In the parade of faces talking about Terri Schiavo last week, two notable authorities were missing: Aristotle and Descartes. Yet their legacy was there.

Beneath the political maneuvering and legal wrangling, the case re-enacted a clash of ideals that has run through the history of Western thought. And in a way, it's the essential question that has been asked by philosophers since the dawn of human civilization. Is every human life precious, no matter how disabled? Or do human beings have the right to self-determination and to decide when life has value?

"The clash is about how we understand the human person," said Samuel Gregg, director of research at the Acton Institute for the Study of Religion and Liberty, a conservative policy group.

The plea last week to prolong Ms. Schiavo's feeding, against the wishes of her husband or what courts determined to be her own expressed inclinations, echoed the teachings of Aristotle, who considered existence itself to be inviolable.

On the other side, the argument that Ms. Schiavo's life could be judged as not worth living echoed Descartes, the Enlightenment philosopher who defined human life not as biological existence - which might be an inviolable gift from God - but as consciousness, about which people can make judgments.

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The philosophical line in this history, then, is not straightforward, but includes a peculiar American twist: The evangelical revival of the 18th and 19th centuries produced the abolition movement, which gave rise to the women's suffrage movement, which inspired the civil rights movement, which led to the patient's rights movement. But now the patient's rights movement faces off with many 21st-century evangelical Christians in the Schiavo case.

At the same time, the scientific legacy of the Enlightenment, which argued that human life resided not in the body but the mind, is now being undermined, as modern neuroscience demystifies elements of thought and personality as heartless biochemical or genetic processes. The mind is simply prisoner to the body's DNA.

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