The term common sense (or, when used attributively as an adjective, commonsense, common-sense or commonsensical), based on a strict deconstruction of the term, refers to what people in common would agree on: that which they intuit ("sense") as their common natural understanding. The term is also used to refer to beliefs or propositions that, in the user’s opinion, would in most people's experience be prudent and of sound judgment, without dependence upon esoteric knowledge, study, or research, but based upon knowledge believed, by the person using the term, to be held by people "in common." It is, however, difficult to come up with an exact definition of common sense, and to identifying particular items of knowledge that are "common sense."
Thomas Reid (1710-1796) developed a philosophical perspective which took common sense as the source and ground of justification for philosophical knowledge. Reid tried to develop a position that could overcome Hume’s skepticism and Berkeley’s solipsism. Reid, Dugald Stewart, and other thinkers formed the Scottish School of Common Sense; the Common Sense school became popular in England, France, and America during the early nineteenth century, but lost popularity in the late nineteenth century. The school did not become popular in Germany due to Kant’s criticism of it. In the early twentieth century, a British philosopher, G. E. Moore developed treatises to defend common sense.
“Common sense” is a perennial topic in epistemology and the term is widely used or referred to by many philosophers. However, it is difficult to come up with an exact definition of common sense, and to identifying particular items of knowledge that are "common sense" — philosophers often avoid using the phrase where precise language is required. Related concepts include intuitions, pre-theoretic belief, ordinary language, the frame problem, foundational beliefs, doxa, and axioms.
Common sense is of interest as a test of the acceptability of metaphysical propositions, which cannot be justified by consistency alone. It also places certain restraints on the creativity of the intellect which logic by itself cannot accomplish. Most philosophers seek a truth which can be applied universally and which can be of value to ordinary human lives. If philosophical principles can not be at least partially validated by common sense, they will be rejected by most people as meaningless. No matter how creatively a philosopher thinks, he is still part of a community and his thought still draws from some of the basic truths accepted by that community.
Common sense ideas tend to relate to events within human experience, and thus commensurate with human scale and observable with ordinary human faculties. Thus, there is no commonsense intuition of, for example, the behavior of the universe at the subatomic level or at speeds approaching that of light.
There are two general meanings to the term "common sense" in philosophy. One is a sense that is common to the others, and the other meaning is a sense of things that is common to humanity. The first meaning, a “sense that is common to the others” was proposed by John Locke in An Essay Concerning Human Understanding. The input from each of the senses must be integrated into a single impression. This is the “common” sense, the sense that unites disparate impressions under a single concept or experience. It is therefore allied with "fancy," and opposed to "judgment," or the capacity to divide like things into separates. Each of the empiricist philosophers examined the problem of the unification of sense data according to an individual’s own manner, giving various names to the operation. However, all believed that there is a sense in the human understanding that sees commonality and does the combining—this is "common sense." It is evident from his writings that Locke regarded the realm of “common sense” as an absence of extremes; he abhorred authoritarianism but did not advocate anarchy, and he was religious without being fanatical.
Two philosophers, Thomas Reid and G. E. Moore, are most famous for advocating the other meaning of "common sense," the view (stated imprecisely) that common sense beliefs are true and form a foundation for philosophical inquiry. Both appealed to common sense to refute skepticism.
The Scottish School of Common Sense, which flourished in Scotland in the late eighteenth and early nineteenth centuries, emerged as a response to the ideas of philosophers such as John Locke, George Berkeley, and David Hume in England and Immanuel Kant in Germany. Berkeley had taken the doctrine of ideas, which Locke had adopted from Descartes, as the foundation of his theory of knowledge, which resolved the external world into ideas, without external reality, directly impressed on the mind by Divine power. Hume contended that all that we know of mind is a succession of states produced by experience, and that there was no ground for assuming that any mental substance existed as a subjective recipient of impressions and ideas. Thus, Berkeley disposed of objectivity and Hume of subjectivity, with the result that philosophy became mere skepticism.
Thomas Reid (1710-1796) defended the common sense, or natural judgment, of human beings, by which the real existence of both subject and object is directly known (natural realism). He argued that if there is no logical or scientific proof of a real external world or continuously existing mind, it is not because they do not exist or cannot be known, but because human consciousness of them is an ultimate fact, which does not require proof but is itself the ground of all proof. Common-sense beliefs automatically govern human lives and thought. Thomas Reid did not give a definition of common sense per se, but offered several "principles of common sense:"
"All knowledge and all science must be built upon principles that are self-evident; and of such principles every man who has common sense is a competent judge" (Thomas Reid, Works, ed. 1863, p. 422).
