Adelard’s theory of vision, that of extramission of “a visible spirit,” is similar to that of Plato in the _Timaeus_, by which he was not unlikely influenced. The visible spirit passes from the brain to the eye through “concave nerves which the Greeks call optic,” and from the eye to the object seen and back again “with marvelous celerity.”[70] It would be interesting to know certainly whether Adelard penned this passage before John of Spain translated into Latin the _De differentia spiritus et animae_, in which Costa ben Luca speaks of “hollow nerves” from the brain to the eye through which the _spiritus_ passes for the purpose of vision.[71] Apparently Adelard was first, since the _Natural Questions_ were finished at some time between 1107 and 1133, while John of Spain is said to have made his translation for Raymond who was archbishop of Toledo from 1130 to 1150. Were the manuscripts not so insistent in naming John as translator,[72] we might think that Adelard had translated the _De differentia spiritus et animae_. Very possibly he had come across it during his study with Arabian masters. But he shows no acquaintance with the optical researches of Al-Hazen or with the treatise on _Optics_ ascribed to Ptolemy, which last is extant only in the twelfth century Latin translation by Eugene of Palermo, admiral of Sicily.[73] However, the fact that three other theories of vision than the one which Adelard accepts are set forth by his nephew suggests that the problem was attracting attention. Pliny’s _Natural History_ gave no theory of vision whatever, although he listed various cases of extraordinary sight. Boethius, on the other hand, briefly adverted to the opposing theories of vision by extramission and intramission in the first chapter of his work on music. As for the marvelous celerity of the visible spirit, Augustine had enlarged upon the vast distance to the sun and back traveled by the visual ray in an instant or twinkling of an eye.[74] [Sidenote: Deductive reasoning from hot and cold, moist and dry.]