  As previously mentioned, Descartes considered the Meditations to contain the principles of his physics. But there is no Meditation labeled “principles of physics.” The principles in question, which are spread through the work, concern the nature of matter (that its essence is extension), the activity of God in creating and conserving the world, the nature of mind (that it is an unextended, thinking substance), mind–body union and interaction, and the ontology of sensory qualities. (Descartes and his followers included topics concerning the nature of the mind and mind–body  interaction within physics or natural philosophy, on which, see Hatfield 2000.)