My aim is to challenge recent attempts at reconciling moral realism and naturalism by pushing ethical naturalists into a dilemma. According to one horn of the dilemma, ethical naturalists must either (a) build unique facts and properties about divergent social structures (or varying moral belief systems) into their subvenient sets of natural facts and properties, and so jeopardize the objectivity of moral truths, or (b) insist, in the face of all possible worlds in which people have different moral beliefs than ours, that they are all mistaken—this despite the fact that the belief-forming mechanism responsible for their moral beliefs was never concerned with the truth of those beliefs in the first place. This will bring me to suggest that moral properties might only weakly supervene upon natural phenomena. But, according to the other horn of the dilemma, weak supervenience is a defeater for moral knowledge.

Une des questions centrales de la métaéthique est celle de savoir si les propriétés morales sont des propriétés naturelles ou non-naturelles. Ce mémoire fait valoir que nous ferions bien de remettre en question une constellation d’arguments en faveur du non-naturalisme moral : l’argument de la question ouverte, l’intuition normative et l’argument du gouffre. L’influent argument de la question ouverte de Moore, d’abord, ne soutient le non-naturalisme que s’il commet une pétition de principe. L’intuition normative commet ou bien le sophisme d’inférer sur la base de sa différence spécifique qu’une chose n’appartient pas à un genre donné, ou bien sous-estime la panoplie de propriétés naturelles qui possèdent les caractéristiques censées être distinctives des propriétés morales et normatives. L’argument du gouffre, quant à lui, sous-estime l’ubiquité du fossé logique et conceptuel censé marquer une discontinuité métaphysique profonde entre les domaines normatif et naturel. / One of the burning questions among metaethical realists is whether moral facts and properties are natural or non-natural. In this thesis, I argue that we should treat a family of arguments for non-naturalism with considerable scepticism: the Open Question Argument, the Normative Intuition, and the argument from the Is-Ought Gap. Moore’s famous Open Question Argument only supports moral non-naturalism if it begs the question against the modest (non-reductionist) naturalist. As for the Normative Intuition, it either commits the fallacy of inferring on the basis of a thing’s specific difference that it does not belong to the genus it putatively belongs to, or it underestimates the breadth of natural properties that possess the features which non-naturalists allege are distinctive of moral and normative properties. The argument from the Is-Ought Gap, for its part, underestimates the ubiquity of the logical and conceptual gap that allegedly marks a deep metaphysical discontinuity between the normative and natural domains.