Descartes and Regius on the Pineal Gland and Animal Spirits, and a Letter of Regius on the True Seat of the Soul

In his concise Cartesian textbook entitled Physiology (1641) Henricus Regius went beyond what could be found in Descartes’ published writings. There is no doubt that Regius completed some of its contents himself, but Descartes generously shared his (unpublished) views with him as well. This article investigates this tension on two specific points that relate to the function of the pineal gland and the animal spirits. First, did Descartes change his mind between the Treatise on Man and the Passions regarding the last step of sensory perception in the brain? It is argued he did not. Second, does the soul move or determine the movement of the animal spirits via the pineal gland? It is argued the soul determines the movement.