Some philosophers think that knowledge or justification is both necessary and sufficient for rational action: they endorse knowledge-action or justification-action biconditionals. This paper offers a novel, metaphysical challenge to these biconditionals, which proceeds with a familiar question: What depends on what? If you know that p iff it is rational for you to act on p, do you know that p partly because it is rational for you to act on p, or is it rational for you to act on (...) p partly because you know that p? And a structurally similar question can be asked regarding justification-action biconditionals. I argue that proponents of these biconditionals cannot give a satisfactory answer to these questions. This is because each direction of these biconditionals strongly supports an opposite order of explanation to the one that is supported by the other direction. Given the traditional assumption that metaphysical explanations—even partial ones—are asymmetric, I argue that these biconditionals should be rejected. While knowledge might be necessary for rational action, and it might be sufficient, it cannot be both. And the same goes for justification. (shrink)