Punctualism v Gradualism
At this point, it looks as though a certain amount of vagueness around the threshold of personhood is inescapable on any account. I think it apparent, though, that those defending the conception threshold typically take a very particular view of such vagueness. The entire thrust of the sorites critique of post-conception thresholds takes it as given that the inability to eliminate vagueness at the margins of per- sonhood is a huge obstacle for any claim about when personhood begins. But we might wonder why anyone would believe such a thing to begin with. The premises underlying the sorites objection are, I think, multi-layered and fairly contentious. But the most fundamental of them is the premise that there is a single moment when human material (be it sperm and ovum together, zygote, fetus, or human infant) is instantaneously and wholly transformed into a morally considerable person. Although it may be impossible to say exactly when, on the microphysical level, that moment occurs, the presumption is that this impossibility is only due to the limits of our knowledge and perception. In other words, the precise moment is certainly there, whether it can be pinpointed or not.
This is the nub of what I will call the ‘punctualist’ thesis about the emergence of personhood. Punctualism is the belief that, to borrow an expression of Warren Quinn’s, the beginning of personhood is something like an ‘existential pop’.2° On one side of the ‘pop’ there exists only human material, and on the other, a being which is essentially and completely a person. Persons do not emerge vaguely and incrementally, like human anatomy does; the beginning of their existence is instead sudden and absolute. Punctualism contrasts starkly with a very different view about how persons begin to exist: what we might call ‘gradualism’. Gradualism eschews the idea that persons come into existence instantaneously and completely—in the manner of an ‘existential pop’—and claims instead that personhood emerges gradually and incrementally. Consequently, there may well be a period of time during which the personhood of a human being is something of a grey area, being partial or indeterminate. The possibility of vagueness around the margins of personhood follows from the ‘gradualist’ thesis about personhood’s emergence.21 But any vagueness of this kind is excluded on the punctualist view, which regards personhood as an ‘all or nothing’ property.
Let us assume for the moment that the punctualist thesis is correct and that new people do begin to exist instantly and completely. The first question to arise is why, given this belief, the conception threshold fares any better than its rivals. The punctualist thesis as I have formulated it simply states that somewhere the lifespan of a new human being is punctuated by the beginning of personhood—the ‘existential pop’. But why could the pop not take place during the later stages of fetal development, or the process of birth? Presumably, one could be committed to the punctualist thesis about personhood’s emergence whilst endorsing birth or some other post-conception milestone as the threshold of personhood. One would simply need a reason to believe that the ‘existential pop’ occurs somewhere between, say, the beginning of birth and its completion, even if one cannot say at exactly which point, down to the millisecond. The defender of the conception threshold presumably just finds no reason to believe that the moment of existential import happens during the change of location from womb to world (or, equally, during the acquisition of independent breathing ability) and good reason to believe that it occurs some time during conception. But what is the basis
- 2° See W Quinn, ‘Abortion: Identity and Loss’ (1984) 13 Philosophy and Public Affairs 24.
- 21 The terms ‘gradualist’ and ‘gradualism’ are not new; they have been employed by a few philosophers to date in discussion about the acquisition of personhood or ‘moral status’ at the beginning of life (see: Margaret Little, ‘Abortion and the Margins of Personhood’ (2008) 39 Rutgers Law Journal 331). Exactly what theory of moral status ‘gradualism’ is taken to denote is not always abundantly clear, and can vary between uses. I use the term here to label the basic proposition that personhood emerges at the beginning of life incrementally or vaguely.
for this assumption? Why is it any more likely that the conception process is the window within which personhood begins? The tempting answer is just to say that conception is an instant, whereas the later developments are not. But we already know that isn’t true.
A different argument might run along the lines of claiming that all human developments subsequent to conception are simply not cataclysmic enough to suggest that somewhere buried within them is such a substantial and meaningful change as the advent of moral personhood. It is often suggested that post-conception developments only ever amount to the acquisition of a little more of one property or another—for example, gaining conscious awareness is only a matter of gaining a few degrees more sentience than shortly before, when a human being is sensitive to its environment in many ways, but not quite fully conscious. On the other hand, it is argued that conception marks the beginning of an essentially new kind of being, rather than the augmenting of certain existing features, like sentience. George and Tollefsen make an argument to this effect in the following passage:
[T]he difference between a being that deserves full moral respect and a being that does not (...) cannot consist only in the fact that, while both have some feature, one has more of it than the other.
