Skepticism is the Western philosophical tradition that maintains that human beings can never arrive at any kind of certain knowledge. Originating in Greece in the middle of the fourth century BC, skepticism and its derivatives are based on the following principles:
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There is no such thing as certainty in human knowledge.
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All human knowledge is only probably true, that is, true most of the time, or not true.
Several non-Western cultures have skeptical traditions, particularly Buddhist philosophy, but properly speaking, skepticism refers only to a Greek philosophical tradition and its Greek, Roman, and European derivatives.
The school of Skeptic philosophers were called the "Skeptikoi" in Greece. The word is derived from the Greek verb, "skeptomai," which means "to look carefully, to reflect." The hallmark of the skeptikoi was caution; they refused to be caught in assertions that could be proven false. In fact, the entire system of skeptic philosophy was to present all knowledge as opinion only, that is, to assert nothing as true.
In this, they were firmly planted in a tradition started a century earlier by Socrates. Socrates claimed that he knew one and only one thing: that he knew nothing. So he would never go about making any assertions or opinions whatsoever. Instead, he set about questioning people who claimed to have knowledge, ostensibly for the purpose of learning from them, using a judicial cross-examination, called elenchus . If someone made an assertion, such as, "Virtue means acting in accordance with public morality, " he would keep questioning the speaker until he had forced him into a contradiction. As in a court of law, this contradiction proved that the speaker was lying in some way, in this case, that the speaker did not really know what they claimed to know. If an assertion can be worked into a contradiction, that means that the original assertion was wrong. While Socrates never claimed that knowledge is impossible, still, at his death, he never claimed to have discovered any piece of knowledge whatsoever.
After its introduction into Greek culture at the end of the fourth century BC, skepticism influenced nearly all other Greek philosophies. Both Hellenistic and Roman philosophies took it as a given that certain knowledge was impossible; the focus of Greek and Roman philosophy, then, turned to probable knowledge, that is, knowledge that is true most of the time.
Christianity, however, introduced a dilemma into Greek and Roman philosophies that were primarily based on skeptical principles. In many ways, the philosophy of Christianity, which insisted on an absolute knowledge of the divine and of ethics, did not fit the Greek and Roman skeptical emphasis on probable knowledge. Paul of Tarsus, one of the original founders of Christianity, answered this question simply: the knowledge of the Romans and Greeks, that is, human knowledge, is the knowledge of fools. Knowledge that rejects human reasoning, which, after all, leads to skepticism, is the knowledge of the wise. Christianity at its inception, then, had a strong anti-rational perspective. This did not, however, make the skeptical problem go away. Much of the history of early Christian philosophy is an attempt to paste Greek and Roman philosophical methods and questions onto the new religion; the first thing that had to go was the insistence on skepticism and probable knowledge. So early Christian thinkers such as Augustine and Boethius took on the epistemological traditions of Greece and Rome to demonstrate that one could arrive at certain knowledge in matters of Christian religion. Augustine devoted an entire book, "Against the Academics," to proving that human beings can indeed arrive at certain knowledge.
Skepticism, however, was radically reintroduced into Western culture in the sixteenth and seventeenth centuries. The break-up of the Christian church and the bloodshed that followed it led people to seriously question religious and philosophical traditions that had long been unquestioned. Thinkers such as Montaigne in France and Francis Bacon in England took as their starting point the idea that they knew nothing for certain, particularly religious truth. Montaigne would invent an entirely new literary format which he called the essai , or "attempt, trial"; this is the origin of the modern-day essay. The "essay" took as its starting point the idea that the writer doesn't really know what he's talking about. Montaigne would propose an issue, walk around the issue for awhile and consider various alternatives, and then end pretty much where he started: uncertain what conclusion to draw. This is why he called his writings, "attempts" (essais in French), for they were attempts at drawing conclusions rather than finished products.
The most radical introduction of Greek skeptical traditions back into the Western tradition occurred in the works of Blaise Pascal and René Descartes. Both thinkers refused to accept any piece of knowledge whatsoever as true, and both tried to rebuild a Christian faith based on this radical questioning of truth.
Descartes set about reinventing Western epistemology with a radical perspective: what if nothing were true? How, if you doubted everything, could you find something—anything—that was true. His conclusion, of course, was the famous cogito : Cogito ergo sum , or, "I think, therefore I am." From this base he built up a series of other true propositions, including the existence of God. In many ways, Descartes was trying to accomplish the same thing that Augustine, Boethius, and other early Christian thinkers were attempting: how do you address the possibility, firmly entrenched in the Western tradition, that there may be no such thing as certain knowledge? How do you reconcile that with religious faith? For that was Descartes' ultimate goal: to prove the existence of God and the validity of the Christian religion.
Although he saw himself as answering old and vexing questions in the Christian tradition, he actually created a radically new way of approaching the world: systematic doubt. The hallmark of Cartesianism is setting up a formal system of doubt, that is, of questioning all propositions and conclusions using a formal system. Once one has arrived at a certain piece of knowledge, that piece of knowledge then becomes the basis for clearing up other doubts. Descartes systematic doubt became the basis of the Enlightenment and modern scientific tradition. One begins with a proposition, or hypothesis, that is in doubt and then tests that proposition until one arrives, more or less, at a certain conclusion. That does not, however, end the story. When confronted by the conclusions of others, one's job is to doubt those conclusions and redo the tests. Once a hypothesis has been tested and retested, then one can conclude that one has arrived at a "scientific truth." That, of course, doesn't end it, for all scientific truths can be doubted sometime in the future. In other words, although scientists speak about certainty and truth all the time, the foundational epistemology is skeptical: doubt anything and everything.