 A related question concerns the role of reason in moral deliberation. If values are incommensurable, must all choices between conflicting values be ultimately subjective or irrational? If so, how does pluralism differ from radical relativism and subjectivism? If not, how, exactly, does moral reasoning work? How can we rationally make choices between values when there is no system or unit of measurement that can be used in making such deliberations? One possible answer to the last question is to offer an account of practical, situational reasoning that is not quantitative or rule-based, but appeals to the moral sense mentioned above. This is what Berlin suggests; but, once again, he does not offer a systematic explanation of the nature of non-systematic reason. (On incommensurability see Chang 1997 and Crowder 2002.)