A key component of much current research in behavioral ecology, cognitive science, and economics is a model of the mind at least partly based on beliefs and desires. However, despite this prevalence, there are still many open questions concerning both the structure and the applicability of this model. This is especially so when it comes to its ‘desire’ part: in particular, it is not yet entirely clear when and why we should expect organisms to be desire-based—understood so as to imply (...) that they consult explicit tokenings of what they ought to do—as opposed to being drive-based—understood so as to imply that they react to the world using behavioral reflexes. In this paper, I present the beginnings of an answer to this question. To do this, I start by showing that an influential recent attempt to address these issues—due to Kim Sterelny—fails to be fully successful, as it does not make sufficiently clear what the relative benefits and disadvantages of drive-based and desire-based cognitive architectures are. I then present an alternative account of this matter based on the idea that organisms that can follow explicit behavioral rules (i.e. which have desires) avoid having to memorize a large set of state of the world–action connections—which can (though need not) be adaptive. Finally, I apply this account to the question of what the cognitive value of mental representations should be seen to be; here, I conclude that—contrary to some recent claims—relying on mental representations can make decision making easier, not harder, but also that—in line with these recent claims—whether it does so depends on the details of the case. (shrink)