, at the very least, that it took an act of God to produce humanity as apart from any other species. Some special creationists believe evolutionary theory explains some species differentiation and just humans came separately, and some take it all the way to saying that no species can come about except by a miraculous act of God. The commenter is therefore a little unfair to those who believe special creation only occurred for humanity to come about, since the comment is directed toward those who believe in special creation at all, and the complaint is that it involves an inability to see how any species could come about through small change.
What was interesting to me about all this, though, even if you ignore the inability on this commenter's part to distinguish between versions of special creation, is the substantial forms issue in relation to humans. This is something that is unfair again on one level but helps explain at least one strain within pro-life thought about personhood and complexity of development.
I don't think everyone in the contemporary pro-life movement holds a view like Aquinas' substantial forms. Aquinas thought there was this component of who we are that makes us what we are. It is our essence. You can take away anything else, what he called accidents, and we'd still be who we are. If the essence is gone, we're no longer there. A number of contemporary philosophers believe that we do have essential properties that are like this, but the difference with Aquinas is that species is part of the essential nature. So my being human is at least part of my substantial form. The matter can be arranged in many different ways, but the substantial form organizes it in such a way that I will develop into an adult human being as long as something doesn't go wrong in the process (e.g. I die first).
I think something along these lines is involved with the pro-life view, in many cases at least. Only for Roman Catholics does it involve anything that would be extended to believing the human substantial form and divine substantial form are in communion wafers, but the first part of the quote above does seem to me to get the view right, except for one small detail, which isn't an issue about whether the quote gets the view right but about whether it gets scientific fact right. A fetus is more sentient than a carrot. There's no question about that, since a fetus has some sort of sensory neural input from the earliest fetal stages. Maybe an embyro doesn't have senses, but a fetus clearly does. A carrot doesn't. A better example would be a lower animal such as a slug. At some stage in development, I'm sure the fetus is less developed than a slug in terms of sensory input and neural complexity. This is, as far as I can tell, the only reason 99% of philosophers today would say that it's perfectly ok to kill a human organism at this stage of development.
Two assumptions could lead to such a conclusion. One is the utilitarian assumption that causing pain would be the only reason to prevent killing something. In some abortion discussions, proponents of the pro-choice position argue that early fetal development doesn't involve as much in the way of pain receptors as other things we kill all the time, e.g. mosquitoes. Utilitarians then say that there's nothing wrong with killing such a creature, since it's so small an amount of pain that it doesn't matter. One problem with this is that by the time most abortions occur there really is much more pain, something more on the level of a cat. The biggest problem, though, is the assumptioin of utilitarianism, which is a dying view among ethical theorists today. The mainstays defending utilitarianism are actually metaphysicians and not ethicists, and I wonder if in some of those cases it's so that they won't have to do the hard work of real ethical discussion. Utilitarianism is so easy. One professor in my department refers to it as a view for children until they can reach maturity to see that it's much more complicated than happiness and unhappiness, and I think that's right.
The second assumption that could give an alternate path to this conclusion is that complexity of development is all the essence of what you are that could matter for whether you can be killed. This, I think, is the part of the orthodoxy of the philosophical world. I don't see why anyone believes it except that they think it follows from materialism. If we're just physical beings with no immaterial component, they can't see how anything could determine whether we're higher beings with moral rights unless we have a more complex makeup than lower beings without those rights. This takes some nuance if it's to be consistent with some kind of animal rights perspective (which I don't think most politically active animal rights proponents have even considered), but that's workable as long as you have a gradation of when killing an animal and killing a fetus are ok. The lower complexity of each makes it less bad to kill. This will of necessity force a limitation on both scope of which animals have rights and on how late an abortion can be justified, but I think such a nuanced view is workable. It's just not going to be the standard pro-choice orthodoxy of the political left.
Pro-life views of personhood do not involve anything to do with levels of complexity. If a fetus is as much a person as I am, then its basis for that personhood and the rights that come with it can't have anything to do with how complex it is. It's true that a fetus has sensory input, but that is much lower at earlier stages, and the pro-life position isn't generally that earlier abortions are ok (though some who call themselves pro-life allow it at earlier stages). The argument isn't usually based on pain, at least not for early-stage fetuses. The argument is usually based on a notion that simply being human involves a distinction with the entirety of the rest of the animal kingdom (and lower forms of life as well). I argued last week that an argument for the presumption of a pro-life position can be mustered very easily without appealing to religious conviction or revelation from God. I still stand by that. What can come in from a religious conviction is the explanation of what is different about a human fetus and an adult orangutan. The fetus is made in the image of God. The ape isn't.
