The two major schools of skepticism, as they originated in ancient Greek philosophy, are often called the Academic and the Pyrrhonian.

(1) Academic skepticism, which derives its name from Plato's Academy, was inspired by the remark attributed to Socrates, "All I know is that I know nothing."; As formulated by Arcesilas (c. 315-241 B.C.) and Carneades (c. 213-129 B.C.), this version of skepticism maintains that nothing can be known - or, more precisely, that nothing can be known for certain.

This claim was based on the standard Greek distinction between knowledge (episteme) and opinion (doxa). In this scheme, if a proposition cannot be demonstrated with complete certainty -- i.e., if it might be false - then it does not qualify as true knowledge and is relegated to the status of mere opinion. According to Academic skepticism, both our senses and our reason are unreliable to some degree, so we can never lay claim to absolute truth, or real knowledge. Since nothing can be known for certain, we must rely instead on opinions that vary in their degrees of probability.

(2) Pyrrhonian skepticism is named after Pyrrho of Elis (c. 360-275 B.C), an obscure figure who is portrayed in secondary accounts as a complete doubter, especially in ethical matters. Rather than endure the mental anguish and unhappiness that comes from seeking knowledge in the realm of values, Pyrrho is said to have suspended judgment, thereby attaining the state of mind known to Greek philosophers as ataraxia (quietude or unperturbedness).

Pyrrhonian skepticism was formulated as a distinct theory by Aenesidemus (c. 100-40 B.C.). It was Aenesidemus and his followers who first adopted the title of "skeptic" (from skeptikos, meaning "inquirer") and "doubters"; (from ephektikos, meaning "one who suspends judgment"). Although both the Academic and Pyrrhonian skeptics attacked "dogmatic"; philosophers (such as the Stoics), who claimed to possess certain knowledge, their approaches differed significantly. ("Dogma," in its original meaning, referred to any proposition that can be known with certainty, not necessarily to a religious tenet that is accepted on faith.) The Academics, according to some Pyrrhonian critics, were not true skeptics, because they claimed to know with certainty that certainty is impossible - a position that is contradictory and therefore self-refuting. Academic skepticism, therefore, was sometimes called dogmatic skepticism, or negative dogmatism.

The Pyrrhonians, in contrast, did not claim that knowledge is impossible; rather, they suspended judgment on all such theoretical questions, thereby avoiding the mental discomfort that comes from taxing one's brain with insoluble problems. For them, skepticism was a mental attitude and a way of life, not an abstract philosophical position. The Pyrrhonist refused to judge or criticize the laws and customs of his society, resolving instead to accept things as they appear to be, without committing himself to any judgment about them. In this way he attained the psychological tranquillity of ataraxia .

Of these two schools of skepticism, it was the Pyrrhonian that was destined to exert a profound influence on the course of Western philosophy, possibly because no major writings of the Academics survived the ravages of time. (Their arguments were largely transmitted through much later secondary accounts, such as those of Cicero and Augustine.) The Pyrrhonists were more fortunate. Around 200 A.D., the Greek physician Sextus Empiricus wrote an excellent and extensive account of Pyrrhonian skepticism. Though virtually unknown to medieval scholars, manuscript copies of Sextus began to circulate during the Italian Renaissance and were eventually disseminated throughout Europe. His works were first published in Latin during the 1560s, and then in English three decades later.

Pyrrhonian skepticism, as summarized by Sextus Empiricus, created a sensation - some called it a crisis - among European intellectuals. Some philosophers, such as Montaigne, enthusiastically embraced Pyrrhonian skepticism, whereas others, such as Descartes, attempted with equal enthusiasm to refute it. But so tremendous was the influence of Sextus Empiricus that, by the end of the seventeenth century, "the divine Sextus" was commonly regarded as the father of modern philosophy.

Only in recent decades have historians of philosophy fully appreciated the impact of Pyrrhonian skepticism on the development of modern religion and philosophy. After the Reformation, many Catholics argued that Protestants, by appealing to personal inspiration and religious experience, were logically destined to land in a hopeless skepticism, where competing knowledge claims cannot be rationally adjudicated. Protestants replied that Catholic doctrine inevitably led to the same destination, owing to its reliance on Church authority to resolve religious disputes. Both sides had a point, and this specter of skepticism was probably behind Descartes' attempt to ground certain knowledge in ``clear and distinct ideas,' without appealing to personal revelation, tradition, or authority.

