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"The worst speculative Sceptic ever I knew, was a much better Man than the best superstitious Devotee & Bigot." --David Hume (Letter to Gilbert Elliot of Minto, March 10, 1751)
Philosophical skepticism systematically questions the notion that absolutely certain knowledge is possible.
Philosophical skepticism is opposed to philosophical dogmatism, which maintains that a certain set of positive statements are authoritative, absolutely certain, and true.
Philosophical skepticism emerged early in ancient Greek philosohy. For example, the Sophist Gorgias (483-378 BCE) claimed that nothing exists or if something exists, 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. Pyrrho (c. 360-c.270 BCE) is generally considered the first philosophical skeptic in western philosophy. Little is known of Pyrrho or his followers. Nor is much known about two other important figures in ancient skepticism: Arcesilaus (ca. 316-241 BCE) and Carneades (214-270 BCE), each of whom headed the Academy founded by the dogmatist Plato (427-347 BCE). The first group of philosophical skeptics are known as Pyrrhonists, the latter are known as the Academics. Neither the Pyrrhonists nor the Academics seem to have advocated the kind of extreme denialism Gorgias maintained. (The distinction between Pyrrhonists and Academic skeptics is not without controversy, but it serves our purpose here.)
Other Sophists can also be seen as philosophical skeptics. For example, Protagoras (480-411 BCE) said that "Man is the measure of all things." This statement is usually interpreted to mean that there are no absolute standards or values and that each person is the standard of truth in all things. When applied to moral rules, this view is known as moral relativism, a type of philosophical skepticism that denies there are any absolute moral values. In its most extreme form, moral relativism is the view that the morality of an act depends on what one believes about the morality of the act. If one believes an act is moral, then that act is moral for you. If one believes an act is immoral, then that act is immoral for you. (Finding a philosopher who defends extreme moral relativism may take a lifetime or two, however. If the reader knows of any, please let me know. Extreme moral relativism is self-refuting. If the same act can be moral and immoral, moral language is pointless.)
The ancient skeptics did not all agree on even the most fundamental matters, such as whether certainty and knowledge are possible. Some believed that they knew certainty was not possible; others claimed that they did not know whether knowledge is possible. The position that one knows that knowledge is impossible seems to be self-refuting. The view that one does not know whether knowledge is possible is consistent with the notion that it makes sense to strive to know, even if one can't be sure that one will arrive at knowledge. And, while some ancient skeptics seem to have advocated that the ideal is to have no strong opinions, most seem to have maintained that when there was a preponderance of evidence supporting the probability of one position rather than another, then belief in the more probable position was desirable. Most ancient skeptics do not seem to have believed that simply because one cannot be absolutely certain about anything, one should therefore suspend judgment on all things. Such a view would be self-refuting. For, according to the principle itself one should not accept it, but suspend judgment on it. Suspending judgment on claims should be reserved for those claims one knows nothing about, or can know nothing about, and for those claims for which the evidence is proportionate on opposing sides. It may be true that nothing is absolutely certain, but it is not true that all claims are equally probable.
The Greek word skeptikoi means seekers or inquirers. Socrates (469-399 BCE), for many the model critical thinker and skeptical inquirer, claimed that the only thing he knew was that he knew nothing. This did not stop him from making inquiries about everything, however. He frequently said "Skepteon," meaning we must investigate this. The Pyrrhonists inquired into the truth of matters, even if most of the time that meant that they sought contrary arguments to dogmatic positions held by other philosophers, such as the Stoics or Epicureans. On those issues where argument and counterargument equaled one another, the Pyrrhonists held that we should suspend judgment. They apparently found that such a stance fit well with their desired goal of peace of mind (ataraxia). For, it is the dogmatist who gets agitated when he doesn't possess the good or truth he knows he should have, or when others refuse to accept what he knows is the truth.
The Academic skeptics rejected their founder's metaphysical dogmatism and defended probabilism: the view that probability rather than absolute certainty is possible in many matters and that probability is sufficient for the important things in life. Both forms of ancient skepticism were revived during the Renaissance with the availability of long-lost works, such as the writings of Sextus Empiricus (ca. 160–210 CE). As in ancient times, some "modern" philosophers rejected the arguments of the skeptics, e.g., Spinoza (1632-1677) and Descartes (1596-1650). Others, such as John Locke (1632-1704), accepted the skeptical arguments, but did not despair of gaining useful knowledge. The advancements of modern science in the seventeenth century were made possible because of the rise of empiricism, the belief that all knowledge is based on sense experience and exists in degrees of probability.