Theologians differ widely on what the image of God consists of. Some think it's something to do with reason. Some think it's a special level of moral development. Some think it's a kind of emotional capacity to connect on a deeper level. Some think it's a spiritual ability to relate to God. Others suggest free will, self-consciousness, something to do with personality, or an ability to relate to God (which sounds to me as just some combination of these, which could well be what was originally meant). A fetus doesn't have most of these properties if the idea is a fully realized level of reason, moral development, etc., but that's not what people mean. They mean all the potential for that development is there, so that any human being is made in God's image, regardless of age or level of development.
Some people in the abortion literature don't care much for the mention of potentiality, because they think reason, moral development, etc. are part of personhood, and therefore a fetus is a mere potential person and not a person. The result is that a fetus has the potential of developing moral rights but doesn't have them yet. The pro-life view is very different. The image of God is what gives the rights. The image of God is itself a potential to develop reason, moral rights, etc., but not everything with that potential has the image of God. An unfertilized human egg has that potential, if it were to be fertilized and developed. A stem cell in my bone has that potential if real cloning, as opposed to what they've done with sheep (which is just genetic engineering of an egg to have the DNA of another being rather than taking a cell of a being and turning it into a duplicate organism), is possible. A stem cell doesn't have the image of God, though, nor does an egg until it is fertilized (it's not even an organism until it has the right chromosomal makeup and capacity to develop). The pro-choice discussions of potentiality seem to me to assume that anything with the potential for certain features will give the potential for rights. The pro-life view seems to me to say that it has the rights because it has the right features that also give it the potential.
I want to suggest a further sense of what the image of God might be that could explain a little more here. This comes right out of some of the most important recent commentaries on Genesis, so it's not as if I'm making this up out of thin air. Gordon Wenham, for instance, considers some of the above ideas about what the image of God might be (noting that these would be the image of God because they're ways we're like God) and then opts for an explanation along different lines. Bruce Waltke takes the additional explanation Wenham gives on top of all the others rather than as exclusive of them. I think Waltke is more likely to be right, but Wenham's version with just this different explanation is sufficient for my purposes.
This further notion is one of standing in a certain relation to God, one ordained by God but not part of the essence of being human other than by God's decree. There are two ways God might decree something. The first is to declare it such that it changes something about the thing. For example, God might declare that there be light, and there will be light. God might decree that I be transformed as a new creation, and the work of the Holy Spirit will bring about change in my life (which of course includes my role in obeying Christ, for instance). The second kind of declaration changes how God related but doesn't change the nature. God might declare that someone would be legally righteous even before any life change has taken place so that the person is becoming more righteous to match that declaration. God might declare that a child is to obey a parent, and that relationship is fixed in some spiritual sense even if there's no biological connection between parent and child, e.g. with adoption. No change takes place in the child due to being adopted, but by virtue of the fact that this child is now the child of a new parent, the child has a relationship declared by God, with certain role relationships, including obedience and love. Something similar takes place when someone is declared married even before the physical consummation gives a real connection between two people. They are declared one flesh even without having become one flesh in a physical sense.
According to this view, which Gordon Wenham defends and Bruce Waltke seems to think is at least part of what the image of God is all about, God therefore says certain things about humanity that define our relationships. One element is that we are God's stewards of all creation. This is something God's declaration makes real but not through changing anything in us. God did give us the abilities to do a better job of this than any lower animals could, but it's the declaration that makes it true of us, not the abilities. This is true no matter what stage of development a human being is at. The responsibilities that will develop and the moral status declared by God to be true of us go along with this. It's stronger than this, though, and reflecting a bit on what being an image in the ancient near east would have been helps a little. People would create images to represent their gods. They weren't foolish enough to think the piece of wood covered with gold trim or the hunk of rock that they bowed down before was really a god. Isaiah makes fun of them by saying that this piece of rock can't save them, because he knows full well that there isn't anything to it more than the hunk of rock, but the people worshiping the god realized that the image was only a symbol. Otherwise, there would be a lot more gods than myths about them, if each hunk of wood was a different god rather than just a different token of the same type, a different instance representing the same god.
In Israel God was represented by an absence. The ark of the covenant contained a setup with cherubim rigged to look like a throne for God to sit on, but no image of God was ever allowed. Nothing was deemed fit to represent God. Yet in Genesis 1 and elsewhere we see these statements that we are made in the image of God. We are made to represent him. Stewardship (in a sense ruling in his place) over creation is one element of that. We're given the responsibility to manage things that God has placed in our laps, his property, for his sake. We're given to rule in his place over whatever sphere we occupy and have authority over. That's almost directly implied by the Hebrew in Gen 1:26 right after humanity is declared to be in his image. That's at least part of being made in the image of God. It's being made to represent God, but this aspect of being made in the image of God isn't ontological. It isn't some aspect of our being that's made a certain way. It's moral and relational. God made us to be this way, but it's not a way we're formed to be. It's a way we're declared to be.