Today the words "skeptic" and "skepticism" are employed in a variety of ways. Some contemporary freethinkers associate skepticism with the tradition of atheism, agnosticism, and other forms of religious dissent. There is some justification for this usage, since some religious skeptics, such as David Hume, did ground their religious doubt in a broader theory of epistemological skepticism, viz., the claim that certain kinds of knowledge (such as knowledge of causation) cannot be rationally justified. This, of course, wreaked havoc with many arguments for the existence of God, most notably the first cause argument.

However, as I argued many years ago in Atheism: The Case Against God, epistemological skepticism will inevitably lend aid and comfort to religious claims and turn against the atheist; because, if no knowledge claims can be rationally justified, then all of them must rely to some extent on "faith." In this case, the claims of the atheist or disbeliever are no more justified that the claims of the theist or religionist. Atheism, according to this position, is as much a matter of faith as is theism.

This position has been explicitly upheld by many "fideists" throughout history, who, beginning with epistemological skepticism, have maintained that all knowledge claims must ultimately rest on faith. Thus the claims of the mystic are held to be as justified (or, more accurately, as unjustified) as the claims of the rationalist. Atheists and freethinkers who embrace epistemological skepticism (especially of the Academic variety) will find themselves hard-pressed to escape this epistemological dead-end of fideism, so they should think carefully before declaring themselves to be "skeptics."

Of course, conventional usage often indicates the meaning of "skepticism" in a particular context. A "psi-skeptic," for example, is generally understood to be a person who does not believe in knowledge claims about paranormal phenomena. It is interesting to note, however, that a doubter of paranormal claims is, technically, no more or less skeptical than the believer of such claims. To believe a proposition logically entails disbelief, or denial of, the contradictory proposition. He who believes that "X is true," necessarily disbelieves, or is skeptical of, the opposite proposition, that "X is not true."

Hence the believer in psi-phenomena is a skeptic in regard to the claims of those who deny or doubt such phenomena. The Christian is likewise a skeptic in regard to the claims of disbelievers, as well as to the competing beliefs of other religions.

Despite my atheism, I have always avoided the term "skeptic" as a label for my own philosophical and religious beliefs, because I reject the various arguments that have been put forth by epistemological skeptics throughout the history of philosophy. (The major reasons for this are presented in my previously mentioned book.) If, to paraphrase Thomas Huxley, I must attach a tail to myself by which I can be identified, I prefer the venerable term "rationalist" - a favorite among nineteenth-century freethinkers. But even this term is liable to misinterpretation, since, in the history of modern philosophy, "rationalists" (such as Descartes and Spinoza) are often contrasted with "empiricists" (such as John Locke and David Hume).

Though I regard this dichotomy, and many similar ones in philosophy, as extremely misleading, I will, if pressed to the wall, side with the empiricist school against the Cartesian rationalists. But this is not what freethinkers have historically meant by the term "rationalist," nor is it what I mean. Generally, a "rationalist" is simply a person who will, to the best of his or her ability, adjudicate knowledge claims in the court of reason, accepting those that can be justified and rejecting those that cannot.

To those skeptics who reply that reason is fallible, that even knowledge claims that seem certain to us now may eventually turn out to be false, I reply: Yes, of course, but so what? It is precisely because we are fallible human beings, that we can easily be misled and deceived, that we need to establish epistemological standards and follow the dictates of reason. Only reason can tell us whether a knowledge claim should be rejected, or perhaps accepted as possible, probable or certain.

An infallible God who is incapable of error would have no need for a philosophy of knowledge, nor would he need to distinguish between degrees of certainty. A theory of knowledge that relies on infallibility as its ideal standard leads us nowhere except into a futile skepticism. Philosophers have made this point repeatedly for several centuries, but Skeptics, it seems, are skeptical of everything except their own skepticism.

George H. Smith is the author of Atheism: The Case Against God.

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