Saletan vs. George/Tollefsen on the Embryo

In Sunday's New York Times, William Saletan reviewed Robert George's and Christopher Tollefsen's new book, Embryo: A Defense of Human Life.  George and Tollefsen responded, and now Saletan has made some additional points in reply.  It's a debate that is helpful and productive, shedding more light than heat.  An excerpt from George and Tollefsen:

In attempting to resist our conclusion that human embryos ought not to be exploited and killed, while at the same time acknowledging their moral standing and the special respect they are owed, Saletan gets himself into a jam.  To meet our argument that a human embryo is, as a matter of scientific fact, a developing human being—i.e., a living member of the species Homo sapiens in the earliest stages of development—and thus, as a matter of basic justice, a possessor of inherent dignity and a right to life, Saletan is driven to deny that human embryos are whole entities, as opposed to mere parts (such as gametes, tissues, or organs).  He denies that embryos are determinate individuals, and he seems to doubt that they are organisms at all.  But if these denials and doubts are warranted, then there is no rational basis for believing that human embryos “deserve our respect” or that “we should never create or destroy them lightly.”  Saletan is trying to find a plot of solid ground lying between the views of radical liberal bioethicists, on the one side, and defenders of the pro-life view, on the other.  The failure of his effort shows that the middle ground is nothing but quicksand.

An excerpt from Saletan:

The virtue of Embryo is that the authors stake their case on science and logic, not religion. What makes you a human being, they argue, isn't a soul, but "a developmental program (including both its DNA and epigenetic factors) oriented toward developing a brain and central nervous system." They believe that this program starts at conception, and therefore, so does personhood.

I like this bet on science. It's scrupulous, brave, and constructive. Let's toss in our chips and call the bet. We'll have to accept what science shows: Conception is, as George and Tollefsen argue, the sharpest line we could draw to mark the onset of moral worth. But they, in turn, will have to accept the other side of what science shows: The lines of embryology are dotted, not solid. Such lines don't warrant severe categorical restrictions on stem-cell research or assisted reproduction.