Scepticism (from the Greek, skeptesthai, 'to examine') is the philosophical view that it is impossible to know anything with absolute certainty, or to know the world as it 'really' is. The word can also mean a general reluctance to accept anything on face value without sufficient proof (as in "He heard that Jim had run the 100m in under ten seconds but he remained sceptical").

However, Scepticism (with a capital 'S') began in the 5th century BC in Greece where certain philosophers came to express doubts about how certain we could be about our knowledge. Protagoras of Abdera (480-411 BC), for instance, is reported to have said that "man is the measure of all things" (i.e. that we make the world in our own image) and Gorgias (485-380 BC) that "nothing exists; if anything does exist, it cannot be known; if anything exists and can be known, it cannot be communicated". Many such thinkers arose from the group known as the Sophists, men who would hire their skills in debate and argument out to anyone for the right fee. From this point of view, this form of scepticism is based on the fact that with enough skill, any argument can sound convincing.

Next came the Pyrrhonists, so called after Pyrrho of Elis, it's founder, who argued that since we can never know true reality we should refrain from making judgements. His pupil, Timon of Philius, followed this by adding that equally good arguments could be made for either side of any argument (so it was impossible to decide). The New Academy of the 2nd century BC, founded by Carneades (214-129 BC), taught only that some arguments were more probable than others. Later sceptics include Aenesidemus (1st century BC), who put forward ten arguments in support of the sceptical position, and the Greek physician Sextus Empiricus (3rd century AD), who argued the use of common sense over abstract theory.

When we reach the Renaissance we can see the influence of Greek scepticism in such thinkers as the French essayist Michel de Montaigne (1553-1592), but the sceptical issues only fully resurfaced with the French philosopher René Descartes ( 1596-1650). Descartes attempted to use sceptical arguments in order to establish a firm ground for knowledge. So, Descartes reasoned, if we attempt to subject everything to doubt we will hopefully discover at some point if there is anything that cannot be doubted. This he claimed to achieve in his assertion that it is impossible to doubt that we are thinking beings - which proves that we exist ( 'Cogito, ergo sum', which is Latin for 'I think, therefore I am'). By employing this 'method of doubt', as he called it, Descartes merely used scepticism as a means to find something certain, and was not therefore actually a sceptic.

The sceptical cause was once again championed by the Scottish empiricist philosopher David Hume (1711-1776), who argued that certain assumptions - such as the link between cause and effect, natural laws, the existence of God and the soul - were far from certain. What little we know that seems certain, Hume argued, was based on observation and habit as opposed to any logical or scientific necessity. The German philosopher Immanuel Kant (1724-1804), influenced by Hume, set limits to human knowledge by arguing that certain things - such as if there was proof for God, or if the world had a beginning - did not make sense to be asked.

The German philosopher Friedrich Nietzsche (1844-1900) argued that objective knowledge did not actually exist, and his scepticism influenced in turn that of French Existentialists such as Jean-Paul Sartre (1905-1980). The American philosopher George Santayana (1863-1952), argued that all belief - even that in oneself - is irrational (even though it seems the most natural thing).

Modern day philosophy, although it does not generally take extreme sceptical arguments very seriously, still retains the influence of earlier sceptical thinkers.

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