People who come to moral theology with a legalistic outlook tend to identify human acts with outward behavior. Thus changes in moral judgment due to change or unfolding of the meaning of an act seem to suggest that the same act now has a different moral value. If one who reaches this conclusion holds a scholastic natural-law theory (criticized in 4‑F), he or she will assume that if the same act has a different moral value, the other term of the relationship which determines morality, human nature itself, has changed. Thus the historicity of action, a misunderstanding of what constitutes a human act, and a poor moral theory lead to the conclusion that human nature changes.

If one also assumes some form of proportionalism, one will suppose that moral norms are merely rules devised to protect and promote human goods in a particular historical context, and that these norms can change when necessary to promote the good or bring about less evil. It is easy at this point to suppose one can use as one’s standard the relative importance given to the basic goods in the life-style of a given society.⁹⁸ The conclusion will be drawn that changing human nature demands a proportionalist transformation of all moral norms, including absolute ones.

The difficulty with this position is that although even basic human goods unfold new dimensions of meaning (7‑D), all human persons have the same nature (see GS 29), and so each good has some invariant meaning. Thus, acts which include a proposal to destroy, damage, or impede some basic human good—for example, the life of an unborn child—can be precisely the same in kind insofar as they are opposed to the invariant aspect of the good of life, although they differ in kind insofar as they are done by people in different cultures and situations, who understand more or less fully the good of human life.

The fundamental requirements of Christian moral life which demand reverence for the basic human goods are not concerned with something merely instrumental. While these requirements gain new and deeper meaning as our understanding of the goods grows, they cannot be contradicted, because they express in a direct way the minimal demands of love in respect to these goods. Because they are truths, not mere rules, these norms cannot be changed.

The Christian norm forbidding adultery, for example, has a depth absent from the Old Testament norm, which was not understood to be protecting so great a good as the sacrament of marriage we know. The exclusion of adultery in itself, moreover, hardly begins to suggest what marital love means. Nevertheless, the absolute prohibition of adultery is always valid and it is extremely important. For this exclusion defines the marital relationship without limiting its power to develop new meaning. If one attempts to define marriage without excluding adultery one will have to invoke some specific, positive form of love. In doing so, one will limit what marriage can be. Such limitation will foreclose the human possibilities of marriage before exploring what is foreclosed.

Just as true negative propositions about God do not define him but preserve the silence in which he reveals himself, and just as definitive condemnations of heresies do not limit our relationship with God but preserve unbroken the line of communication we have with him, so absolute moral prohibitions do not limit human freedom to pursue the good but preserve intact our understanding and love of the basic human goods.

To assume that one can criticize and perhaps revise such moral norms by taking as a standard the relative importance assigned to the basic human goods in the life-style of a given society—for example, that of the contemporary nations of the West—is to lose one’s historical consciousness. Historicity does not exclude but demands insight into the unity of each basic good of human persons, a real unity over time and place which cannot be limited without arbitrariness to the contingent conditions of the here-and-now which delimit one’s present point of view. Only with insight into the unity of the human goods can the various ages and conditions of humankind be understood as a history—as the one universal history of salvation, to which our Lord Jesus is always and everywhere present—rather than as a disjoined succession of arbitrarily delimited sociocultural points of view.

To understand how negative norms protect human goods and so permit them to be creatively unfolded helps one understand another important aspect of legitimate development in Christian moral thought. Today everyone realizes that enslaving anyone—and buying and selling people in general—are great crimes; the Church condemns these and many other acts against the person (see GS 27). Surely, one thinks, slavery always was wrong. Yet the Church did not always condemn it, and many teachers in the Church at times defended it as licit. How could Christian morality be so inadequate?

The answer is that although the liberty violated by slavery does pertain to justice, and so is a good which always deserves reverence, the aspects of justice which involve liberty were much less well understood in times past than they have come to be in recent centuries. The chief obstacle to clarity was that much slavery was related to punishment, and penal servitude can be just. The errors involved in condoning slavery related to confusions between the public and private spheres, as well as misplacement upon children of responsibility for the wrongdoing of their parents or more remote ancestors.

Gradually, these matters were clarified There are many reasons for this development of understanding; it was powerfully advanced by secular humanists. Secular humanism, one must remember, is a Christian heresy; the principles of the human individual liberty it promotes belong to the Christian conception of redemption and deification (see Gal 3.28; Phlm 15–16; Jas 2.12–13). However, Christians did not at once draw all the conclusions from their principles. (Secular humanists, of course, draw some false conclusions from them.)

The development of moral doctrine with respect to slavery was from condoning it to forbidding it. The change concerning slavery can be seen as possible because of the unfolding understanding of the human good; now that the development has occurred, we find it hard to see why it did not occur much sooner. The supposed development which radically dissenting theologians proposed would have been from forbidding kinds of acts to permitting them—indeed, in cases in which they were considered the “lesser evil,” enjoining them. Such a change could not occur because the received prohibitions are based on aspects of the human goods which are already understood. One can no more learn something new which would lead one to find adultery good than one can learn something new which would lead one to find slavery good.