(Contemporary philosophical skepticism is a complex topic and will not be discussed here. See the entry on this topic in the Internet Encyclopedia of Philosophy. What is sometimes called scientific skepticism will be discussed below.)
Gorgias's skepticism was based on the belief, later shared by the empiricists, that all knowledge originates in sense experience and is not constant or uniform. Sense experience varies from person to person, moment to moment. His view might be called sensory skepticism, the philosophical position that we cannot have absolute certainty about anything that is based solely on sense experience. Throughout the history of philosophy, arguments demonstrating the unreliability of sense experience have flourished, especially among philosophical dogmatists who looked elsewhere for absolute certainty. One common argument is that what appears to us via the senses cannot be a reliable guide as to what is really beyond those appearances. The materialist Democritus (460-370 BCE), a contemporary of Gorgias and not generally considered a philosophical skeptic, made such an argument.
Throughout the history of philosophy, sensory skeptics have argued that we perceive only things as they appear to us and cannot know what, if anything, causes those appearances. Thus, if there is sense knowledge, it is always personal, immediate, and mutable. Any inferences from appearances are subject to error, and we are without a method to know whether the inferences or judgments we make are correct. As noted already, these arguments did not prevent Academic skeptics from putting forth a defense of probabilism with regard to empirical knowledge. Nor has sensory skepticism hindered dogmatists from seeking absolute truth elsewhere, namely in reason or logic.
Philosophical skepticism did not originate as a guide for practical living, but as an antidote to philosophical dogmatism. The earliest philosophical skeptics did not allow vicious dogs to bite them on the ground that their senses might be deceiving them, however. Even if it cannot be proved with absolute certainty that any phenomenal object is real, experience is a good guide as to the probability of what will happen if one allows a vicious dog to tear into one's leg. Skeptics don't deny the reality of sense perception. Dog bites hurt and honey tastes sweet. What the skeptics deny is that beyond the appearances of the biting dog there is a "dog essence" or that the experience of sweetness when tasting honey justifies inferring that "sweetness" is part of the essence of honey. Skeptics don't deny appearances and subjective knowledge. They don't deny that one bitten by a dog feels real pain and knows he or she is in pain. Skeptics deny that it is justifiable to infer from subjective experience indubitable propositions about a reality beyond those appearances. Any inference to "objective reality," a reality that transcends immediate experience, should be couched in probabilistic language at best.
Nevertheless, ancient skepticism was considered a guide for living by its advocates. Their goal was to achieve a state of being completely unperturbed. Denying appearances would not serve such a goal. Rejecting dogmatism did. Absolute certainty is not needed, according to skeptics, either for science or for daily living. Science can do quite well even if limited to appearances and to probabilities. We can find guides for daily living, including moral principles, without needing absolute certainty. We can figure out what principles are likely to lead us to what we desire: a peaceful, happy life. Many philosophical skeptics of the Greco-Roman period advocated a very conservative lifestyle, maintaining that nature and custom know best. They advocated following the laws and customs, including the religious customs, of one's native country. They believed that following our natural appetites is a generally reliable guide to living well.
Perhaps the strongest criticism of the possibility of absolute truth is to be found in the skeptic's argument regarding the criterion of truth. Any criterion used to judge the truth of a claim can be challenged because a further criterion is needed by which to judge the present criterion, and so on ad infinitum. This argument did not deter philosophers such as Plato and Descartes from claiming to have found an absolutely impeccable criterion of truth. Descartes' famous dictum cogito ergo sum was arrived at in his Meditations during his pursuit of a criterion for truth that didn't need another criterion to validate it. He thought he found such a criterion in the notion of the "clear and distinct perception." Skeptics rejected Descartes' argument and the notion of infallible criteria for empirical claims. Many Academic skeptics, however, accept that there are absolutely certain claims, but that these are matters of logic or definition, and have nothing to do with establishing the certainty of any claim that goes beyond immediate perception.
