This paper argues that functionalist metasemantic views, such as Conceptual Role Semantics and Interpretivism, face a Euthyphro challenge. The challenge, put roughly, is this: functionalist metasemantic views reverse the order of explanation. According to such views, representational mental states have the contents that they do partly because they play certain roles in our mental lives. According to an intuitive picture of the roles that representational mental states play in our mental lives, however, these states play the roles they do partly (...) because they have the contents that they do. Since explanations—even partial ones—are asymmetric (call this principle ‘the Euthyphro Principle’), we can’t accept both this intuitive picture and a functionalist metasemantics. Hence, we should reject all versions of the latter. This paper can be seen as an attempt to better articulate an intuition that has been suggested in various forms in the literature. The paper proceeds as follows. First, it shows that functionalist metasemantic views imply certain ‘partly because’ claims. Second, it argues for the Euthyphro Principle. Third, it defends the intuitive picture of representational mental states and shows that it implies ‘partly because’ claims that conflict with those made by functionalist metasemantic views. Finally, it considers ways of relaxing the intuitive picture and finds them all wanting. (shrink)