Some maintain that moral judgments are nothing more than the opinion of the judge.
The evidence for this is the fact that people make incompatible moral judgments.
The observable variation in moral opinion is not the only relevant evidence. It is also necessary to explain why morality is universal and why it is more effective to say that something is wrong than it is to say that you don’t like it. If morality were just opinions, it would constitute a superfluous idiom. If morality were just opinions, its effectiveness would be inexplicable.
The relativist hypothesis is that different moral codes are all basic rather than explicable in terms of underlying factors.
But we could expect differences in moral codes and moral opinions even if the basis of morality was shared by all moral agents.
The cause of the differences would be things like ignorance and error. There are so many relevant causal factors that the differences are inevitable.
In view of all the relevant observations and possible explanations, it is irrational to be a relativist.