 Having set aside the issue of whether natures exist and for present purposes posits that they do, Avicenna now unpacks Aristotle’s definition. Before considering his analysis, however, it should be noted that there is a certain ambiguity in Aristotle’s overall treatment of nature. The ambiguity involves whether nature should be taken in a passive sense: is nature a cause of a substance’s being moved? Or, alternatively, should nature be taken in an active sense: is a nature a cause of a substance’s (self) motion? In Physics, 2.1, the text where Aristotle defines nature, he suggests, although does not explicitly claim, that nature can be understood in both senses: a passive nature, which corresponds with a substance’s matter, and an active nature, which corresponds with the form of that substance, and so is a cause of its motion. In contrast, later in the Physics, he argues that natures should be understood exclusively as passive (Aristotle, Physics, 8.4, 225b29–256a3). This conclusion follows upon Aristotle’s principle that everything that is moved must be moved by something, which he established at Physics, 7. The principle is significant for a key element in Aristotle’s his natural philosophy, namely, his proof for an unmoved mover. (The unmoved mover is intended to explain why there is motion at all.) The aforementioned principle is significant because if all natural substances were capable of self motion, it is no longer clear that there is a need for an unmoved mover to account for the motion of the cosmos. This ambiguity in Aristotle account of nature, namely, whether nature is an active source of moving or a passive factor of being moved, was the source of much discussion among Aristotle’s subsequent Greek commentators, who included  Alexander of Aphrodisias  (ca. 200 CE) and the late Christian, Neoplatonic Philosopher  John Philoponus  (ca. 490–570). (For more detailed studies of this tradition see Macierowski &amp; Hassing 1988, Lang 1992, 97–124 and Lammer  2015, 2018, ch. 4). 