Epistemology is supposed to guide our reason. This means that epistemology is practically important because our beliefs so often play a decisive role in how we decide to act. We argue that in order to be effectively action-guiding, traditional epistemological theories must be supplemented with an account of significance--of what makes one reasoning problem significant and another reasoning problem insignificant. We sketch the beginning of an account of significance that employs Signal Detection Theory. We then argue that any internalist theory of justification is not going to fit smoothly with a plausible account of significance. This calls into question the power of internalist theories of justification to effectively guide reason and action.