The second debate related to the problem of the origins of knowledge is that between rationalism and empiricism. According to rationalists, the ultimate source of human knowledge is the faculty of reason; according to empiricists, it is experience. The nature of reason is a difficult problem, but it is generally assumed to be a unique feature or faculty of the mind through which truths about reality may be grasped. Such a thesis is double-sided: it holds, on the one hand, that reality is in principle knowable and, on the other hand, that there is a human faculty (or set of faculties) capable of knowing it. One thus might define rationalism as the theory that there is an isomorphism (a mirroring relationship) between reason and reality that makes it possible for the former to apprehend the latter just as it is. Rationalists contend that if such a correspondence were lacking, it would be impossible for human beings to understand the world.
Almost no philosopher has been a strict, thoroughgoing empiricist—i.e., one who holds that literally all knowledge comes from experience. Even John Locke (1632–1704), considered the father of modern empiricism, thought that there is some knowledge that does not derive from experience, though he held that it was “trifling” and empty of content. Hume held similar views.
Empiricism thus generally acknowledges the existence of a priori knowledge but denies its significance. Accordingly, it is more accurately defined as the theory that all significant or factual propositions are known through experience. Even defined in that way, however, it continues to contrast significantly with rationalism. Rationalists hold that human beings have knowledge that is prior to experience and yet significant. Empiricists deny that that is possible.
The term experience is usually understood to refer to ordinary physical sensations—or, in Hume’s parlance, “impressions.” For strict empiricists, that definition has the implication that the human mind is passive—a “tabula rasa” that receives impressions and more or less records them as they are.
The conception of the mind as a tabula rasa posed serious challenges for empiricists. It raised the question, for example, of how one can have knowledge of entities, such as dragons, that cannot be found in experience. The response of classical empiricists such as Locke and Hume was to show that the complex concept of a dragon can be reduced to simple concepts (such as wings, the body of a snake, the head of a horse), all of which derive from impressions. On such a view, the mind is still considered primarily passive, but it is conceded that the mind has the power to combine simple ideas into complex ones.
But there are further difficulties. Empiricists must explain how abstract ideas, such as the concept of a perfect triangle, can be reduced to elements apprehended by the senses when no perfect triangles are found in nature. They must also give an account of how general concepts are possible. It is obvious that one does not experience “humankind” through the senses, yet such concepts are meaningful, and propositions containing them are known to be true. The same difficulty applies to colour concepts. Some empiricists have argued that one arrives at the concept of red, for example, by mentally abstracting from one’s experience of individual red items. The difficulty with that suggestion is that one cannot know what to count as an experience of red unless one already has a concept of red in mind. If it is replied that the concept of red and others like it are acquired when we are taught the word red in childhood, a similar difficulty arises. The teaching process, according to the empiricist, consists of pointing to a red object and telling the child “This is red.” That process is repeated a number of times until the child forms the concept of red by abstracting from the series of examples shown. But such examples are necessarily very limited: they do not include even a fraction of the shades of red the child might ever see. Consequently, it is possible for the child to abstract or generalize from them in a variety of different ways, only some of which would correspond to the way the community of adult language users happens to apply the term red. How then does the child know which abstraction is the “right” one to draw from the examples? According to the rationalist, the only way to account for the child’s selection of the correct concept is to suppose that at least part of it is innate. Skepticism
Many philosophers, as well as many people studying philosophy for the first time, have been struck by the seemingly indecisive nature of philosophical argumentation. For every argument there seems to be a counterargument, and for every position a counterposition. To a considerable extent, skepticism is born of such reflection. Some ancient skeptics contended that all arguments are equally bad and, accordingly, that nothing can be proved. The contemporary American philosopher Benson Mates, who claimed to be a modern representative of that tradition, held that all philosophical arguments are equally good.
Ironically, skepticism itself is a kind of philosophy, and the question has been raised whether it manages to escape its own criticisms. The answer to that question depends on what is meant by skepticism. Historically, the term has referred to a variety of different views and practices. But however it is understood, skepticism represents a challenge to the claim that human beings possess or can acquire knowledge.
In giving even that minimal characterization, it is important to emphasize that skeptics and nonskeptics alike accept the same definition of knowledge, one that implies two things: (1) if A knows that p, then p is true, and (2) if A knows that p, then A cannot be mistaken (i.e., it is logically impossible that A is wrong. Thus, if people say that they know Smith will arrive at nine o’clock and Smith does not arrive at nine o’clock, then they must withdraw their claim to know. They might say instead that they thought they knew or that they felt sure, but they cannot rationally continue to insist that they knew if what they claimed to know turns out to be false.
Given the foregoing definition of knowledge, in order for the skeptical challenge to succeed, it is not necessary to show that the person who claims to know that p is in fact mistaken; it is enough to show that a mistake is logically possible. That condition corresponds to the second of the two clauses mentioned above. If skeptics can establish that the clause is false in the case of a person’s claim to know that p, they will have proved that the person does not know that p. Thus arises skeptics’ practice of searching for possible counterexamples to ordinary knowledge claims.
One variety of radical skepticism claims that there is no such thing as knowledge of an external world. According to that view, it is at least logically possible that one is merely a brain in a vat and that one’s sense experiences of apparently real objects (e.g., the sight of a tree) are produced by carefully engineered electrical stimulations. Again, given the definition of knowledge above, that kind of argument is sound, because it shows that there is a logical gap between knowledge claims about the external world and the sense experiences that can be adduced as evidence to support them. No matter how much evidence of this sort one has, it is always logically possible that the corresponding knowledge claim is false. Avrum Stroll The history of epistemology Ancient philosophy The pre-Socratics
The central focus of ancient Greek philosophy was the problem of motion. Many pre-Socratic philosophers thought that no logically coherent account of motion and change could be given. Although the problem was primarily a concern of metaphysics, not epistemology, it had the consequence that all major Greek philosophers held that knowledge must not itself change or be changeable in any respect. That requirement motivated Parmenides (flourished 5th century bce), for example, to hold that thinking is identical with “being” (i.e., all objects of thought exist and are unchanging) and that it is impossible to think of “nonbeing” or “becoming” in any way. Plato