This paper argues that naturalistic moral realism is vulnerable to a 'Hard Problem' that has gone largely unrecognised. This problem is to explain how natural moral properties are detected by the folk. I argue that Thomas Nagel's persuasive case for moral realism founded on the priority of first-order moral evaluations over second-order reflection is not conclusive - a certain type of moral agnosticism which I call Meta-Ethical Pyrrhonism can account for our inability to think of first-order moral evaluations as merely subjective or relative. Although unsatisfactory as metaphysics, Meta-Ethical Pyrrhonism is arguably all that a moral naturalist is entitled to by way of a meta-ethical theory.