Jonathan Schaffer, among others, has argued that metaphysics should deal primarily with relations of "grounding." I will follow John Heil in arguing that this view of metaphysics is problematic, for it draws on ambiguous notions of grounding and fundamentality that are unilluminating as metaphysical explanations. I take Heil's objections to presuppose that "grounding" relations do not form a natural class, where a natural class is one where some member of that class has (analytic or contingent a posteriori) priority among others and explains order among other members in the class. To strengthen Heil's criticism that "grounding" is a non-natural class of relations, I will draw on an unlikely ally. Thomas Aquinas's "analogy of being" doctrine, if accurate, offers reasons that no categorical relations (like grounding relations) form a natural class.