Andrew Sullivan has a very interesting piece on Terri Schiavo.

I have used the Catholic arguments because this case involved a Catholic, but they apply more generally. What we have lost today is the prudence and moderation of old moral teaching. Thomas Aquinas, the father of Catholic natural law theology, argued that a human being did not exist as such immediately after conception. He believed that the soul entered at about the time of “quickening” — roughly the first trimester.

Even today we accept that a fertile procreating woman spontaneously aborts countless fertilised eggs after conception. That does not make her guilty of involuntary manslaughter on a massive scale. So the abortion of a foetus the morning after conception is intuitively different from an abortion in the third trimester. But that insight was dropped by the church in its fierce opposition to every aspect of modernity in the last part of the 19th century.

Relative judgments get turned into absolute ones when religion feels threatened by new technology or new ways of life. We live in an era of two great trends. One is the miracle of modern science and its awesome capacity to prolong and better life. The other is the rise of religious fundamentalism in which bewilderment at technology, global change and cultural and moral diversity understandably leads some to cling to the most absolute of moral claims.