A statement such as this asserts that certain behaviour is always right, or always wrong (at least in our society), and not just a matter of personal preference. Although people may disagree about whether a particular assertion is true or false, each group believes that it knows the truth and that those who disagree are in error. Those who believe the earth is flat (scientists say) are simply wrong – this is a matter of scientific truth, not a matter of opinion. In the same way, those who believe slavery is morally acceptable are wrong – this is a moral truth, not a matter of opinion.
Moral rules cannot be proved by scientific experiment, of course, but then the law of gravity cannot be proved by moral reasoning! Some moral philosophers (called realists) see moral assertions as inherently true or inherently false: there may be uncertainty and argument about their truth, but they have an eternal truth or falsity independent of changes in society. Other philosophers (called relativists) argue that moral truths may change from time to time and from place to place.
Three hundred years ago it was morally acceptable for a husband to beat his wife if she misbehaved – he would have been failing in his duty if he did not – but such a thing would clearly be immoral today. Polygamy is morally acceptable in Muslim countries, but drinking alcohol is not; in Western Christian countries the moral rules are reversed. But the moral rules, say the relativists, are determined by the general attitudes of the society and not by the opinions of each individual separately.
Unless we accept the inherent existence of moral rules independent of personal opinion, it makes no sense to criticise as immoral or "wrong" anything that anyone else does. Hitler and the Nazis sincerely believed that Jews were sub-human and that they had a moral duty to rid the world of that "inferior race": we have no hesitation in saying that the Nazis' views and actions were morally wrong, regardless of their own opinions on the matter. Most people agree that theft is immoral regardless of what the thief thinks about it – they claim this as a general moral rule, not a mere matter of opinion.
Legal rules and moral rules The relationship between law and morality is not an easy one. The legal system presupposes a certain amount of morality, because if law is not essentially moral there is no easy explanation of the obligation to obey. Moral rules and legal rules have some similarities: like all rules, according to Hart, they share a general (though not necessarily universal) habit of obedience within the society to which they apply, and a "critical reflexive attitude" (a sense of "ought-ness"). These matters are discussed further on another webpage.
But moral rules and legal rules are certainly not the same: there are some legal rules that are not moral rules, and vice versa. Moral rules, said Hart, can be distinguished from legal rules and other social rules in several ways. There can be no doubt that the law of every modern state is strongly influenced by moral considerations. That is not to say that law and morality always coincide: clearly they do not. Whether a wholly immoral legal system is still a legal system is a question discussed in more depth in the context of Natural Law theories, but even if we say morality is essential we have still to decide which morality.
Is the morality of the slave-owners, the Aryans or the white voters good enough? Whether we are realists or relativists we must decide what the moral rules are: morality itself may or may not change, but the public understanding of morality certainly does. We take it for granted now that all human beings are entitled to the same human rights, but only two hundred years ago the prevailing morality of Western Europe and America was that black people were less than human.
Will the society of two hundred years hence look back in horror at our refusal to grant equal rights to primates of non-human species, merely because they cannot talk? Certainly there are some long-established rules that are legal rules as well as moral rules, and were probably adopted as part of common law as much for moral as for practical reasons. (This view would be disputed by some adherents of American realism, who suggest morality may have been the result of a law backed by sanctions, rather than vice versa.)
The moral rule that "thou shalt not kill" finds its legal expression in the common law offence of murder, and the moral rule against stealing coincides with the legal prohibition of theft, another very ancient crime even though now codified. The moral prohibition against bearing false witness against a neighbour is reflected in the tort of defamation and the offence of perjury, and blasphemy against Christ or the Christian religion is a (controversial) common law offence.
Nearly all western countries (including the United Kingdom) prohibit the practice of euthanasia, thereby giving effect to the supposed moral rule that deliberately killing another human being is wrong even when that other has consented to (or perhaps positively asked for) the killing. Some of these countries (this time excluding the UK) have no qualms about killing criminals who have not consented to the killing, but the moral exception justifying capital punishment is not easy to identify. In April 2001 the Netherlands Parliament broke new ground by enacting a law making euthanasia lawful under certain circumstances.
For this to apply, the patient must be suffering continuous, unbearable and incurable pain (confirmed by two doctors), must be of sound mind, and must voluntarily and persistently have asked to be killed. This new law goes further than the semi-official "blind eye" policy of the past twenty years or so. No other western country allows euthanasia, though some (such as Sweden and Switzerland) allow doctors to assist a patient's suicide and several (including the United Kingdom) allow doctors to withhold lifesaving treatment even though that may lead to death.
Belgium is thought likely to follow the Netherlands' example by legalising active killing. Some moral rules have been given effect by statute. The moral censure of those who deal in pornography is given legal effect by the Obscene Publications Act 1959, making it illegal to possess any obscene material with a view to its sale or other publication, and the widespread condemnation of incest (seen by many people as morally wrong even when both parties are adult and consenting) led to its being criminalised by the Punishment of Incest Act 1908.