I have just sketched two competing conceptions of the emergence of personhood at the beginning of human life and some rough implications of what I am calling the ‘punctualist’ thesis. It is possible to see why someone committed to the punctualist thesis would worry about arbitrariness of the sorites kind at the threshold of personhood and might, for that reason, want to reject any putative threshold which cannot be non-arbitrarily pinpointed, even though, as I have said, this would present problems for all possible thresholds. I believe, however, that there are good reasons to reject the punctualist thesis and to accept its antithesis, gradualism.
The main challenge to punctualism can be put in the form of an adapted version of a thought-experiment proposed by the philosopher Derek Parfit, originally used in the service of defending a particular view of personal identity, or, what it means to remain the same person over a period of timeTh Let me call the adapted experiment the ‘Brain Cell Spectrum’. It runs as follows: