The Problem with Personhood, Part 2:
"There are three positions that can be defended without quickly falling into logical inconsistency.
The first is that human beings are in no morally relevant way different from other creatures and therefore have no special dignity.
The second is that human beings have an inherent and equal dignity; each and every human being possesses it simply by virtue of his or her humanity.
The third is that some, but not all, human beings have dignity; those who have it possess it by virtue of some quality or set of qualities that they happen to possess that other human beings do not possess (or do not yet possess, or no longer possess).
Anyone who believes that stepping on an ant is not a grave moral wrong but murdering your grandmother to prevent her spending down your inheritance is one, has already rejected the first position. Anyone who accepts the third position will, in fairly short order, find himself driven by the force of logical argumentation into the positions infamously defended by Peter Singer. Assuming one doesn't want to embrace Singerism, that leaves the second position.
Now, once one adopts that position, the key question in the debate over embryo-destructive research is "When does the life of a human being begin?" To answer this question is to decide whether or not human embryos are, in fact, human beings and, as such, possessors of inherent human dignity. Where do we go to find the answer? Not to the Catechism of the Catholic Church... Not to the Bible, which says nothing about human embryos. Not to the Talmud, which (like the Bible) was composed centuries before the discovery of the ovum----a time when almost nothing was known about embryogenesis. Not to the Koran. Not to our "moral intuitions." Rather, we go to the standard texts of modern human embryology and developmental biology---for example, the texts by Keith Moore and T.V.N. Persaud; Bruce M. Carlson; Ronan O'Rahilly and Fabiola Mueller; and William J. Larsen. When we consult these works, we find little or nothing in the way of scientific mystery or dispute. The texts tell the same story and answer the key question in the same way. Anyone who wishes to know when he or she as a distinct living member of the species Homo sapiens came into existence need only open any of these books and look up the answer." (R. George in The Corner, NRO On-Line, 8/8/05)
Science, in other words, does not tell me the embryo is valuable. It tells me what kind of thing the embryo is. After that, it's up to philosophy and metaphysics to help me determine how we should value human beings, both inside and outside the womb.