In the Fourth Meditation, Descartes famously treats the indifference of the will (roughly, ambivalence of reasons) as the source of error, which many read as oddly suggesting that the will judges arbitrarily. In his letter to Elisabeth dated 1st September 1645, however, he expressly takes passions to be the source of error, saying that passions move the will to judge erroneously by misrepresenting the value of objects. Although these two accounts focus on different kinds of error – theoretical and practical (...) error, respectively – I argue that Descartes is best understood as extending the second account also to the source of theoretical error. On my reading, the first account does not imply that erroneous judgements are simply arbitrary, but it leaves out an explanation why we judge at the time we do, when we could (and should) continue to inquire insofar as we have not yet gathered sufficient evidence. The second account fills in this lacuna by giving an explanation in terms of passions. I further argue that the schematic nature of the first account is due partly to the structure of the Meditations, but mainly to the fact that Descartes has not yet systematically examined the nature of passions there. (shrink)