  The main source of disagreement between Descartes and Regius was their respective accounts of human nature. This had emerged as early as 1640, when Regius commented on a draft of the Meditations, and more explicitly in the disputation (December 1641), which proposed the thesis that the human mind and body are united in an ‘accidental unity.’ In July 1645, when Descartes responded to a draft of the Fundamenta physices, he argued that Regius had gone too far in the opposite direction by making the union of mind and body so close that the mind was merely a ‘mode’ of the body—which was an even greater error than his earlier mistake (AT IV, 250: Bos 2002, 188). This issue, of how to conceive of the mind and body as united and yet separable, dominated the series of claims and counter-claims made by both philosophers up to 1648. Between 1648 and February 1650, when Descartes died, the two philosophers did not engage directly with each other. Descartes discussed a range of questions with Burman in 1648, and in response to one query he provided another reminder of the extent to which Regius allegedly misunderstood his metaphysics. “As far as Regius is concerned … in physics he has always been anxious to follow the views of the author [i.e., Descartes] … whereas in metaphysics he has done everything possible to contradict the author’s views” (Descartes CB, 38: AT V, 170).  Descartes’ last publication, Les Passions de l’Ame (1649: AT, vol. XI), may be read as his final attempt to address the source of their disagreement, and Regius 1650 may be seen as Regius’ response (Verbeek 2017).