Many philosophers, from Plato and Aristotle through to contemporary writers such as Russ Shafer-Landau and Judith Thomson, have endorsed some version of moral objectivism.
One important problem for objectivism turns on the deep differences in moral views we find between cultures. For instance, many societies have accepted forms of slavery, which most today find abhorrent. There are also, of course, deep differences in moral views within cultures.
According to 20th century philosopher John Mackie, these observations undermine objectivism. He writes:
“variation in moral codes from one society to another and from one period to another, and also the differences in moral beliefs between groups and classes within a complex community… make it difficult to treat those judgments as apprehensions of objective truths” (Ethics, p.36).