Thanks for the response. I realize that this is a very belated reply, and that it would have done a lot more good prior to the release of your How-To-PSC essay. Nevertheless, I’ll respond to a few of your points.
For one thing, an embryo that was conceived from the gametes of two humans doesn’t “grow into a human” or “develop into a human”; it is a human. I’m not saying that this necessarily confers moral worth, but it does jog the question of which trait does, and you don’t provide a strong alternative.
In defense of the ZEF’s potentiality: before fertilization, an arbitrary pair of sperm and egg isn’t a coherent object any more than union of my left sock and the moon is a coherent object. In contrast, after fertilization, it’s the sperm and the egg that cease to be coherent objects. The egg releases chemical signals to reject additional sperm, the successful sperm’s cell membrane disintegrates, and the former contents of the gametes are bound together within one structure: the zygote.
I think that natural pregnancies are more nuanced than that, although I do agree that it involves an ongoing moral disaster to some extent. I don’t think it’s immoral for a woman to become pregnant despite the high miscarriage rate—just as I don’t believe it was immoral for a woman living 1,000 years ago to become pregnant, even though a third of her children who were born would die by the age of 5. Instead, I think that there’s an imperative on society to develop medical technology that prevents (pre)natal deaths.
Does anyone here have qualms about the moral status of the embryos that are discarded in this process? I’m aware of the OP’s views on the issue, and I recently addressed them elsewhere, but I’m curious about the average viewer of this page.