Realism and the objectivity of moral facts
Moral realism holds that moral facts exist objectively, independent of what anyone believes. Torturing children for entertainment is wrong not because we feel disgusted but because torture is genuinely, factually wrong. This view takes morality seriously and explains why we talk as if moral disagreements can be resolved and some people are simply mistaken.
Anti-realism denies objective moral facts. Error theory agrees that our moral language presupposes objectivity but concludes we are all mistaken because no objective moral facts exist. Expressivism argues that moral statements express attitudes or commitments rather than describing facts. Constructivism occupies middle ground: moral facts are real but constructed from human practices and reasoning rather than discovered in the world.
Implications for moral disagreement
If realism is true, moral disagreement is a problem demanding resolution, like scientific disagreement. If expressivism is true, moral disagreement is more like aesthetic disagreement: your assertion that something is beautiful expresses your attitude; his assertion that it is ugly expresses his. Neither is wrong, just different.
Constructivism suggests moral disagreement tracks real differences in frameworks, practices, and reasoning, so some moral positions can be better-supported than others without being objectively true in a realist sense. Which meta-ethical position you hold reshapes how you approach moral disputes. Understanding the distinction between realism, anti-realism, error theory, expressivism, and constructivism clarifies what is actually at stake in debates about what is morally true.