What is a Person?

So much of the disagreement on abortion hinges on one particular issue: When does a person begin to exist, with all the rights of a person? (Certainly, even the “right-to-life” terminology, taking its words from the Declaration of Independence, with its statement that “all men [we would now say ‘persons’] are created equal, that they are endowed by their Creator with certain inalienable rights, that among these are life, liberty, and the pursuit of happiness,” depends on this question as to whether a fetus is a person!) Certainly, my leg or my finger is not a person, though it is alive and all of the cells therein are definitely human, so we have never heard of “pro-life” activists protesting surgical amputations. I have heard that the people who insist that a fetus is a person use, as the basis for this claim, that it has a new genome, different from either parent. The problem with this “independent genome” argument is that by this criterion, two identical twins (the technical term, actually, is “monozygotic” twins) are not separate persons. Their genomes are alike. (I have pointed this out in a previous post, which has been read by many visitors.) This is why I have thought that the “independent viability” criterion is the one to use; if a purported person can be separated from any other human being and can survive, then it counts as a separate human being. The only thing, this definition has problems too. Consider “Siamese” twins. They can usually be separated and both survive. But in some cases, they share a vital organ. One could not separate them without killing one. Yet I certainly would admit that these are two separate persons. (The “independent genome” definition, favored by “pro-lifers,” fails just as well; these twins certainly have a common genome!)

I don't know. How do you define a person in a way that would take care of such a case? I'd be interested in others' ideas.