As best as I can tell, this law appears to apply peculiarly to human beings. Though other animals may at times appear to show glimmerings of a moral sense, they are certainly not widespread, and in many instances other species' behavior seems to be in dramatic contrast to any sense of universal rightness. It is the awareness of right and wrong, along with the development of language, awareness of self, and the ability to imagine the future, to which scientists generally refer when trying to enumerate the special qualities of Homo sapiens.
But is this sense of right and wrong an intrinsic quality of being human, or just a consequence of cultural traditions? Some have argued that cultures have such widely differing norms for behavior that any conclusion about a shared Moral Law is unfounded. Lewis, a student of many cultures, calls this "a lie, a good resounding lie. If a man will go into a library and spend a few days with the Encyclopedia of Religion and Ethics, he will soon discover the massive unanimity of the practical reason in man. From the Babylonian Hymn to Samos, from the laws of Manu, the Book of the Dead, the Analects, the Stoics, the Platonists, from Australian aborigines and Redskins, he will collect the same triumphantly monotonous denunciations of oppression, murder, treachery and falsehood; the same injunctions of kindness to the aged, the young, and the weak, of almsgiving and impartiality and honesty." In some unusual cultures the law takes on surprising trappings -- consider witch burning in seventeenth-century America. Yet when surveyed closely, these apparent aberrations can be seen to arise from strongly held but misguided conclusions about who or what is good or evil. If you firmly believed that a witch is the personification of evil on earth, an apostle of the devil himself, would it not then seem justified to take such drastic action?
Let me stop here to point out that the conclusion that the Moral Law exists is in serious conflict with the current postmodernist philosophy, which argues that there are no absolute rights or wrongs, and all ethical decisions are relative. This view, which seems widespread among modern philosophers but which mystifies most members of the general public, faces a series of logical Catch-22s. If there is no absolute truth, can postmodernism itself be true? Indeed, if there is no right or wrong, then there is no reason to argue for the discipline of ethics in the first place.
Others will object that the Moral Law is simply a consequence of evolutionary pressures. This objection arises from the new field of sociobiology, and attempts to provide explanations for altruistic behavior on the basis of its positive value in Darwinian selection. If this argument could be shown to hold up, the interpretation of many of the requirements of the Moral Law as a signpost to God would potentially be in trouble -- so it is worth examining this point of view in more detail.
Consider a major example of the force we feel from the Moral Law -- the altruistic impulse, the voice of conscience calling us to help others even if nothing is received in return. Not all of the requirements of the Moral Law reduce to altruism, of course; for instance, the pang of conscience one feels after a minor distortion of the facts on a tax return can hardly be ascribed to a sense of having damaged another identifiable human being.