§14. Scientia in Theory and Practice
Aristotle’s Posterior Analytics lays down a demanding standard for science. A science is an axiomatized deductive system comprising a finite set of connected demonstrations (apodeixeis).
A demonstration is a syllogism distinguished by dependence on principles that are true, necessary, universal, and explanatory of the conclusion, which also has to be true, necessary, and universal. Suppose we ask why dogs have incisors. Here is the explanation:Having incisors necessarily belongs to every carnivore;
Being carnivorous necessarily belongs to every dog;
So having incisors necessarily belongs to every dog.123
The example is not Aristotle’s. In fact, nowhere in his entire corpus does Aristotle offer one perfect example of science to his standard. His own scientific treatises (e.g., on meteorology, the senses, sleep, memory, respiration, and the parts and generation of animals) emphasize the provisional nature of the results and the need for further investigation, much as did empirical and medical authors like Democritus and Hippocrates. We have, then, a highly formalized theory of scientific explanation combined with a fruitful research practice that never rises to science by his own standard, but which relies instead on a method left unformalized though consistent with methods pioneered in medicine.
In Generation of Animals Aristotle maintains that “credit must be given to observation rather than to theories, and to theories only if what they affirm agrees with the observed facts.” According to Prior Analytics, “It falls to experience to provide the principles of any subject. In astronomy, for instance, it was astronomical experience that provided the principles of the science, for it was only when the phenomena were adequately grasped that proofs in astronomy were discovered. And the same is true of any art or science whatever.” On the Heavens criticizes Empedocles and Democritus for inconsistent empiricism.
They deny Aristotle’s contention that elemental earth, air, fire, and water can pass into each other and transform among themselves, a denial he considers to be “absurd because unreasonable... and also contrary to the observed data of sense.” He concludes with a passage that epitomizes the empirical tenor of his natural philosophy.Perceptible things require perceptible principles, eternal things eternal principles, corruptible things corruptible principles; and, in general, every subject matter principles homogeneous with itself. But they [Empedocles and Democritus], owing to their love of their principles, fall into the attitude of men who undertake the defenses of a position in argument. In the confidence that the principles are true they are ready to accept any consequences of their application. As though some principles did not require to be judged from their results, and particularly from their final issue! And that issue, which in the case of productive knowledge is the product, in the knowledge of nature is the unimpeachable evidence of the senses as to each fact.124
This commitment to the value of experience in natural science is not expressed merely on a philosophical plane, as Aristotle shows what it means for research. A prominent feature of his explanations is to use analogy, showing one thing to be relevantly similar to something whose causes are better understood. For instance, he explains pulse: “The effect is like boiling.” The formation of skin is likened to a crust, and the action of semen to that of rennet. He knows of fluxes (rheumata) that descend from the brain to the lower body, and he explains them as like rain, formed of vapors that rise from the earth and become condensed by cold air. Analogously, food in the body creates vapors that rise to the brain, which Aristotle understands to be an organ of refrigeration. There they condense, change to liquid, and spread through the body.125
A more involved example of his empiricism is an explanation of the saltiness of the sea.
He observes that a man who drinks only pure water produces urine and sweat that are salty, also that urine left in a container precipitates a salt, and that water passed through ash becomes bitter. From these observations he induces the explanation that “saltiness is produced by a body which is formed in the water and is earthy.” Water becomes salty when it mixes with earth and acquires earth-like particles, despite the mixture being invisible. We need not dwell on the merit of the explanation, wondering for instance why rivers and lakes are not salty. The value of the example is how Aristotle uses experience to reason about nature. An invisible cause (invisible earth) is introduced to explain a phenomenon (salty sea). Having the explanation is good for nothing else, which is an objection medical empiricists raise against such “hypotheses.” Even if they were true they do nothing for the physician. To Aristotle though, the uselessness of science is a point of honor.126The passage from On the Heavens where Aristotle discussed this also contains one of his few proposals for an experiment. He does not claim to have performed it, but implies that it would confirm his explanation. Seal a vase with wax and submerse it in the sea for a day and a night. Aristotle expects that on retrieval it will be filled with pure water, the wax having acted as a filter. A different experiment that he cites in two works is enrolled in the demonstration that yolk and egg white are different substances. The experiment is to mix several bird eggs in a container and apply gentle heat. He claims that yolks will conglomerate into a single whole, as will whites, and that the white will harden and the yolk remain soft. The observation (if it is one) bears on Aristotle’s theory of avian reproduction, which (by our lights correctly) assigns yolk the function of nourishment and regards the white as the arche of bird growth, a homogeneous mass that develops into the infant animal.127
Aristotle invokes experience to refute a hypothesis from Empedocles, who held that vision occurs when fire, springing from the eyes, lights up the surroundings.
Empedocles describes the eye: “primeval fire wrapped in membranes and delicate garments.” Aristotle deduced that we should see in the dark, which we do not. He carefully argues that nothing about darkness per se extinguishes the fire supposedly flowing from the eye, because to extinguish fire requires the opposite qualities of wet and cold, and darkness is not wet or cold.128Aristotle evidently had an inkling of experiments, though his use of them does not impress an advocate of experimental natural philosophy like Francis Bacon, whose judgment is severe but just. He says Aristotle’s practice “was not to seek information from unfettered experiment, but to exhibit experience captive and bound. He did not introduce a wide and impartial survey of experience to assist his investigation of truth; he brought in a carefully schooled and selected experience to justify his conclusions.”129