 Not all were pleased with Avicenna’s interpretation of Aristotle’s definition of motion and some authors challenged not merely Avicenna but even the need for Aristotle’s own recondite definition of motion in term of an entelekheia. Such an attitude was certainly the case in the Islamic East during the period immediately following Avicenna as witnessed in the works of Abū l-Barakāt al-Baghdādī (d. 1165) and Fakhr al-Dīn al-Rāzī (d. 1209). Abū l-Barakāt, although part of the falsafa tradition, was also a critical reader of both Aristotle and Avicenna, whereas Fakhr al-Dīn al-Rāzī, while working within the kalām tradition seriously engaged the philosophers and even wrote a commentary on one of Avicenna’s shorter philosophical encyclopedias. They preferred to define motion as a gradual or not-all-at-once emergence from potency to act. Avicenna himself had considered this definition but found it wanting. Specifically, Avicenna complained that this definition is ultimately circular (Avicenna, 2009, II.1, 3). Thus he noted that gradual/not-all-at-once are understood in terms of time, and time, following Aristotle, is defined in terms of motion, namely, as the measure of motion with respect to before an after. Consequently, complains Avicenna, an understanding of time requires that one already understand motion, but the suggested understanding of motion appeals to an understanding of time. In response to this objection, Abū l-Barakāt noted that both Aristotle (Posterior Analytics, 1.13, 78a22–b32) and Avicenna (Kitāb al-burhān, III.2, 202–3) recognize a difference between something’s being better known to us and being better known by nature. Something is better known to us, if it is a matter of direct sensible observation, whereas it is better known by nature if one grasps the underlying causal explanation. The standard example is drawn from the observation that what is near does not twinkle, and the planets do not twinkling. Thus Aristotle and Avicenna observe that from the fact that the planets do not twinkle (a fact better known to us) one can infer that the planets are near. One can then convert this conclusion and use it a demonstration that causally explains why the planets do not twinkle, namely, because they are near (now a fact better known by nature). The appearance of circularity is purportedly illusory, since in the two inferences, the premises are not being used in the same way: in one case the premise explains something better known to us and in the other something better known by nature. Whether one accepts Aristotle and Avicenna’s analysis is less important than that both of them seemed committed to it, and Abū l-Barakāt exploits this fact. He argues that the supposed circularity underlying defining motion in terms of the gradual emergence from potency to actuality is nothing more than a case of a conversion from something better-known-to-us to something better-known-by-nature.