The ‘Sentience Spectrum’
A serum has been invented which, when injected into kittens, can heighten their sentience and eventually transform them into beings that are as rational and self-aware as typical humans. If a full dose of the serum is injected, the kitten will acquire the ability for selfawareness and higher thinking equal to that of a mature human being. If only one drop of serum is injected, the kitten will become just slightly more self-conscious and rational than it was before. If all but the last drop of serum is injected, the kitten will be almost as fully sentient as a typical human being—only slightly less self-aware and rationally capacitous. A scientist injects a kitten with one drop of the standard dose at a time. With every additional drop, the kitten increases in sentience until, with the addition of the final drop, it is brought up to the same sentience level as the typical adult human. There are 100 drops in the standard dose.28
If punctualism is correct, what follows about the kitten’s acquisition of personhood on the ‘Sentience Spectrum’? At the near end of the spectrum, the kitten is certainly not a person, and at the far end it certainly is one. Again, if punctualism is true, one would have to hold that at all intermediate points on the spectrum, the kitten either is or is not yet fully and completely a person. One must believe that at all points, the question ‘is this kitten a person now?’ has a determinate, all-or-nothing kind of an answer, and moreover, that there is some additional drop of serum—a 
critical quantity—which corresponds with the beginning of personhood. Yet again, it is hard to believe when surveying the spectrum that any of these things are true, because there is no ground for believing that it could be true of any particular drop of serum.
It is worth emphasizing that the purely theoretical nature of these thought- experiments does not diminish their usefulness in drawing out clashes between what follows from punctualism and what it is reasonable to believe about person- hood’s emergence. If we really believed that there is a sharp borderline between human material and persons, we could comfortably claim that if any of these hypothetical continuums were possible, there would be a precise variation with which personhood begins, even if we cannot say for sure which it is. But it is exactly this claim that the spectrum thought-experiments show to be implausible, for there is simply no reason to believe that a sharp borderline exists between any two neighbouring variations on any of the spectrums.
Interestingly, the existence of sorites arbitrariness between the variations in the spectrums is the very thing which makes punctualism seem so unreasonable. The sorites-s usceptibility of each individual point on the personhood continuums cuts against the belief that personhood begins at a non-arbitrarily distinguishable ‘moment’. None of this shows punctualism to be utterly incoherent. It might still be true that in each of the spectrums there is a sharp borderline when personhood obtains or vanishes. It is simply difficult to square that notion with the fact that there is no single increment on the spectrum of brain-functioning, genetic humanity, or sentience, which it seems reasonable to think marks the sudden emergence or disappearance of a person.
Finally, the very same judgements which place the punctualist thesis in doubt also seem to lend some measure of support to the gradualist thesis. To reiterate, the central tenet of gradualism is that personhood at the beginning of life emerges progressively, and that personhood status can, therefore, be indeterminate at points. Gradualism does not posit a single point of complete metaphysical change. It supposes, to the contrary, that no such moment exists. The spectrum experiments show that whichever properties upon which personhood is thought to supervene, physical or psychological, it does not seem to admit of sharp boundaries.
-  Although the thought-experiment is merely theoretical, the existence of grey areas between species which the ‘Transgenic Spectrum’ demonstrates are not at all fantastical, see John Dupre, ‘NaturalKinds and Biological Taxa’ (1981) 90 Philosophical Review 66. 28 The example is an adaptation of a thought-experiment constructed by Michael Tooley (Tooley(n 10) 60-1).