While probabilism in empirical matters was defended as reasonable by philosophical skeptics, such an attitude was considered unreasonable with regard to metaphysics. One particular type of metaphysical skepticism (also known as positivism) is noteworthy: theological skepticism. A theological skeptic raises doubts regarding the possibility of knowledge about any gods. A theological skeptic may be an atheist, but the two positions are distinct. A theological skeptic may be a theist or an agnostic. The theological skeptic maintains that we cannot know for certain whether any gods exist. Such a view does not entail the notion that we should be atheists. Some theological skeptics will defend atheism on the grounds that there is much more support for the probability that a god does not exist than for the probability that a god does exist. A theist might disagree and think the probability is greater for theism, or he might admit that belief in a god is not based on probabilities at all, but is an act of pure faith. Such a position is known as fideism and it was the position of one of the most renowned proponents of scientific skepticism (see below), Martin Gardner (1914-2010). An agnostic, as distinguished from a theological skeptic, would hold that neither theism nor atheism is more probable than the other.
Theological skepticism is based on beliefs about the nature of supernatural claims and the nature of the human mind. Supernatural claims, it is said, transcend the limits of human knowledge. It is for this reason that some skeptics assert that revelation from a god is necessary. Skeptics may be atheists and be completely unaware of the arguments of theological skepticism, however. A skeptic may be an atheist simply because he or she perceives little, if any, evidence for the belief in a god. Some scientific skeptics (e.g., Richard Dawkins) believe that theological claims have empirical implications and can be assessed for their probability. In Dawkins' view, the probability of the existence of the Abrahamic god (or any other god, for that matter) is near zero.
In addition to providing philosophical doubts about dogmatic metaphysics, some philosophical skeptics aimed their arguments at specific types of claims. One of the most important figures in the history of skepticism is David Hume (1711-1776), whose skeptical argument against belief in miracles is still considered by many skeptics to be the best single argument in the history of skepticism. In fact, Hume hoped his argument would serve as "an everlasting check to all kinds of superstitious delusion." Hume argues that for the same reason it is justifiable to avoid the vicious dog trying to bite us, it is justifiable to reject miraculous claims. Miraculous claims assert that a violation of the laws of nature has occurred. Laws of nature are based on uniform experience. Experience is our guide in avoiding the vicious dog and must be our guide in judging the miraculous event. To accept an event as miraculous is to accept that experience is not a reliable guide. However, it is our only guide in such matters, unless we abandon reason and believe on pure faith. As he so eloquently and succinctly puts it:
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.
What other kinds of superstitious delusions would Hume's argument apply to? It seems that it would apply to things such as homeopathy, channeling, astral projection, levitation, past life regression, psychic surgery, map dowsing, and other things that require us to abandon uniform experience as a guide. Claims about ESP, however, would not be covered by the argument, unless advocates maintain that ESP occurs outside the realm of the laws of nature. As long as the ESP advocate claims that ESP follows natural laws that we haven't discovered yet, then Hume's argument would not apply.
In addition to fighting dogmatism, skeptics of all types have traditionally been critical of those who are gullible and credulous; thus, many skeptics have devoted themselves to debunking irrational traditional beliefs and superstitions. Even so, philosophical skeptics can be quite gullible. Most of what we know about ancient philosophical skepticism comes from Sextus Empiricus who believed, among other things, that some animals bypass fertilization in reproduction and originate in fire, fermented wine, mud, slime, donkeys, cabbage, fruit, and putrefied animals.
Many philosophical skeptics would agree that logic is an area where dogmatism is justified. The principle of contradiction, that a statement is either true or false but not both, is accepted by many skeptics as true but empirically empty. That is, such a truth reveals nothing about the world of experience. In addition to formal truths, such as the principle of contradiction or the principle of identity, most skeptics would probably accept that there are semantic truths, i.e., some statements that are true by definition. "A bachelor is an unmarried male," is true and does provide information about the world of experience, namely, how a certain word is used in a certain language. But the statement is a matter of convention, not discovery.
Dogmatic philosophies have become rare. The age of metaphysics is long gone, indicating that the skeptics have won the war with the dogmatists. Logic is about the only philosophical area left where professional philosophers still speak of absolute certainty with a straight face. The chance of another Plato or Hegel arising in the 21st century seems very slim. Most philosophers today content themselves with probabilistic arguments based on empirical knowledge and the application of logical principles to concepts. They are guided by the methods and discoveries of modern science.
Skepticism is not restricted to professional philosophers who devote themselves to epistemological disputes regarding the origin, nature, limits, and kinds of knowledge. Outside of the academy, there has been a significant social movement that characterizes itself as skeptical. The movement is worldwide and is diverse. Three of the more important organizations in the United States* that promote skepticism today are the Committee for Skeptical Inquiry (CSI), The Skeptics Society, and The James Randi Educational Foundation (JREF).
