 Among the specifics of prophecy theory, the working of miracles received most attention in the Latin West. al-Kindī’s and Avicenna’s explanation of miracles is naturalistic in the sense that neither theory involves divine factors. al-Kindī argues in On Rays (De radiis, extant only in Latin) that if a person conceives a corporeal image in his imagination, the image assumes a material existence in the spiritus that belongs to the faculty. The spiritus in turn sends out rays, which alter objects in the external world (d’Alverny &amp; Hudry 1975, 230–1). An alternative to this extramission theory is formulated by Avicenna, who claims that persons who have perfected their body and soul are able to affect directly the external matter of the world and may produce rain, fertile seasons, and the like – by sheer power of the will. Avicenna arrives at this conclusion by generalizing the principle that there are psychic causes for material effects (De anima IV.4).