In this paper, I propose a critique of the naturalist version of the Desire-Based Reasons Model. I first set the scene by spelling out the connection between naturalism and the Model. After this, I introduce Christine Korsgaard’s circularity argument against what she calls the instrumental principle. Since Korsgaard’s targets, officially, were non-naturalist advocates of the principle, I show why and how the circularity charge can be extended to cover the naturalist Model. Once this is done, I go on to investigate (...) in some detail the different ways of responding to the circularity challenge. I argue that none of these responses succeed, at least not without serious costs to their advocates. I then end the paper with a brief summary and some concluding remarks. (shrink)