The two main forms of skepticism about morality are skepticism about moral truths and skepticism about reasons to comply with moral considerations. These doctrines challenge the cognitive significance or rational authority of morality.

Skepticism about moral truths denies that there are—or that we can know that there are—true moral propositions (or facts) that entail that something has a moral attribute. This form of skepticism seems to imply that rational and informed agents would give moral claims no credence. It has been supported by a variety of arguments, including arguments about moral disagreement. One deep motivation for it is the difficulty of explaining the normativity or action-guiding nature of moral claims.

Noncognitivists attempt to explain the normativity of moral judgments by supposing that their function is to express states of the speaker and to affect behavior rather than to express propositions. Noncognitivists would agree that...