 With the distinction between nature as active and nature as passive in place, it becomes clear why Avicenna identifies the proper subject matter of natural philosophy with body insofar as it is subject to motion, for the science of physics studies nature in both of its active and passive senses. Nature as passive refers to that which is subject to motion, namely, bodies, while nature as active refers to the causes and conditions of the motions and actions that bodies naturally undergo. In this section, I begin with Avicenna’s general introduction to bodies. I then take up his critique of atomism, specifically as that theory was developed within Kalām. Thereafter, I turn to Avicenna’s theory of the continuity of magnitudes and his conception of infinity, concluding with a brief survey of his theory of the elements. 