The exact precedents of Al Farabi's influential philosophical scheme, in which "nous" (Arabic "ʿaql") plays an important role, are no longer perfectly clear because of the great loss of texts in the Middle Ages which he would have had access to. He was apparently innovative in at least some points. He was clearly influenced by the same late classical world as neoplatonism, neopythagoreanism, but exactly how is less clear. Plotinus, Themistius and Alexander of Aphrodisias are generally accepted to have been influences. However while these three all placed the active intellect "at or near the top of the hierarchy of being", Al Farabi was clear in making it the lowest ranking in a series of distinct transcendental intelligences. He is the first known person to have done this in a clear way. He was also the first philosopher known to have assumed the existence of a causal hierarchy of celestial spheres, and the incorporeal intelligences parallel to those spheres. Al Farabi also fitted an explanation of prophecy into this scheme, in two levels. According to Davidson (p. 59):The lower of the two levels, labeled specifically as "prophecy" ("nubuwwa"), is enjoyed by men who have not yet perfected their intellect, whereas the higher, which Alfarabi sometimes specifically names "revelation" ("w-ḥ-y"), comes exclusively to those who stand at the stage of acquired intellect.