Bruce Waltke gives one more element of being made in the image of God, so please bear with me before I get back to what the pork this has to do with abortion. One thing I discovered after learning a lot about the abortion debates that makes them all seem a little strange is that the origin of the term 'person' was in theological discussions of the Trinity. When theologians were coining terminology to explain how God can be a complete unity of being while also being three in some other important sense (without being three gods), they used the Latin ancestor of 'person' to say that God is what we now in English as refer to as three persons but one being or substance. That was the origin of the word person. Now butterflies have nothing to do with butter or flies, so we don't want to commit the fallacy of taking a word's origin to guide us into an exact meaning today, but there's something important about the Trinity in Christian theology that I think does have a bearing on the image of God, which in turn has a bearing on personhood, so hear me out.
The Genesis passage doesn't say that male and female together make up God's image, as if each is half of it (or part of it if it's somehow not supposed to be 50-50). It says that each is made in God's image, male and female. God is fully represented in each. Each represents God. Yet the wording of it might be important. Waltke cites Gen 5:1-3 and Gen 9:6 together show that it's not a partial image in each of male and female (from 9:6), but being made in his image involves being male and female (5:1-3). Both are true. We are unity in diversity, and neither overrules the other. Those who overstress the difference between male and female ignore biblical themes of unity and equality, such as Paul's statement in Galatians that there is no male or female, slave or free, etc.
Yet some overstress the sameness based on that one verse Galatians, including such magnificent commentators as F.F. Bruce and Gordon Fee. This ignores the plain teaching that there aren't just differences between men and women but sometimes role distinctions that serve a distinct purpose, role distinctions that don't imply lack of equality on what counts. Paul bases some of those differences on the creation order in I Tim 2 and I Cor 11, so they're part of the good creation and not a fallen disorder from the curse of Gen 3. These two truths together form a very interesting way of imaging and representing God. We as humanity are in his likeness because we are unity in diversity, in the most biological differences humans can have, that of sexual distinction. There's a unity in duality in male and female that represents the unity in Trinity in God. Father, Son, and Holy Spirit have clear role distinctions. Nowhere does the Father submit to the Son, and the Spirit doesn't send the Father or the Son. Prayers are to be directed to the Father, not the Son or the Spirit, though the Son is clearly to be worshiped explicitly, as in Revelation 4-5 (I'm a little unclear as to whether the Bible says anything about worshiping the Spirit). After being given all glory upon full ascendance to kingship, Jesus will turn it back over to the Father in submission, according to I Corinthians 15.
If Waltke is right that this unity in diversity is part of how we represent God, as Paul seems to say explicitly in Ephesians 5 and I Corinthians 11 (with wives submitting to husbands as to the Lord and husbands loving their wives as Christ does the church, dying for her; with husbands the heads of their wives as Christ of husbands, and God of Christ). The analogies are all there. I'm not making them up, and neither is Waltke. It's one way humanity represents God iin our relationships in a structural way, even if there are other ways each person represents God directly. Corporate truth is as important for the Semitic mindset as individual truth, and it's something we Westerners don't consider enough. It's fundamental to some of the themes in scripture.
So now what does this have to do with the personhood and the image of God? What I hope I've done a decent job presenting is an account of ways the image of God is present not just in a potential way to be realized later, though I've argued that that's sufficient as long as it's the image of God that carries rights and not the properties the image of God will eventually lead to that give rights. This second way of conceiving of the image of God or of representing God has to do with what God has declared about us. That's true of human beings simply because God has declared it about human beings. It's not true of other species, no matter how cognitively or emotionally developed they are. That doesn't mean we have no obligation to be kind to animals. It does mean there's something different about human beings because God has declared us to be different. I've given only two ways we're to be different in representing God, but the details of that aren't as important as the fact that these are declarations that make us different and make the nature of our relationships different. The fact that God has spoken us into certain relationships of moral concern is enough that every human being has those relationships regardless of whether we recognize it and independent of what capacities they've achieved. I'm not claiming to have established this about us, but I am claiming that this is the scriptural view of human beings, and those who believe the Christian scriptures have an account of personhood that can lie in these elements that can then explain moral rights and responsibilities even with regard to human embryos. I think this will go a long way toward explaining why it could be wrong to destroy something that has much less perceptual ability than a slug, no self-concept, something far short of even the kind of reasoning a mosquito can do, and no ability to control how its life might go. None of this involved immaterial souls or substantial forms of any sort.