 Aristotle’s definition of nature as it came down to Avicenna in Arabic literally translates into English as “the primary principle of motion and rest in that to which it belongs essentially rather than accidentally” (quoted in Avicenna, [Ph], 1.5  [4]).[3]  A “principle of motion,” Avicenna tells us, means “an efficient cause from which proceeds the production of motion in another, namely, the moved body” (ibid). Immediately we recognize that a nature for Avicenna can be understood as an active cause that produces motion. Such a position is to be contrasted with Aristotle’s claim at Physics 8.4 that a nature is not an internal cause of a thing’s motion, but of its being  moved.[4]  Next we learn that “primary” in the definition means that the nature causes the motion in the body without some further intermediary cause. As for “essentially,” (Gk. kath hauto, Ar. bi-dhātihi) it is an equivocal notion with two senses, Avicenna tells us. In one sense, “essentially” is predicated relative to the mover, while in another sense it is predicated relative to what is subject to motion, namely, the body. Thus, on the one hand, when “essentially” is said relative to the mover, a substance’s nature, as efficient cause, essentially produces those motions and actions that typify the substance as the kind that it is. On the other hand, when “essentially” is said relative to the body, it refers to the body’s essentially being subject to those naturally characteristic motions and actions. In effect, by distinguishing the two relative senses of “essentially,” Avicenna has made room for Aristotle’s definition to include nature as an active and as passive principle. To sum up, for Avicenna, a substance’s nature is the immediate efficient cause for all of the naturally characteristic actions and motions it produces (active nature) as well as explaining why the body is subject to those characteristic actions and motions (passive nature). 