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.[51] 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.[52] Al Farabi also fitted an explanation of prophecy into this scheme, in two levels.