In the Republic of Ireland, though, abortions are not readily available even when the life of a mother is at risk. But moral relativists defend an even stronger claim that our main moral values vary in at least some cultures. According to many relativists, then, moral standards of different cultures are like isolated islands of values, each of which gains its justification through the social customs of that particular culture. Critics of moral relativism sometimes challenge the idea of cultural variation, defined above. For example in Elements of Moral Philosophy James Rachels attacks moral relativism arguing that there is in fact a core set of values that are common to all societies and are in fact necessary for any society to exist.
Critics also point out problems of consistency with moral relativism.
For example, if moral relativism is true, then we could no longer say that customs of other societies such as slavery are morally inferior to our own.