A brief history of Skepticism
by Massimo Pigliucci - University of Tennessee

Skepticism is a very ancient concept. In philosophy, it is the doctrine that absolute knowledge is not achievable. Therefore, inquiry must proceed by doubt and acquisition of approximate or relative knowledge. This is pretty much the way the modern scientific method works, even though many scientists would not consider themselves "skeptics" in a philosophical sense. Interestingly, one of the first modern books on the scientific method was entitled A skeptical chemist, and was authored by the father of modern chemistry, the Anglo-Irish physicist Robert Boyle (1627-1691). 

The very first skeptics were perhaps the Greek sophists of the 5th century BC. They were philosophers of pre-Socratic time, inquiring about theology, metaphysics, natural sciences and mathematics. They specialized in dialectic and rhetoric, from which they got their bad reputation as specious thinkers. In fact, sophists believed that one should exercise his/her mind by taking different sides of the same argument and attempting to defending all with equal effectiveness. They are probably the precursors of modern lawyers...

Protagoras (490-421 BC) was the main skeptic of ancient times. He was a Greek philosopher, leader of the sophist movement. His most famous statements is perhaps "Man is the measure of all things". By that he meant that truth is relative to the individual who maintains it, i.e., that there are no universal truths. He denied the possibility of objective knowledge, and went so far as to deny a difference between reason and sense. No work by Protagoras has actually reached us directly, but his ideas are presented in some of Plato's dialogues.

The first "modern" skeptic was David Hume (1711-1776), a Scottish philosopher. He elaborated on the philosophy of two other great thinkers: the empiricists John Locke (1632-1704), and George Berkeley (1685-1753). Let us take a quick look at these two, before going back to Hume. The English Locke opposed the existence of innate ideas, suggesting instead that the human mind is born as a blank board (tabula rasa) on which experiences inscribes the character. According to Locke, science is possible because the primary qualities of objects in the world affect human sense organs mechanically. The Irish Berkeley pushed Locke's ideas a step further. His subjective idealism maintains that matter does not exist independently of perception. We perceive a "real" world, according to Berkeley, only because of the mind of God. Hume went beyond both Locke and Berkeley, with his notion of radical skepticism. He suggested that the mind only perceives sensations, and that therefore there is no certain knowledge. A corollary was the implication that our perception of cause and effect is just that, a perception. As for religion, Hume rejected any form of theology, natural or rational.

Immanuel Kant (1724-1804) and his agnostic doctrine is probably more close to a modern understanding of skepticism. A German philosopher, Kant openly acknowledged having being turned into a critical philosopher by reading Hume. He thought of his work as of a synthesis of Hume's skepticism and Leibnitz's rationalism (Gottfried Wilhelm, baron von Leibnitz was a German mathematician born in 1646, who died in 1716. His philosophy was quite naive, claiming that the existing world was the best of all possible worlds, a view satirized by the French philosopher Francois Marie Arouet de Voltaire (1694-1778) in his hilarious novel Candide). But let us get back to Kant. According to Kant, objective reality is known only to the extent that it conforms to the structure of our mind. He made a distinction between phenomena, which are knowable by experience, and noumena, which lie beyond experience and therefore cannot be demonstrated to exist or not exist. The main consequence - discussed in his Critique of pure reason - is that God, freedom, and similar can simply not be investigated by scientific means. 

The first author to use skepticism as a method of inquiry was the French Rene` Descartes (1596-1650), chiefly exposed in his Discourse on method. Descartes idea was to extend the mathematical approach to discovery to every realm of human endeavor. His method therefore started out by doubting everything, and attempting to derive everything from the only thing that could not be doubted, that it doubt itself. If that is correct, then the doubter must exist, hence his famous cogito ergo sum. He then expanded gradually his inquiry, to include the existence of God as the first cause of the universe, and the existence of the physical universe itself. 

The modern scientific method, as mentioned above, is to some degree based on skepticism. It needs to be remembered, however, that scientific positivism requires that material effect is possible only with a material cause. A spin off of skepticism is the philosophy of logical positivism, established in the 1920s by a group of thinkers known as the Vienna circle (including people like Kurt Godel, Bertrand Russell, and Ludwig Wittgenstein). Logical positivists maintain that metaphysical speculation is nonsensical, and that moral and value statement are merely emotive. Russell, in particular, suggested that individual facts are logically independent, while knowledge is dependent on data from experience.