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Moral realism holds that there are objective, mind-independent moral facts, and that moral claims can be straightforwardly true or false in virtue of those facts. On this view, 'torture for fun is wrong' is not merely an expression of preference but a claim about reality that would be true even if everyone disagreed. Major variants include naturalistic realism (moral properties reduce to natural ones), non-naturalism (moral properties are sui generis, as in Moore's view), and Cornell realism. Moral realism faces challenges from the queerness argument (Mackie) and the epistemological problem of how we access non-natural facts.
Read J.L. Mackie's 'The Subjectivity of Values' for the strongest anti-realist challenge, then read a response by Russ Shafer-Landau or David Enoch. Carefully separate the semantic thesis (moral claims are truth-apt), the metaphysical thesis (moral facts exist mind-independently), and the epistemological thesis (we can know them).
Moral realism is best understood by contrasting it with the alternatives from metaethics. When you say "slavery is wrong," what kind of claim are you making? Three main options: (1) you're expressing a feeling — expressivism; (2) you're making a claim that is systematically false because no moral facts exist — error theory; or (3) you're making a claim that can be straightforwardly true or false about mind-independent facts — moral realism. Moral realism takes moral discourse at face value: moral sentences are genuine assertions, and some of them are true.
The semantic thesis says moral claims are truth-apt — they have truth conditions. Connecting to first-order semantics: just as "the cat is on the mat" is true if and only if there's a cat on a mat, "torture for fun is wrong" is true if and only if torture for fun has the property of wrongness. The metaphysical thesis goes further: those truth conditions are satisfied or not independently of what anyone believes, feels, or desires. Moral facts are mind-independent in the same way mathematical facts are — they don't depend on our attitudes. The epistemological thesis then asks how we can know these facts, which is where moral realism faces its deepest challenges. Keeping these three theses distinct is the essential skill in evaluating realist positions.
The debate among realists concerns what moral properties are. Naturalistic moral realism holds that moral properties — rightness, goodness, wrongness — just are natural properties, describable by empirical science or ordinary observation. Identifying "goodness" with "what promotes wellbeing" is a naturalist move. Non-naturalism (G.E. Moore's position) holds that moral properties are sui generis — not reducible to any natural property. Moore thought "good" was a simple, indefinable property detectable by a kind of moral intuition. His "open question argument" was the weapon: for any natural property N you propose to identify with goodness, you can coherently ask "but is N actually good?" — suggesting good cannot be identical to N. Cornell realism occupies middle ground, holding that moral properties are natural but multiply realizable and not reducible to any single property.
The most powerful challenge to moral realism comes from J.L. Mackie's queerness argument: moral facts, if they existed, would be metaphysically strange entities unlike anything in the natural world — they would be intrinsically motivating, demanding action just by being known. Nothing in our ordinary ontological categories (universals, particulars, relations) resembles such an entity. Mackie concluded we should be moral anti-realists. Realists respond in several ways: some deny that moral facts must be intrinsically motivating (externalism about moral motivation); others argue that the "queerness" is no greater than the queerness already admitted in mathematics or logic. The exchange reveals that moral realism's tenability depends partly on resolving questions in philosophy of mind (what motivates action?) and epistemology (what counts as credible access to facts?).
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