Skepticism (or Scepticism in the UK spelling) is a Hellenistic school of philosophy. At its simplest, Skepticism holds that one should refrain from making truth claims, and avoid the postulation of final truths. This is not necessarily quite the same as claiming that truth is impossible (which would itself be a truth claim), but is often also used to cover the position that there is no such thing as certainty in human knowledge (sometimes referred to as Academic Skepticism). See the section on the doctrine of Skepticism for more details.
Possibly the earliest Skeptic, Gorgias claimed that nothing exists; or, if something does exist, then it cannot be known; or if something does exist and can be known, it cannot be communicated. Gorgias, however, is known primarily as a Sophist rather than as a philosophical skeptic.
Socrates claimed that he knew one and only one thing: that he knew nothing. Thus, rather than making assertions or opinions, he set about questioning people who claimed to have knowledge, ostensibly for the purpose of learning from them. Although he never claimed that knowledge is impossible, he never claimed to have discovered any piece of knowledge whatsoever, even at his death.
The first Skeptic proper, however, was Pyrrho of Elis (although he was perhaps not actually a "skeptic" in the later sense of the word), and the Skeptic movement which subsequently grew up was largely based around his early ideas. Pyrrho traveled and studied as far as India, but he became overwhelmed by his inability to determine rationally which of the various competing schools of thought of the time was correct. Upon admitting this to himself, he finally achieved the inner peace (or "ataraxia") that he had been seeking (and which became the ultimate goal of the early Skeptikoi), and he propounded the adoption of what he called "practical skepticism". Pyrrho himself wrote nothing, and even the satiric writings of his pupil Timon of Phlius are mostly lost. Today, his ideas are known mainly through the book "Outlines of Pyrrhonism" by the Greek physician Sextus Empiricus in the early 3rd Century A.D.
Later thinkers took up and extended Pyrrho's approach, accusing the Stoics of dogmatism, and arguing that the logical mode of argument was untenable, as it relied on propositions which could not be said to be either true or false without relying on further propositions. They did not believe that truth was necessarily unobtainable, but rather an idea which did not yet exist in a pure form, or had not yet been discovered. Thus, they viewed dogmatism as a disease of the mind and vowed to continue their inquiry.
Around 266 B.C., Arcesilaus (c. 316 - 241 B.C.) became head of Plato's Academy in Athens, and he strongly changed the Academy's emphasis from Platonism to Skepticism, and it remained the center of "Academic Skepticism" for the next two centuries. Carneades (c. 214 - 129 B.C.), who became the fourth Academy scholarch in succession after Arcesilaus in 155 B.C., was one of the best known of the Academic Skeptics, and he famously claimed that "Nothing can be known, not even this". He was followed as head of the Academy by Clitomachus (187 - 109 B.C.) in 129 B.C., and by Philo of Larissa (c. 159 - 84 B.C.) who became the last undisputed head of the Academy in 110 B.C. until the Roman occupation in 84 B.C.
During the 1st Century B.C., Aenesidemus rejected many of the theories of the Academy and founded a separate Pyrrhonian Skepticism school, which revived the principle of epoche" (or "suspended judgment") originally proposed by Pyrrho and Timon, as a solution to what he considered to be the insoluble problems of Epistemology.
Later followers of Pyrrho and Carneades developed more theoretical perspectives, and Sextus Empiricus (c. 200 A.D.) in particular incorporated aspects of Empiricism (the idea that the origin of all knowledge is sense experience) into the basis for asserting knowledge. Sextus and his followers considered both the claims to know and not to know to be equally dogmatic, and claimed neither. Instead, despite the apparent conflict with the goal of ataraxia, they claimed to continue searching for something that might be knowable.
After centuries of religious dogmatism throughout the Middle Ages, Skepticism again resurfaced during the Age of Reason and the Enlightenment of the 17th and 18th Century. Michel de Montaigne (1533 - 1592) in France and Francis Bacon in England both took as their starting point the skeptical viewpoint that they knew nothing for certain, as did Blaise Pascal and René Descartes, although these early pioneers were careful not to jettison their Christian beliefs.
Descartes established a methodological skepticism (also known as Cartesian Skepticism) in which he rejected any idea that can be doubted, and then attempted to re-establish it in order to acquire a firm foundation for genuine knowledge. His famous formulation "Cogito, ergo sum" is sometimes stated as "Dubito, ergo cogito, ergo sum" ("I doubt, therefore I think, therefore I am").