 La Forge was among the first commentators to attribute to Descartes an unambiguous dualism of mind and body. He defined a substance as ‘a thing in which some property, quality or attribute (of which we have a real idea in ourselves) resides immediately as in a subject and by which it exists’ (H preface ù3). His Remarks were designed to extend the explanatory scope of the machine model of the body to include all the actions of non-human animals and, in the case of the human body, to explain how ideas arise in the brain, how they are stored in memory, and how they are re-activated in the imagination. Accordingly, he initially used the word ‘idea’, as Descartes had done, to refer to the brain-states that are associated with the occurrence of thoughts in the mind (H 262), and he defined memory as the physical capacity of the brain to facilitate the repeated occurrence of such traces. Since the relevant brain-events were understood as patterns in the flow of animal spirits, memory was explained as an acquired disposition of the paths through which animal spirits flowed to re-open on subsequent occasions when appropriately stimulated (Sutton 1998).