When Elisabeth requests from Descartes a definition of the soul ‘more particular than in your Metaphysic, that is to say, of its substance separate from its action, thought’ in order to explain how ‘the soul of a human being is able to determine the spirits of the body to produce voluntary actions’ her suspicion is that nothing can move or be moved by a body without being a body itself (6/16 May 1643, AT iii, 66). Previously persuaded by Descartes' arguments for a real distinction of mind and body, Elisabeth begins to doubt the coherence of the idea that these two heterogeneous substances might nonetheless interact. As noted in chapter 1, Descartes does not address Elisabeth's objection directly, but the Passions represents something of an attempt to ground the interaction between mind and body in a way that is, at least, perfectly consistent with his dualism. In light of their exchange, it is not surprising that the Passions begins on what would have seemed to be a distinctively metaphysical note. In the first two articles, Descartes invokes the (Aristotelian) categories of action and passion, which specify the conceptual relationship that obtains between substances or within a single substance when a natural change occurs.