Returning to the Natural Questions, we may note that, like the Problems of Aristotle, they vary from such crude queries as might occur to any curious person without scientific training to others that imply some previous theory or knowledge. A list of some of them will illustrate the scope of the scientific curiosity of the time. When one tree is grafted upon another, why is all the fruit of the nature of the grafted portion? Why do some brutes ruminate; why are some animals without stomachs; and why do some which drink make no water? Why do men grow bald in front? Why do some animals see better in the night than in the day and why can a man standing in the dark see objects that are in the light, while a man standing in the light cannot see objects that are in the dark? Why are the fingers of the human hand of unequal length and the palm hollow? Why don’t babies walk as soon as they are born, and why are they at first nourished upon milk, and why doesn’t milk agree equally with old and young? Why do we fear dead bodies?[64] A number of questions are devoted to each of the topics, vision, hearing, and heat, while the senses of taste, smell, and touch are dismissed in a single question and answer.[65]
Theory of sound.
The discussion of sound and vision may be noted more fully. The nephew has already learned from his Boethius something similar to the wave theory of sound. He states that when the air has been formed by the mouth of the speaker and impelled by the tongue, it impresses the same form upon that which is next to it, and that this process is repeated over and over just as concentric circles are formed when a stone is thrown into water.[66] Vitruvius had given the same explanation in discussing the acoustics of a theater.[67] But when the nephew asks his uncle how the voice can penetrate an iron wall, Adelard replies that every metal body, no matter how solid, has some pores through which the air can pass.[68] Thus he appears to regard air as the only substance which can transmit or conduct sound waves. His notion that air can pass through solids reminds one a little of the milder theory of Hero of Alexandria that heat and light consist of material particles which penetrate the interstices between the atoms composing air and water.[69] But it hardly seems as if Adelard could have derived his notion from Hero, since the impermeability of metal vessels to air is a fundamental hypothesis in many of the devices of Hero’s Pneumatics.
Theory of vision.
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]
Deductive reasoning from hot and cold, moist and dry.
Throughout the Natural Questions Adelard’s explanations and answers are based in large measure upon the familiar hypothesis of the four elements and of the four qualities, hot and cold, dry and moist. When asked, for instance, why all ruminating animals begin to lie down with their hind legs, he explains that their scanty animal heat is the cause of their ruminating to aid digestion, and that there is more frigidity in their posterior members, which are consequently heavier and so are bent first in reclining. The nephew thinks that here he has caught his uncle napping, and asks why is it then that in rising they lift themselves first onto their hind legs. But Adelard is not to be so easily nonplussed, and explains that after they have lain down and rested, they feel so refreshed that they lift their heavier limbs first.[75] Again, asked why persons of quick perception often have faulty memories, Adelard suggests that a moist brain is more conducive to intelligence, but a dry one to memory. Thus moist potter’s clay receives impressions more readily but also easily loses them; what is drier receives the impression with more difficulty but retains it.[76] In a third passage, Adelard explains his nephew’s weeping in his joy at seeing his uncle safely returned by the theory that his excessive delight overheated his brain and distilled moisture thence.[77]
Refinement of the four elements hypothesis.
Adelard, however, like Galen, Constantinus Africanus, Basil, and other writers before him, finds it advisable to refine the theory of the four elements. He is at pains in his answer to the nephew’s very first question to explain that what we commonly call earth is not the element earth, and that no one ever touched the pure element water, or saw the elements air and fire. Every particular object contains all four elements and we deal in daily life only with compounds. In an herb, for instance, unless there were fire, there would be no growth upward; unless there were water or air, no spreading out; and without earth, no consistency. Moreover, when Adelard is asked why some herbs are spoken of as hot by nature, although all plants have more earth than fire in their composition, he says that while earth predominates quantitatively, efficaciously they are more fiery, just as his green cloak is larger than his green emerald, but much less potent.[78] Thus comes in the theory of occult virtue to help out the inadequate and unsatisfactory hypothesis of four elements and four qualities. We shall find our subsequent authors often resorting to the same explanation.
Animal intelligence doubted.