What Is Moral Skepticism? Definition & Arguments

Direct Answer

Moral skepticism is the view that we cannot have moral knowledge — or, in its stronger forms, that there are no moral truths to know. The position comes in two main varieties. Epistemological moral skepticism concedes, for the sake of argument, that moral truths might exist, but argues that we cannot know them: our moral beliefs are so shaped by culture, upbringing, and bias, and moral disagreements are so persistent, that no moral belief can be justified as knowledge. Metaphysical moral skepticism — usually called error theory or moral nihilism — denies that moral truths exist at all. Its most famous modern defender is J.L. Mackie, who argued in Ethics: Inventing Right and Wrong (1977) that moral judgments presuppose "objective prescriptivity" — objective values that exist in the world and command us — and that no such values exist. Since the presuppositions of moral language are false, all positive moral claims are false. The two forms of skepticism are independent: one could hold that moral truths exist but are unknowable, or that they do not exist and hence are trivially unknowable.

Historical Context

Moral skepticism has ancient roots. The Sophists argued that moral and legal norms are conventions rather than truths, and Socrates' elenctic questioning regularly exposed the fragility of his interlocutors' moral certainties. The modern skeptical tradition draws on the Humean separation of facts and values: if morality is grounded in sentiment rather than reason, and sentiments vary, the objective status of moral claims is immediately threatened. The decisive modern statement came from Mackie in 1977, who combined the argument from queerness with the argument from disagreement to conclude that the "fabric" of objective values is an illusion. The twentieth century also saw non-cognitivism — the view that moral judgments are expressions of attitude rather than truth-apt beliefs — which is a form of skepticism about moral knowledge in the sense that it denies there is any moral knowledge in the factual mode. Against the skeptics, the realist tradition — from Plato and Aristotle through Kant to contemporary realists such as Thomas Nagel and Peter Railton — has insisted that moral knowledge is possible.

Key Arguments & Debates

The skeptic's arsenal contains two main arguments. The argument from disagreement runs: moral beliefs differ radically across cultures, across history, and even within a single society; these disagreements are not resolvable by investigation the way scientific disagreements are; if moral truths existed and were accessible, we would expect convergence; so there is no moral knowledge. Realists respond that disagreement exists in science too, that much apparent moral disagreement dissolves once factual questions are settled, and that some moral claims — "gratuitous cruelty is wrong" — command near-universal assent. The argument from queerness runs: objective moral properties would be "queer" entities, unlike anything in the natural world, and the faculty by which we supposedly know them would be equally queer; we have no reason to believe in either. Naturalists respond that moral properties can be reduced to natural properties, avoiding queerness; intuitionists respond that non-natural normative knowledge is no queerer than mathematical knowledge. The debate also engages the is-ought problem: if values cannot be derived from facts, how could moral truths be known? The outcome of these disputes determines whether moral knowledge — and with it, moral responsibility, human rights, and moral progress — can be rationally grounded.

Contemporary Relevance

Moral skepticism is not merely an academic position. Popular doubt about morality — "right and wrong is just a matter of opinion," "morality is a human invention," "we can't know what's truly right" — is amateur moral skepticism, and it has real consequences for how people approach moral argument, moral education, and moral commitment. At the same time, sophisticated skepticism raises serious questions about the foundations of human rights: if there are no objective moral truths, what grounds the claim that torture is objectively wrong? Philosophers answer in different ways — some embrace skepticism and try to build morality on other foundations, others defend realism against the skeptical challenge. Understanding the arguments on both sides allows you to see what is at stake in the popular claim that morality is "just an opinion."

Related Concepts

Moral skepticism is closely related to, but distinct from, moral nihilism (the denial of moral value) and the error theory (the claim that moral judgments are systematically false). It is the skeptical counterpart of moral epistemology, which asks how moral knowledge is possible, and it draws on the is-ought problem. The debate is framed by moral realism and anti-realism.

Further Learning

The Stanford Encyclopedia of Philosophy's entry on moral skepticism is the authoritative survey. The Internet Encyclopedia of Philosophy's entry on moral knowledge and moral skepticism offers a clear introduction. Mackie's Ethics: Inventing Right and Wrong (Penguin, 1977) is the classic modern statement.