Detail of a Roman copy (2nd century bc) of a Greek alabaster portrait bust of Aristotle (c. 325 bc); in the collection of the Museo Nazionale Romano, Rome.A. Dagli Orti/©De Agostini Editore/age fotostock
Much of what Aristotle says about knowledge is part of his doctrine about the nature of the soul, and in particular the human soul. As he uses the term, the soul (psyche) of a thing is what makes it alive; thus, every living thing, including plant life, has a soul. The mind or intellect (nous) can be described variously as a power, faculty, part, or aspect of the human soul. It should be noted that for Aristotle “soul” and “intellect” are scientific terms.
In an enigmatic passage, Aristotle claims that “actual knowledge is identical with its object.” By that he seems to mean something like the following. When people learn something, they “acquire” it in some sense. What they acquire must be either different from the thing they know or identical with it. If it is different, then there is a discrepancy between what they have in mind and the object of their knowledge. But such a discrepancy seems to be incompatible with the existence of knowledge, for knowledge, which must be true and accurate, cannot deviate from its object in any way. One cannot know that blue is a colour, for example, if the object of that knowledge is something other than that blue is a colour. That idea, that knowledge is identical with its object, is dimly reflected in the modern formula for expressing one of the necessary conditions of knowledge: A knows that p only if it is true that p.
To assert that knowledge and its object must be identical raises a question: In what way is knowledge “in” a person? Suppose that Smith knows what dogs are—i.e., he knows what it is to be a dog. Then, in some sense, dogs, or being a dog, must be in the mind of Smith. But how can that be? Aristotle derives his answer from his general theory of reality. According to him, all (terrestrial) substances are composed of two principles: form and matter. All dogs, for example, consist of a form—the form of being a dog—and matter, which is the stuff out of which they are made. The form of an object makes it the kind of thing it is. Matter, on the other hand, is literally unintelligible. Consequently, what is in the knower when he knows what dogs are is just the form of being a dog.
In his sketchy account of the process of thinking in De anima (On the Soul), Aristotle says that the intellect, like everything else, must have two parts: something analogous to matter and something analogous to form. The first is the passive intellect, the second the active intellect, of which Aristotle speaks tersely. “Intellect in this sense is separable, impassible, unmixed, since it is in its essential nature activity.…When intellect is set free from its present conditions, it appears as just what it is and nothing more: it alone is immortal and eternal,…and without it nothing thinks.”
The foregoing part of Aristotle’s views about knowledge is an extension of what he says about sensation. According to him, sensation occurs when the sense organ is stimulated by the sense object, typically through some medium, such as light for vision and air for hearing. That stimulation causes a “sensible species” to be generated in the sense organ itself. The “species” is some sort of representation of the object sensed. As Aristotle describes the process, the sense organ receives “the form of sensible objects without the matter, just as the wax receives the impression of the signet-ring without the iron or the gold.” Ancient Skepticism
After the death of Aristotle the next significant development in the history of epistemology was the rise of Skepticism, of which there were at least two kinds. The first, Academic Skepticism, arose in the Academy (the school founded by Plato) in the 3rd century bce and was propounded by the Greek philosopher Arcesilaus (c. 315–c. 240 bce), about whom Cicero (106–43 bce), Sextus Empiricus (flourished 3rd century ce), and Diogenes Laërtius (flourished 3rd century ce) provide information. The Academic Skeptics, who are sometimes called “dogmatic” Skeptics, argued that nothing could be known with certainty. That form of Skepticism seems susceptible to the objection, raised by the Stoic Antipater (flourished c. 135 bce) and others, that the view is self-contradictory. To know that knowledge is impossible is to know something. Hence, dogmatic Skepticism must be false.
Carneades (c. 213–129 bce), also a member of the Academy, developed a subtle reply to the charge. Academic Skepticism, he insisted, is not a theory about knowledge or the world but rather a kind of argumentative strategy. According to the strategy, the Skeptic does not try to prove that he knows nothing. Instead, he simply assumes that he knows nothing and defends that assumption against attack. The burden of proof, in other words, is on those who believe that knowledge is possible.
Carneades’ interpretation of Academic Skepticism renders it very similar to the other major kind, Pyrrhonism, which takes its name from Pyrrhon of Elis (c. 365–275 bce). Pyrrhonists, while not asserting or denying anything, attempted to show that one ought to suspend judgment and avoid making any knowledge claims at all, even the negative claim that nothing is known. The Pyrrhonist’s strategy was to show that for every proposition supported by some evidence, there is an opposite proposition supported by evidence that is equally good. Such arguments, which are designed to refute both sides of an issue, are known as “tropes.” The judgment that a tower is round when seen at a distance, for example, is contradicted by the judgment that the tower is square when seen up close. The judgment that Providence cares for all things, which is supported by the orderliness of the heavenly bodies, is contradicted by the judgment that many good people suffer misery and many bad people enjoy happiness. The judgment that apples have many properties—shape, colour, taste, and aroma—each of which affects a sense organ, is contradicted by the equally good possibility that apples have only one property that affects each sense organ differently.
What is at stake in such arguments is “the problem of the criterion”—i.e., the problem of determining a justifiable standard against which to measure the worth or validity of judgments, or claims to knowledge. According to the Pyrrhonists, every possible criterion is either groundless or inconclusive. Thus, suppose that something is offered as a criterion. The Pyrrhonist will ask what justification there is for it. If no justification is offered, then the criterion is groundless. If, on the other hand, a justification is produced, then the justification itself is either justified or it is not. If it is not justified, then again the criterion is groundless. If it is justified, then there must be some criterion that justifies it. But this is just what the dogmatist was supposed to have provided in the first place.
If the Pyrrhonist needed to make judgments in order to survive, he would be in trouble. In fact, however, there is a way of living that bypasses judgment. One can live quite nicely, according to Sextus, by following custom and accepting things as they appear. In doing so, one does not judge the correctness of anything but merely accepts appearances for what they are.
Ancient Pyrrhonism is not strictly an epistemology, since it has no theory of knowledge and is content to undermine the dogmatic epistemologies of others, especially Stoicism and Epicureanism. Pyrrho himself was said to have had ethical motives for attacking dogmatists: being reconciled to not knowing anything, Pyrrho thought, induced serenity (ataraxia). St. Augustine