CSI's mission "is to promote scientific inquiry, critical investigation, and the use of reason in examining controversial and extraordinary claims." CSI publishes Skeptical Inquirer, a bimonthly magazine. The Skeptic Society's mission is to promote science and "to serve as an educational tool for those seeking clarification and viewpoints on" controversial ideas, extraordinary claims, and revolutionary ideas. The Skeptic Society publishes Skeptic magazine, a quarterly journal. The JREF's mission "is to promote critical thinking by reaching out to the public and media with reliable information about paranormal and supernatural ideas so widespread in our society today." The JREF publishes an online newsletter.
In his "Skeptic's Manifesto," co-founder of the Skeptics Society Michael Shermer describes skepticism as applying scientific methods and providing naturalistic explanations for apparently supernatural events. He quotes the OED's definition of 'skeptic':
One who doubts the validity of what claims to be knowledge in some particular department of inquiry; one who maintains a doubting attitude with reference to some particular question or statement.
The OED definition captures what most people mean when they say they are a skeptic. Shermer also disconnects the modern skeptical movement—whose beginning he traces to Martin Gardner's 1952 classic, In the Name of Science—from ancient denialism and nihilism.
The modern skeptical movement has a direct link to academic or mitigated skepticism in its belief that even though absolute knowledge and certainty is not possible, empirical claims can be justifiably assented to or rejected based on their probabilities. Most skeptics today would agree with David Hume's concluding remark on "academical or skeptical philosophy":
When we run over libraries, persuaded of these principles, what havoc must we make? If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion.*
Most non-academic skeptics today are probably not familiar with Hume's argument that our perceptions are all we know; we have no way of knowing whether those perceptions correspond to anything outside of us. All our empirical knowledge, including all scientific knowledge, is based on the assumption that there is an external reality that bears enough similarity to our perceptions to allow us to say that our perceptions provide a good basis for understanding the empirical world. In addition to the assumption that things outside us cause our perceptions, there is the additional problem regarding our idea of cause and effect. We think that causes are necessarily connected to their effects. But the idea of necessary connection can't be justified by any particular perception. We perceive and remember a regular contiguity of causes and effects, but we never perceive the necessary connection. According to Hume, we can't help adding the notion of necessary connection, but the concept is based on our natural disposition to do so. The core notion of our idea of causality, in short, is not based on perception, but on habit or instinct. Why do we add this notion? Hume would say that Nature made us this way. Today, skeptics would say we've evolved to see patterns and ascribe such connections to the patterns we observe.
Hume's epistemological skepticism was a direct descendent of John Locke's empirical philosophy and George Berkeley's (1685-1753) immaterialism. Modern skepticism has benefited from the progress of science since the 17th century. In particular, it has benefited from the methods that science has developed to determine whether observed patterns are indicative of causal connections. We may not be able to answer Hume's inquiry into the impossibility of proving the existence of the external world or that causality is a feature of that world and not just a psychological imposition on our subjective perceptions. Furthermore, we may not be able to solve a problem Hume identified, usually called the problem of induction: the only justification we have for believing that the patterns of the past will continue in the future is that in the past like things under like circumstances produced like results. We assume the future will be like the past because in the past the future has been like the past, but this really doesn't guarantee that the future we're considering now will in fact be like the past. Or, as we might put it today, there is no guarantee that the laws of nature will continue forever unchanged. The modern skeptic, having the benefit of enormous scientific advancement since Hume's time, shrugs. Today's skepticism must be seen within the context of that scientific development. In fact, the kind of skepticism advocated by the major groups calling themselves skeptical today is often referred to as scientific skepticism. Scientific skepticism isn't bothered by the epistemological problems Hume posed.
The term 'scientific skepticism' may have originated with Carl Sagan (1934-1996), an astronomer who spent many years promoting science and skepticism. Science since Hume's day has proven by its results that the assumptions it makes about the existence of an external world, the reality of causality, and the stability of natural laws are justified and are grounded in more than just our psychological disposition to believe them. The success of science, in part due to advancements in technology over the past century, has cast doubt on the validity of the pessimism of empiricists like John Locke, who despaired that we would ever know the intimate details about the constituents of the physical universe.
