From First Things:
The natural-law tradition is neither limited to Roman clerics and Protestant academics nor alien to American political life. The American Founding is largely based on natural law principles understood as “self-evident truths.” And the American civil rights movement can serve as a template for how religious reasoning should be brought to the public square and how it can result in meaningful political change. Consider how Martin Luther King, Jr.’s “Letter from a Birmingham Jail” quotes St. Augustine’s declaration that “an unjust law is no law at all.” He delves deeper into the Christian tradition to explain his point: “A just law is a man-made code that squares with the moral law or the law of God. . . . To put it in terms of St. Thomas Aquinas: An unjust law is a human law that is not rooted in eternal law and natural law.”
Underlying King’s argument, and that of the Christian tradition, is the proposition that human reason can know the moral law, the natural law, because human reason participates in eternal reason, the eternal law. Rather than argue from God’s commands down to human endeavors, social conservatives should place their emphasis on human flourishing and the moral principles that protect it. King put it best when he said: “Any law that uplifts human personality is just. Any law that degrades human personality is unjust.” Citing the Jewish philosopher Martin Buber, he went on to argue that segregation “substitutes an ‘I-it’ relationship for an ‘I-thou’ relationship and ends up relegating persons to the status of things.” This is the precise argument that social conservatives should be making when it comes to abortion, human cloning, and embryo-destructive research.
Of course, we need not make moral arguments alone. If Aquinas, Martin Luther King, Jr., and John Paul II are correct to say that true morality is about protecting human flourishing, then when true moral norms have been eviscerated we can expect to find the social fallout. With abortion the results need no social-science research: the fetal corpse is evidence enough. Yet social science indicates that the widespread practice of abortion—initially to be used in only the most tragic and desperate of situations—has led to practices that truly devalue human life: abortion on demand as birth control, selective abortion to reduce the number of children when twins or triplets result from IVF, eugenic abortion to do away with genetically “defective” children, and now the practice of embryo destruction for biomedical research, human cloning, and animal-human hybrids. These are the fruits of the abortion seed.