Dugald Stewart (1753-1828), Dr. Thomas Brook (1778-1820), and Sir James Mackintosh (1765-1832) further developed the principles of common sense. Sir William Hamilton (1788-1856), who was influenced by Kant, introduced distinctions which the Common Sense School had not recognized. James Oswald (l727-1793) used Reid's principles to support religious belief, and James Beattie (1735-1803) to support the existence of a moral faculty in man. The Scottish School of Common Sense influenced philosophers, including the American pragmatist C. S. Peirce, in Europe and in the United States.
The British philosopher G. E. Moore, who did important work in epistemology, ethics, and other fields near the beginning of the twentieth century, is known for a programmatic essay, "A Defense of Common Sense," (1925) which had a profound effect on the methodology of much twentieth century Anglo-American philosophy. In this essay, Moore listed several seemingly very obvious truths, such as "There exists at this time a living human body which is my body"; "My body has existed continuously on or near the earth, at various distances from or in contact with other existing things, including other living human beings"; and other such platitudes. He argued that these propositions are much more obviously true than the premises of many philosophical claims which entail their falsehood, such as the claim of J. M. E. McTaggart) that time does not exist.
In another essay, “Proof of an External World” (1939), Moore used the fact that he knew he had two hands as proof that an external world exists. He gave three requirements for a successful proof: the premises must be different from the conclusion; the premises must be demonstrated; and the conclusion must follow from the premises. This idea strongly influenced Wittgenstein, who spent his final weeks working out a new approach to it, published posthumously in “On Certainty.”
Appeal to common sense is characteristic of a general epistemological orientation called "epistemological particularism" (a term coined by Roderick Chisholm). The particularist gathers a list of propositions that seem obvious and unassailable and then requires consistency with this set of propositions as a condition of adequacy for any abstract philosophical theory. Any entry on the list, however, may be eventually rejected for inconsistency with other, seemingly more secure, entries. Epistemological methodism, on the other hand, begins with a theory of cognition or justification and then applies it to see which of our pre-theoretical beliefs survive. Reid and Moore were paradigmatic particularists, while Descartes and Hume were paradigmatic methodists. Methodism tended toward skepticism because the rules for acceptable or rational belief were so restrictive (being incapable of doubt for Descartes, or being constructible entirely from impressions and ideas for Hume). Particularist methodology, on the other hand, tended toward a kind of conservatism, granting what was perhaps undeserved authority to particular beliefs. Particularism applied to ethics and politics carries the risk of entrenching prejudice and socially inculcated stereotypes. However, logic, metaphysics, and epistemology may require some assumptions of common sense as a starting point.
The topic of common sense raises interesting and important questions in "meta-philosophy," a field closely related to epistemology and philosophy of language which examines, the rules, contexts, and purposes of philosophy. What is common sense? If a precise characterization of it cannot be given, does that mean a philosophical argument cannot appeal to common sense? Why should one care whether a belief is a matter of common sense or not? When and how can common sense change? Under what circumstances, if any, is it permissible to advocate a view that seems to run contrary to common sense? Should considerations of common sense play any decisive role in philosophy? If not common sense, then should any other similar concept such as "intuition" play such a role? In general, are there "philosophical starting points," and if so, how might one characterize them? Supposing that there are no beliefs we are willing to hold on to under any circumstances, are there some we ought to hold on to more strongly than others?
Common sense is sometimes regarded as an impediment to abstract and even logical thinking, especially in mathematics and physics, where human intuition often conflicts with provably correct or experimentally verified results. A definition attributed to Albert Einstein states: Common sense is the collection of prejudices acquired by age eighteen."
Common sense is sometimes appealed to in political debates, particularly when other arguments have been exhausted. Common sense, in this sense, simply means a popular belief, which requires further reflection and examination.
The Cyc project is an attempt to provide a basis of commonsense knowledge for artificial intelligence systems. The Open Mind Common Sense project is similar except that it, like other on-line collaborative projects like Wikipedia, was built from the contributions of thousands of individuals across the internet.
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