Fertilization is a gradual process, so how can it be the “line” marking the beginning of personhood?

Fertilization is indeed a gradual process, unfolding over the course of several hours. Because human organisms (typically) come into being during that process, I often say that personhood begins at fertilization for the sake of convenience. However, a more precise formulation of my position is that personhood begins when a human organism begins. Because organism-hood is binary, this transition is necessarily a discrete moment in time. Similarly, personhood ends when a human organism ends, which is also a discrete moment in time.

I am not prepared to lay out an exhaustive rubric that will allow me to pinpoint, in any circumstance, the exact moment at which a human body expires. I am willing to let more knowledgeable biologists and legal experts sort that out, and I am willing to update my understanding as the science of biology progresses. Except in rare circumstances (which fall outside the scope of this discussion), a little fuzziness in our knowledge of when the human body has died is morally irrelevant. Similarly, I am willing to defer to the biologists and philosophers of science regarding the exact moment during the process of fertilization at which the entity first starts acting as a self-integrating unit. I have heard advocates of a few different positions on this question, separated by a matter of hours. The extent of the uncertainty—perhaps a few hours, given our current scientific knowledge—is even less likely to be relevant to moral conundrums than the uncertainty surrounding death. If a moral decision emerges that rises or falls on this question, I would be willing to consider it in more detail. For the moment, I am content to wait for the science of embryology to progress further and, if necessary, play it safe at the margin.

Thus, the fact that fertilization is a process does not defeat my position because my position never relied on fertilization being a discrete moment. My position does rely on the transition from non-person to person being a discrete moment, but I define this as the (equally discrete) transition from non-organism to organism, not as the “moment” of fertilization.