One of the chief innovations in medieval adaptations of Aristotelian psychology was the expansion of Aristotle's notion of imagination or phantasia to include a variety of distinct perceptual powers known collectively as the internal senses (hawâss bâtinah). Amongst medieval philosophers in the Arabic world, Avicenna (Ibn Sînâ, 980–1037) offers one of the most complex and sophisticated accounts of the internal senses. Within his list of internal senses, Avicenna includes a faculty known as “estimation” (wahm), to which various functions are assigned in a wide variety of contexts. Although many philosophers in the Arabic world as well as in the Latin West accepted Avicenna's positing of an estimative faculty, Avicenna's best-known critics, al-Ghazâlî (1058–1111) and Averroes (Ibn Rushd, 1126–1198), found Avicenna's arguments in support of a distinct estimative faculty problematic. For different reasons, both Averroes and Ghazâlî raised the basic question of whether one needed to posit a distinct faculty of estimation to supplement the perceptual abilities of the other internal senses, and whether the notion of an estimative power as defined by Avicenna was internally coherent. Such criticisms suggest that the Avicennian conception of estimation is not entirely unambiguous, and that a correct understanding of Avicenna's motivations for delineating an estimative power requires a careful study of the diverse activities assigned to it throughout Avicenna's philosophical writings.