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So "right to life" as a slogan is a question-begging term. The command not to kill is directed at the killing of persons, and the issue in abortion is this: When does the fetus become a person? The answer to that is not given by church teaching. Even St. Thomas Aquinas, who thought that a soul was infused into the body, could only guess when that infusion took place (and he did not guess "at fertilization"). St. Augustine confessed an agnosticism about the human status of the fetus.
Natural reason must use natural tools to deal with this question — philosophy, neurobiology, psychology, medicine. When is the fetus "viable," and viable as what? Does personality come only with responsibility, with personal communication? On none of these do the bishops have special expertise. John Henry Newman said, "The pope, who comes of Revelation, has no jurisdiction over Nature."
The evidence from natural sources of knowledge has been interpreted in various ways, by people of good intentions and good information. If natural law teaching were clear on the matter, a consensus would have been formed by those with natural reason. The fact that the problem is unsettled by them does not mean that a theological authority can be resorted to. An invalid authority (theology) does not become valid faute de mieux.
Church authorities have not acted on their own claims. Aborted fetuses, if they are persons, should be baptized, just as infants are, and buried in consecrated ground. But that has not been regular church practice. If abortion kills a person, then the woman who undergoes an abortion should be punished as a murderer — and the worst kind of murderer, a filicide. Church authorities have not demanded such punishment.
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http://www.nytimes.com/2004/06/27/opinion/27WILL.html?th