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 there are no true moral propositions, since they hold that moral claims do not express propositions. Yet they do not view moral claims as defective. According to noncognitivists, one who makes a claim, such as "Truthfulness is morally required." expresses a moral attitude or acceptance of a moral norm (Ayer,  1946; Gibbard, 1990; cf. Hume, [1739–1740] 1978).
Cognitivists object that our moral thinking cannot be understood except on the assumption that moral claims express propositions. To avoid skepticism, cognitivists must believe that there are moral properties that are sometimes exemplified. For if no moral property exists, or if none is exemplified, it follows that there are no moral requirements, no moral goods or bads, no moral virtues or vices. It may follow that there are no honest persons, for example, although there may be truthful persons.
A skeptic might hold that moral properties exist but that none is exemplified. This position seems implausible, however, for if there is the property of wrongness, it would be astonishing if nothing were ever wrong. Alternatively, a skeptic might argue that there are no moral properties. According to widely accepted views about propositions, however, the proposition that lying is wrong, for example, would attribute the property wrongness to acts of lying. The property would be a constituent of the proposition. Hence, if there are no moral properties, these views about propositions may lead to the conclusion that no proposition is expressed by sentences such as "Lying is wrong."
J. L. Mackie argued that there are no moral properties (1977). We conceive of moral properties as intrinsic; if an action is wrong, it is wrong "as it is in itself." But we also conceive of moral properties as intrinsically action guiding; we can be motivated to act in an appropriate way simply by coming to know that an action would be wrong, regardless of any antecedent motivations. Yet, Mackie thought, it is not intelligible that it be intrinsic to an action's having an intrinsic property that the mere recognition that the action has the property could motivate a person. The idea of a moral property is not intelligible; moral properties would be metaphysically "queer."
Gilbert Harman (1977) argued for an epistemic version of skepticism about moral truths. He argued that there seems to be no good reason to affirm any moral proposition, for moral hypotheses are never part of the best explanation of any observation. There is always a better nonmoral explanation. The belief that there are true moral propositions is therefore unwarranted.
Skepticism about moral truth appears to have a life of its own in secular cultures, independent of skeptical arguments. Some people believe that moral truths are grounded in God's commands. A secular culture would tend to think, however, that all substantive facts are empirical and "natural." And natural facts do not seem to be normative in the way moral facts are normative. It is therefore difficult to see how a natural fact could be a moral fact.
The second skeptical doctrine is the thesis that there need be no reason to comply with moral considerations. According to this thesis, rational agents would not give attention to moral considerations, as such, in deciding how to live their lives. To be sure, we may desire to live morally, and this desire may give us a reason to live morally. Or we may find ourselves in a context in which living morally is in our interest. Yet these possibilities do not show that there is necessarily a reason to comply with moral considerations (Nielsen, 1974); they do not distinguish moral considerations from considerations of etiquette, for example.
Skepticism about compliance is typically motivated by the idea that morality can require actions that are not to the agent's advantage. Assuming that there are reasons for one to do something just in case it would be to one's advantage, this idea implies that there may be no reason to comply with morality.
The two main skeptical doctrines are closely linked, on certain ways of thinking. First, it may seem, we cannot be guaranteed to have reasons to comply with moral considerations unless there are moral truths of which we have knowledge. Second, a kind of "internalist" theory holds that moral facts are "constituted" by reasons. On this view there are no moral facts unless there are reasons of a relevant kind.
Internalist antiskeptical theories attempt to defeat both skeptical doctrines at once. Immanuel Kant held, in effect, that if a moral imperative corresponds to a truth, it does so in virtue of the fact that it would be complied with by any fully rational agent (Kant,  1981). "Externalist" theories attempt to deal with skepticism about moral truths independently from skepticism about compliance (Sturgeon, 1985). Those who believe that moral truths are grounded in God's commands may suppose, for example, that God necessarily gives us reasons to comply.
Philosophers who accept one of the skeptical doctrines typically try to defuse it. Skeptics about rational compliance may argue that people with normal psychologies invariably have reasons to comply with morality. Skeptics about moral truth may argue that there nevertheless are reasons to engage in the practice of judging things morally.
Ayer, A. J. Language, Truth, and Logic (1936). London: Gollancz, 1946.
Copp, D. "Moral Skepticism." Philosophical Studies 62 (1991): 203–233.
Gibbard, A. Wise Choices, Apt Feelings: A Theory of Normative Judgment. Cambridge, MA: Harvard University Press, 1990.
Hume, D. A Treatise of Human Nature (1739–1740). Edited by P. H. Nidditch. Oxford: Clarendon Press, 1978.
Kant, I. Grounding for the Metaphysics of Morals (1785). Translated by James W. Ellington. Indianapolis: Hackett, 1981.
Mackie, J. L. Ethics: Inventing Right and Wrong. Harmondsworth, U.K.: Penguin, 1977.
Nielsen, K. "Why Should I Be Moral?" In Introductory Readings in Ethics, edited by W. K. Frankena and J. T. Granrose. Englewood Cliffs, NJ: Prentice-Hall, 1974.
Nietzsche, F. Basic Writings of Nietzsche. Edited and translated by Walter Kaufmann. New York: Modern Library, 1968. See The Genealogy of Morals and Beyond Good and Evil.
Sturgeon, N. "Moral Explanations." In Morality, Reason, and Truth, edited by D. Copp and D. Zimmerman. Totowa, NJ: Rowman and Allanheld, 1985.
David Copp (1996)