In other words, a mere quantitative difference (having more or less of the same feature, such as the development of a basic natural capacity) cannot by itself be a justificatory basis for treating different entities in radically different ways. Between the ovum and the approaching thousands of sperm, on the one hand, and the embryonic human being on the other, there is a clear difference in kind . But between the embryonic human being and that same human being at any later stage of its maturation, there is only a difference of degree. 
It jumps the gun somewhat in the present discussion to accept, without argument, that conception is a qualitative change and every development following it merely quantitative, or that quantitative differences cannot add up to qualitative or ‘intrinsic’ differences. The most meaningful differences between typical adult humans and chimpanzees consist in only a few more degrees of intelligence, sociability, and emotional breadth. Presumably, George and Tollefsen would not dismiss those differences as meaningless—as being merely quantitative.23 Conception is certainly not quantitative in the sense that it does not continue to augment during the life of a human being, but neither does the acquisition of consciousness, or birth, once those benchmarks are passed. One does not become any more conscious after acquiring the capacity for conscious thought, and the characteristic of ‘having been born’ does not come in greater and lesser degrees. These developments do come about incrementally, in many successive moments, but as we saw, so does conception. Thus, if the incremental nature of their coming to be is what makes these milestones ‘quantitative’, so is conception.
George and Tollefsen are in fact independently committed to the view that a life form with a complete set of human DNA is fully a person. Believing this, they might have reason to think, if accepting punctualism, that the ‘existential pop’ must have happened by the end of conception, even if they can find no ground for placing it at the millisecond during which the sperm first penetrated the ovum, the millisecond during which the gametes’ nuclei first begin to fuse, or any of the milliseconds in between or after. But just as George and Tollefsen are convinced that conception is a more fundamental event in the life of an early human being than any other, so the proponent of the consciousness, viability, or birth thresholds could argue that the change taking place by the completion of these processes marks the beginning of a morally different kind of being. At issue here is whether there is better reason to believe that conception, rather than some other process of human development, is punctuated by the beginning of personhood on the ground that conception is a definitive moment, whereas the others are not. But if sorites-susceptibility is the thing which disqualifies post-conception processes in this inquiry, conception is in the same boat.
Be that as it may, punctualism, if true, may have extremely important implications for discussion about the advent of personhood. For one, whether it recommends the conception threshold or not, the punctualist thesis can make certain worries intelligible, including concerns which arise out of sorites arbitrariness. A belief in punctualism may, for instance, provide an explanation as to why sorites arbitrariness surrounding a proposed personhood threshold is found so troubling.
Vagueness is everywhere in the world (consider: at what precise moment does day turn into night?), but in few other places is it ever thought to be as worrisome as at the margins of personhood. Only philosophers are vexed by the question of how many grains of sand constitute a heap. But when the same puzzle is apprehended at the threshold of personhood, it is regarded by many as far more than a theoretical curiosity. If punctualism were correct, the seriousness of the sorites problem at the borderline of personhood might be explicable, since it posits that there is a determinate answer to the question whether the fetus is already a person at twenty-three weeks and six days, or becomes one at some time during the following day. This view effectively loads up the stakes of the sorites problem, by suggesting that it is possible to mistake the threshold of personhood by the most marginal fraction, and that the consequences of such a mistake are gravely serious: the killing of a complete, fUlly realized person, instead of a cluster of human biological material. It is as if the punctualist thesis pictures a policy-maker engaged in the enterprise of line-drawing as being like the protagonist in an action movie who is forced to decide whether to snip the red or blue wire of a ticking bomb, with nothing to choose between them and everything on the line. It is possible to see how this picture of what is entailed by settling on a threshold of personhood can make the sorites problem significant in a way that it is not in the context of other vague thresholds, like the difference between night time and dusk.
-  George and Tollefsen (n 3) 120.
-  Incidentally, George and Tollefsen think that, by this point, they have already established theclaim that conception marks a difference in kind in an earlier chapter (ibid, chapter 2). It is notentirely clear why, however, given that the relevant chapter is in fact only a detailed exposition of thefertilization process which features no argument from the biological facts considered to the moralconclusion that conception is morally significant in a way that later human developments could notpossibly be.