 Closely related to causes and principles is the notion of a nature (Gk. phusis, Ar. ṭabīʿa), for Aristotle defined a nature as “a principle of being moved and being at rest in that to which it belongs primarily, not accidentally but essentially” (Aristotle, Physics, 2.1, 192b21–3). Before commenting Aristotle’s definition of nature, Avicenna begins his chapter “On Defining Nature” from The Cure (Avicenna, [Ph], 1.5) by distinguishing between the motions and actions that proceed from a substance owing to external causes and those that proceed from a substance owing to the substance itself. For instance, water becomes hot as a result of some external heat sources, whereas it becomes cool of itself. Avicenna next identifies two sets of parameters for describing those motions and actions that proceed from a substance owing to that substance itself. They may either proceed from the substance as a result of volition or without volition. Additionally they may proceed either uniformly without deviation or non-uniformly with deviation. Thus there are four very general ways for dividing and describing all the motions and actions that are found in the cosmos.