Isaiah Berlin is accredited with having done the first substantial work on value pluralism, bringing it to the attention of general academia. The idea is that two or more moral values can both be correct (ultimate), but they can be at odds with each other.

Also, it says that in many situations, values that aren’t compatible with each other may be rationally incommensurable. So, value pluralism is a theory in metaethics, not a set of values or an ethical theory.