If you have a taste for realist doctrines, suppose that the mind is a store of real, efficacious beliefs, desires and propositional attitudes generally. Why should anyone agree that propositional attitudes exist? For much the same reasons that lead us to endorse other scientifically reputable entities. Our behavior is largely explicable by reference to the propositional attitudes we have, variation in behavior devolving from variation in propositional attitudes. This leads to two questions. First, how is it that if behavior is driven by propositional attitudes, it is typically appropriate to the circumstances of its production? And second, if variation in behavior falls to variation in propositional attitudes, what accounts for variation among propositional attitudes?