Philosophical scepticism is the critical examination of whether the knowledge and perceptions one has are true, and whether or not one can ever be said to have knowledge. More colloquially, scepticism is the challenging of accepted beliefs.
There are two kinds of scepticism -- "global"and "local". If a sceptic believes that knowledge of anything at all is impossible, then that is "global scepticism". Global scepticism is rare, since it presents something of a paradox -- if knowledge of anything is impossible, then we cannot know even if scepticism is correct, and cannot use any knowledge (even of logic) to argue or refute the question. But, on the other hand, if the sceptic believes that only knowledge of a particular area is impossible, then that is "local scepticism". There are "local"sceptics in almost every area of philosophical inquiry. But two main areas of local scepticism are "the external world"and "other minds". The external world sceptic will deny that we can ever know that there is an external world. The other mind sceptic will deny that we can ever know that other people have minds.
Philosophical scepticism has played a significant role in the development of philosophical thought for three reasons.
Firstly, philosophical scepticism challenges the fundamental beliefs of philosophical dogmatism, which maintains that some truths are authoritative, and certainly true. For example, most western philosophical thought after the Greeks and before The Enlightenment (or Age of Reason) was constrained by the dogmatism of Christian (and Islamic) beliefs. It can be argued that the sceptical challenges of philosophers like David Hume (1711-1776) were fundamental to the nurturing of the kind of open-minded attitudes that were the foundation of the Age of Reason. In presenting an argument that he hoped would serve as "an everlasting check to all kinds of superstitious delusion", Hume argued that
"A miracle is a violation of the laws of nature; and as a firm and unalterable experience has established these laws, the proof against a miracle, from the very nature of the fact, is as entire as any argument from experience can possibly be imagined."
The strength of this kind of sceptical challenge to generally accepted dogmatic beliefs created serious problems for the defenders of those beliefs. The sceptics were sufficiently successful against the various dogmatic philosophies, that such philosophies have become quite rare (at least among professional philosophers, if not amongst the general population). Logic (and its partner, mathematics) is about the only area left where modern philosophers still write about truths that are certain.
Secondly, the arguments presented by the various sceptical philosophers have forced philosophers to investigate and properly identify the nature of knowledge, and the nature of sensory experience. The sceptical challenge is based on one or both of two premises -- (a) that everything we experience might not be real (or, equivalently, might be in error); and (b) that knowledge demands certainty.
The efforts of philosophers to refute the sceptical premise that knowledge demands certainty generated the field of epistemology. Epistemology is that branch of philosophy that is concerned with theories of the sources, nature, and limits of knowledge, truth, concepts, and belief. The response to the sceptic has been to correct our understanding of the meaning of the term "knowledge"to more accurately reflect how the concept is actually used. The general result has been a concept of knowledge based on probabilities, preponderance of the evidence, absence of defeaters, and other such ideas. Thus, contrary to the argument of the sceptic, knowledge does not require certainty. So the existence of possible, yet highly improbable, sources of doubt with no supporting evidence, does not in fact defeat claims of knowledge. The remote possibility that I might be a "brain in a vat", or wired into a pod in The Matrix world, does not defeat the claims I make to have knowledge of Reality.
The efforts of philosophers to refute the sceptical premise that everything we experience might not be real have taken longer to show results. The highly persuasive arguments presented by Rene Descartes (1596-1650) in his Meditations convinced several generations of philosophers to buy into the idea of the "Cartesian Theatre"and the "sense-data"theory of perception. It has only been fairly recently that realist theories of perception have become more widely accepted. Although a direct realist theory of perception dates all the way back to Thomas Reid (1710-1796), it has only been in the last 40 years or so that the realist theory of perception has appeared to receive any significant attention. (See, for example, David Kelley - The Evidence of the Senses: A Realist Theory of Perception).
