 While identifying time with a certain possibility to traverse some spatial magnitude, provides one with a criterion for recognizing time, it does not provide a proper scientific definition of what time essentially is, that is, a definition in terms of genus and difference. Thus, having shown that time exists, Avicenna next turns to defining time and showing what time essentially is. In general, Avicenna follows Aristotle and defines time as a “number of motion when it is differentiated into prior and posterior parts” (cf. Aristotle, Physics 4.1, 219b1–2). Time’s genus, then, is a certain number, that is, some measurable aspect or magnitude, belonging to motion. Its difference is having essentially prior and posterior elements. Here is it important to note that for Avicenna, priority and posterity (taqaddum and taʾakhkhur) are generic notions, with a number of different species, like priority in eminence or location or time. For Avicenna, the prior and posterior in Aristotle’s definition refers to what is prior and posterior in the distance traversed as determined by the directedness of the motion over that distance. Thus, the charges of some, particularly Galen, that Aristotle’s definition of time is circular, since it appeals to purportedly temporal notions of before and after, is avoided. (See Lammer 2018, ch. 6.2).