Skepticism refers to the philosophic position holding that the possibility of knowledge is limited either because of the limitations of the mind or because of the inaccessibility of its object. It is more loosely used to denote any questioning attitude. Extreme skepticism holds that no knowledge is possible, but this is logically untenable since the statement contradicts itself. During the Renaissance the influence of ancient skepticism was reflected preeminently in the writings of the 16th-century French philosophical essayist Michel de Montaigne. The greatest exponent of modern skepticism was the 18th-century Scottish empiricist philosopher David Hume. In his Treatise of Human Nature (1739-1740) and An Enquiry Concerning Human Understanding (1748), Hume questions the possibility of demonstrating the truth of beliefs about the external world, causal connections, future events, or such metaphysical entities as the soul and God. The 18th-century German philosopher Immanuel Kant, while attempting to overcome Hume's skepticism, denied the possibility of knowing things in themselves or of achieving metaphysical knowledge. In the 19th century, the German philosopher Friedrich Nietzsche denied the possibility of complete objectivity, and thus of objective knowledge, in any field. The 20th-century American philosopher George Santayana, claiming to have taken Hume's skepticism a step further, maintained, in his work Scepticism and Animal Faith (1923), that belief in the existence of anything, including oneself, rests on a natural, but irrational impulse. Elements of skepticism may be found in other modern schools of philosophy, including pragmatism, analytic and linguistic philosophy, and existentialism.
In philosophical skepticism, pyrrhonism is a position that refrains from making truth claims. A philosophical skeptic does not claim that truth is impossible (which would be a truth claim). The label is commonly used to describe other philosophies which appear similar to philosophical skepticism, such as "academic" skepticism, an ancient variant of Platonism that claimed knowledge of truth was impossible. Empiricism is a closely related, but not identical, position to philosophical skepticism. Empiricists see empiricism as a pragmatic compromise between philosophical skepticism and nomothetic science; philosophical skepticism is in turn sometimes referred to as "radical empiricism."
Philosophical skepticism originated in ancient Greek philosophy. One of its first proponents was Pyrrho of Elis (c. 360-275 B.C.), who traveled and studied as far as India, and propounded the adoption of 'practical' skepticism. Subsequently, in the 'New Academy' Arcesilaos (c. 315-241 B.C.) and Carneades (c. 213-129 B.C.) developed more theoretical perspectives, by which conceptions of absolute truth and falsity were refuted. Carneades criticized the views of the Dogmatists, especially supporters of Stoicism, asserting that absolute certainty of knowledge is impossible. Sextus Empiricus (c. A.D. 200), the main authority for Greek skepticism, developed the position further, incorporating aspects of empiricism into the basis for asserting knowledge.
Greek skeptics criticized the Stoics, accusing them of dogmatism. For the skeptics, 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. This was the regress argument, whereby every proposition must rely on other propositions in order to maintain its validity. In addition, the skeptics argued that two propositions could not rely on each other, as this would create a circular argument (as p implies q and q implies p). For the skeptics such logic was thus an inadequate measure of truth which could create as many problems as it claimed to have solved. Truth was not, however, necessarily unobtainable, but rather an idea which did not yet exist in a pure form. Although skepticism was accused of denying the possibility of truth, in actual fact it appears to have mainly been a critical school which merely claimed that logicians had not discovered truth.
A scientific (or empirical) skeptic is one who questions the reliability of certain kinds of claims by subjecting them to a systematic investigation. The scientific method details the specific process by which this investigation of reality is conducted. Considering the rigor of the scientific method, science itself may simply be thought of as an organized form skepticism. This does not mean that the scientific skeptic is necessarily a scientist who conducts live experiments (though this may be the case), but that the skeptic generally accepts claims that are in his/her view likely to be true based on testable hypotheses and critical thinking.
Common topics that scientifically-skeptical literature questions include health claims surrounding certain foods, procedures, and medicines, such as homeopathy, Reiki, Thought Field Therapy (TFT), vertebral subluxations; the plausibility of supernatural entities (such as ghosts, poltergeists, angels, and gods); as well as the existence of ESP/telekinesis, psychic powers, and telepathy (and thus the credibility of parapsychology); topics in cryptozoology, Bigfoot, the Loch Ness monster, UFOs, crop circles, astrology, repressed memories, creationism, dowsing, conspiracy theories, and other claims the skeptic sees as unlikely to be true on scientific grounds.
Most empirical or scientific skeptics do not profess philosophical skepticism. Whereas a philosophical skeptic may deny the very existence of knowledge, an empirical skeptic merely seeks likely proof before accepting that knowledge.