Some of the questions which Peter raises one might expect him to solve by an appeal to occult virtue, sympathy and antipathy existing between things in nature, or the superior influence of the stars. This, however, he almost never does; his reasoning is based rather on the prevalence of the four qualities, hot and cold, dry and moist, in natural objects. Thus the property of dove’s blood of removing spots from the eye is attributed to its heat and humidity, not to any occult power.[1672] And sparrows, although epileptics themselves, are said to cure epilepsy because they are very hot and dry and consume the humors or vapors which cause epilepsy, not by antipathy or because like cures like.[1673] Even less does one note any instances of ligatures and suspensions, characters and incantations. One or two passages show, however, that Peter believes in occult virtue and the rule of the stars. He states that in addition to the four simple elements “there is another simplicity ... namely, from qualities, and it is not in the elements but in the celestial bodies.”[1674] And in his prologue to the Particular Diets he affirms that celestials are the cause of inferiors and quotes the words addressed by God to the stars in Plato’s Timaeus, “O gods of gods, whose father and creator I am, by your nature you are dissoluble but by my will indissoluble,” to prove how much more corruptible inferior bodies are.[1675]
Incorrect ideas about Nature.
Despite the almost complete absence of superstitious practices and of astrological and magical doctrine from the Commentaries on Isaac it is of course true that Peter harbors many incorrect notions such as that fish lack bones and that hot water freezes harder and quicker than cold.[1676] This last, however, he supports by the authority and arguments of Aristotle and Avicenna. Peter also throughout the work displays faith in the validity of compound medicines, although he raises the suggestive question, why compound foods are more often injurious than compound medicines. He also accepts various stories of animal remedies and sagacity for which he finds support in Aristotle’s History of Animals, such as that the serpent eats fennel to restore or sharpen its sight,[1677] and that the bone in the heart of the stag is especially beneficial for heart disease because “the stag is very ingenious and astute and so eats potent herbs which especially affect the heart like parsley and origanum,” and from these the bone forms.[1678] In explaining why deer shed their horns and hide them, Peter incorrectly makes Aristotle say that they secrete the right horn with more care, whereas the History of Animals states, “It is said that no one has ever seen the left horn, for he conceals it as if it had some medicinal (or, magic) power.”[1679]
Reason and experience.
Finally, despite the scholastic form of Peter’s Commentary, it contains a long passage on the importance of experimental method or “the way of experience” (via experimenti),[1680] f which it couples with “the path of reason” (via rationis) as the two methods by which dietary science may be investigated. First Peter distinguishes between the two, then shows the necessity of the way of experience, and third that it can and should be confirmed by reason. Galen says that experience is weak without reason, and so is reason when not joined to experience. Some say that reason should precede experience, that first we should seek rationally, then test by experience. In any case the way of experience proceeds through effects, the way of reason through causes. The one method is inductive; the other, syllogistic; the one based on immediate effects, the other on mediate effects. “Experimental method pays no attention to causes; rational method considers causes and principles; experience makes use of the senses; reason, of the intellect” and arguments.
Via experimenti.
After listing various arguments pro and con as to whether the via experiment “is of art and in art, or precedes art,” Peter gives his solution to the effect that experimentum is threefold. As a method of attaining knowledge it antecedes all arts and sciences. As a method of making known the objects of scientific inquiry, it is a part of science. As an application of scientific doctrine to practical life and industry, it follows science. Furthermore, experience “may proceed regularly through science and doctrine and in this way it can be rational.” Or it may be irregular and not syllogistic in method. The experimental discoveries of brutes, as when the serpent restores its sight with fennel, come to them from nature, but ours are acquired by art and confirmed by reason, although man too possesses the experimental instinct. Peter further distinguishes the experiences of rustics, which are unregulated by reason, from the experiments of skilled men which are regulated by reason. Moreover, “experiences are not without their reasons, and idle is the experiment which does not rest on reason.” Finally, Peter gives six conditions requisite in medical experimentation which are somewhat similar to the seven conditions stated by his contemporary, John of St. Amand. First, the medicine administered should be free from all foreign substance.[1681] Second, the patient taking it should have the disease for which it is especially intended. Third, it should be given alone without admixture of other medicine. Fourth, it should be of the opposite degree to the disease. Fifth, “we should test it not once only but many times.” Sixth, “the experiments should be with the proper body, as on the body of a man and not of an ass.”
Question of Peter’s relation to Roger Bacon and Galen.
Peter’s discussion of the via experimenti is in several respects similar to Roger Bacon’s discussion of experimental science, but is probably quite independent of it. Peter died before Roger in 1277, and his Commentary on Isaac was probably composed before the works which Bacon addressed to the pope in and around the year 1267. The influence of Galen, who had discussed the part played by reason and experience in his own work on food values, upon Peter is fairly evident.
John of St. Amand on medical experimentation.