 Time is defined by Aristotle as the number of motion in respect of before and after (Phys. IV.11, 219b1–2). Averroes expounds that ‘number’ is like the logical genus and ‘before and after’ the specific difference (LC 181F TC101). The definition was subject to discussion among Greek and Arab commentators, and Averroes refers to Alexander of Aphrodisias and Themistius, and to al-Fārābī. For Averroes the definition of time about which there is no doubt is that “time is the numerable one of the before and after existing in motion” (numeratum prioris et posterioris) (LC 181H TC101), that is, before and after is a condition existing in motion, and its numerability is time. Averroes has the same explanation in his Short Commentary: 