The basic argument of the realist theory of perception is that there is no "Cartesian Theatre"where "sense-data"are presented to the perceiver for interpretation and comprehension. Instead, the senses provide direct (although complex) awareness of the external world in the same manner that one's self-awareness provides direct awareness of one's internal mental states. If one feels pain, one feels pain directly (the qualitative feel of the pain is the pain) and the awareness of that pain is incorrigible. Similarly, if one sees something with the eyes, the awareness of that thing is direct and incorrigible (under normal conditions). So the argument is essentially that, contrary to the premise of the sceptic, everything we experience is indeed real and cannot (under normal conditions) be in error. (Although we often err when we draw inferences from what we perceive. Only partly because the conditions are often not "normal".)
The third major reason that philosophical scepticism has played a significant role in the on-going development of philosophical thought, is a negative consequence rather than a positive one. In response to the challenge of the sceptics that knowledge of the external world was impossible, various philosophers developed schools of thought that are today loosely known as "Modern Idealism". The most interesting of the early forays in this direction was by Bishop Berkeley (1685-1753), the Irish philosopher and clergyman, generally regarded as the founder of the modern school of idealism. He proposed that the apparently objective world has its existence in the consciousness of individuals, and that everything we experience is simply an idea in the mind of God. It neatly side-stepped the challenges raised by the sceptic by making the sensory inputs irrelevant, and the foundation of knowledge the incorrigible omniscience of God.
The most famous of the modern idealists is probably Immanuel Kant (1724-1804). But the movement was populated also by the likes of the Existentialists (such as Søren Kierkegaard, Martin Heidegger, Jean-Paul Sartre), the Transcendentalists (such as Ralph Waldo Emerson, Henry David Thoreau), the Intuitionists (such as Baruch Spinoza, Henri Bergson), and the Pre-Marxists (such as Georg Wilhelm Friedrich Hegel, Saint-Simon, Louis Blanc) -- to name just a few of the more notable personalities.
Despite the wide variation in the philosophies of this movement, all of them share a common core. They all attempt to deal with the sceptical challenge to knowledge by accepting the sceptical conclusion. They all start from the premise that we cannot really know anything about the external world, or that it exists. Kant, as the archetypical example, argued that instead of assuming that, to be true, our ideas must conform to an external reality independent of our knowing, objective reality is known only insofar as it conforms to the essential structure of the knowing mind. He maintained that objects of experience -- phenomena -- may be known, but that things lying beyond the realm of possible experience -- noumena, or things-in-themselves -- are unknowable. In other words, it is the nature of the conscious mind that determines what we know of reality and truth.
Of course, given the critical disconnect between the external world in which we (allegedly) live and the philosophical structures erected by the members of the various Modern Idealist philosophies, it is not surprising to find that they tended towards the wild and woolly. So we have the philosophies of Nietzsche, Marx, Husserl, Heidegger, Sartre, and so forth. All essentially disconnecting the mind from reality. All laying claim to some source of certain truth independent of experience. And all fostering the popular impression (technically incorrect though it is) that the truth is whatever you want it to be.
On balance, I would suggest that the positive consequences of Scepticism outweigh the negative consequences. The sceptic's challenge to dogmatic philosophies has worked just as well against the dogmatisms of the Modern Idealists as it did against the Pre-Enlightenment philosophies. Both the firmer foundation of epistemology and the clarified realist theory of perception have been a clear benefit to Philosophy. Which is not to suggest that the sceptic has been totally routed. Sceptics still contribute in many areas of philosophy. Challenging of accepted beliefs is part and parcel of good philosophical thinking.
Hume, David; An Enquiry Concerning Human Understanding
Kelley, David; The Evidence of the Senses: A Realist Theory of Perception
Reid, Thomas; Thomas Reid's Inquiry and Essays
Russell, Bertrand; A History of Western Philosophy
Stanford Encyclopedia of Philosophy (http://plato.stanford.edu)
MicroSoft BookSelf Basics: The American Heritage® Dictionary of the English Language, Third Edition.
The Columbia Dictionary of Quotations
Encarta® 98 Desk Encyclopedia
http://www.encyclopedia.com (Columbia Encyclopedia, Sixth Edition, 2003)
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