Iain, regarding your post yesterday: You are quite right to suggest that science cannot tell us how to treat other living human beings. I don’t think Professor George suggested otherwise. In his initial posting on this topic, he identified three basic moral positions, none of which is dictated by science. One of them–the one he finds most plausible on philosophical grounds–holds that all human beings possess a profound, inherent, and equal fundamental dignity. That leaves the question: Are human embryos human beings or creatures of some other kind? Or, to put it a different way, when does the life of a new human being begin? Prof. George invoked the findings and authority of science to answer these questions. His point is that the scientific disciplines of human embryology and developmental biology have exploded the idea that there is some impenetrable mystery about when the life of a human being begins. The facts are in, and have been for some time. So I think there is less disagreement about the relation between scientific knowledge and moral conclusions than meets the eye.
Except, however, when it comes to this passage of yours: “Science certainly tells us that the zygote, blastocyst and embryo are potential human beings. It also tells us that, unless I am very much mistaken, actually a majority of zygotes will not become human beings (and so one might suggest, if one took a probabilistic approach, that the science tells us, on balance, we should not treat them as such).” If human beings are taken to be living organisms of the human species, then science tells us more than that human zygotes and blastocysts are “potential” human beings. They are nothing less than human beings at the earliest developmental stage. And in suggesting that science tells us that we should not treat them as such, I’m afraid that you are inadvertently mixing up science with (bad) philosophy.