Scientific skepticism takes it for granted that the methods of science are the best methods for gaining knowledge and that skepticism is warranted when knowledge claims are made that reject the methods of science, contradict well-established scientific facts or principles, or go beyond the limits of science. Thus, scientific skepticism is particularly critical of paranormal and supernatural claims, and of what is often referred to as pseudoscience. By extension, scientific skepticism considers all extraordinary claims as dubious. Such claims are not to be dismissed as false, however. "Extraordinary claims require extraordinary proof" is a commonly expressed mantra among advocates of scientific skepticism.
As Michael Shermer points out, skepticism does not stop with doubting extraordinary or "weird" (as he calls them) claims. Scientific skepticism uses scientific methods to investigate such claims. Scientific knowledge regarding the nature of perception, memory, and human testimony, as well as the tools of critical thinking, are a part of the skeptical inquiry into dubious claims. Finally, scientific skepticism rejects the notion that empirical matters should be taken on faith; beliefs should be based on sufficient evidence, not intuition, authority, or tradition.
Because so many "extraordinary" or "weird" claims are based on a poor understanding of perception, memory, testimony, and science, it is only natural that scientific skepticism should also involve the promotion of science and critical thinking. The modern skeptical movement, therefore, is not in the business of debunking for debunking's sake or of denying for denying's sake. Scientific skepticism does lead to the debunking of many claims. Debunking, however, involves demonstrating where the claim goes wrong and thus provides the positive benefit of exemplifying critical thinking and scientific investigation in action. Denialism, on the other hand, is not a part of scientific skepticism. Among other things, denialism is a polemical tactic that uses uncertainty to cast doubt on consensus viewpoints by belittling the value of probabilism. As such, modern denialists are more akin to the ancient Sophists than to the ancient skeptics. Scientific skepticism, on the other hand, recognizes that even though some uncertainty may exist, the sum of the evidence may preponderantly support some claims. A reasonable person accepts what is most likely the case rather than demanding that we not give our assent to any proposition until one can say there is no doubt that this is the case. Critical thinkers recognize that the precautionary principle can be paralyzing. Science may not be immune to error and it may not provide us with infallible truth, but it is the best method we've discovered so far for getting at the most reasonable beliefs about the world we live in.
See also 10 Facts About Skepticism, Skeptical Investigations, empiricism, Belief Armor, Sharon Begley: Skeptics Think They're Intellectually Superior, Evaluating Evidence, 2010: A Time for Reflection, Clever Irrationality, Defending Falsehoods, Evaluating Personal Experience, Why Do People Believe in the Palpably Untrue?, Why I am not a real (true) skeptic, My Opinion on Opinions, Teaching Critical Thinking, The War on Science, and Using ghost stories to teach critical thinking.
Hume, David. An Inquiry Concerning Human Understanding, Section x "Of Miracles," (1748), Bobbs-Merrill, Library of Liberal Arts edition.
Hume, David. An Inquiry Concerning Human Understanding, Section xii "Of the Academical or Skeptical Philosophy," (1748), Bobbs-Merrill, Library of Liberal Arts edition.
Novella, Steven and David Bloomberg. (1999). Scientific Skepticism, CSICOP, and the Local Groups. Skeptical Inquirer. July/August. Scientific skepticism defines skepticism around the principles of scientific investigation. Specifically, scientific skepticism addresses testable claims; untestable claims are simply outside the realm of science.
Popkin, Richard H. "Skepticism" in The Encyclopedia of Philosophy, vol 7, pp. 449-461, ed. Paul Edwards (Macmillan, Inc., 1967).
Ridolfo, Heather. Amy Baxter, and Jeffrey W. Lucas. 2009. "Social Influences On Paranormal Belief: Popular Versus Scientific Support." Current Research in Social Psychology. "Contrary to predictions, participants [160 students at a large public university] appeared to react against the views of science when evaluating claims, particularly when they believed those claims were unpopular. This finding may reflect decreasing trust in the institution of science."
Timmer, John. 2010. "When science clashes with beliefs? Make science impotent." Ars Technica. "It's hardly a secret that large segments of the population choose not to accept scientific data because it conflicts with their predefined beliefs: economic, political, religious, or otherwise. But many studies have indicated that these same people aren't happy with viewing themselves as anti-science, which can create a state of cognitive dissonance."Ancient Greek Skepticism - Internet Encyclopedia of PhilosophyContemporary Skepticism - Internet Encyclopedia of Philosophy
A Skeptical Manifesto by Michael Shermer
Scientific Skepticism - Knowledge Rush
Last updated 14-Mar-2015