In Philosophy, 'common sense' (or 'commonsense') describes beliefs or propositions that seem, to most people, to be as obviously true as any beliefs can be. This, however, cannot be considered a definition of 'common sense'; the issue of what common sense is is vexed, and its vexedness is one reason why many philosophers eschew the word altogether. Whatever it does mean, it does not mean homespun (and sometimes dubious) truths such as "Chicken soup is good for colds." Nonetheless, common sense is a perennial topic in epistemology and widely used or referred to, in one form or another, by very many philosophers. Some related concepts include intuitions, pre-theoretic belief, ordinary language, foundational beliefs, endoxa, and axioms.
The great Scottish philosopher Thomas Reid, a contemporary of Hume and the founder of the so-called Scottish School of Common Sense, devotes considerable space in his Inquiry and the Intellectual Powers developing a theory of common sense. While he never gives a definition, per se, he does offer a number of so-called "earmarks" of common sense (which he sometimes calls "principles of common sense"), such as:
- Principles of common sense are believed universally (with the apparent exceptions of some philosophers and the insane).
- It is appropriate to ridicule the denial of common sense.
- The denial of principles of common sense leads to contradictions.
Of course, each of these is stated and explained by Reid much more carefully that is done here.
The British philosopher G. E. Moore, who did important work in epistemology, ethics, and other fields near the beginning of the twentieth century, is famous for a programmatic essay, "A Defence of Common Sense." This essay had a profound effect on the methodology of much twentieth-century Anglo-American philosophy. In this essay, Moore lists 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 many other such platitudes. He argues (as Reid did before him) that these propositions are much more obviously true than the premises of many philosophical claims which entail their falsehood (such as the claim that time does not exist, a claim of A. N. Whitehead's).
Both Reid and Moore, individually, are famous for appealing to common sense to refute skepticism.
Appeal to common sense is characteristic of a general epistemological orientation called epistemological particularism (the appelation comes from Roderick Chisholm), which orientation is contrasted with epistemological methodism. 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. (An entry on the list, however, may be eventually rejected for inconsistency with other, seemingly more secure, entries.) Methodists, on the other hand, begin with a theory of cognition or justification and then apply it to see which of our pre-theoretical beliefs survive. Reid and Moore are paradigmatic particularists, while Descartes and Hume are paradigmatic mehodists. Methodist methodology tends to toward skepticism, as the rules for acceptable or rational belief tend to be very restrictive (for instance, being incapable of doubt for Descartes, or being constructible entirely from impressions and ideas for Hume). Particularist methodology, on the other hand, tends toward a kind of conservatism, granting perhaps an undue privilege to beliefs we happen to be confident about.
An interesting question is whether the methodologies can be mixed. For instance, it seems impossible to do logic, metaphysics and epistemology without beginning with some assumptions of common sense. However, particularism applied to ethics and politics often seems simply to entrench prejudice and other contingent products of social inculcation. Is there a way to provide a principled distinction between areas of inquiry where reliance on the dictates of common sense is legitimate (because necessary) and areas where it is illegitimate because it is an obstruction to intellectual and practical progress?
The topic of common sense raises interesting and important questions in a field closely related to epistemology and philosophy of language called "meta-philosophy." Various questions might be raised in a meta-philosophical discussion of common sense: what is common sense? Supposing that a precise characterization of it cannot be given, does that mean appeal to common sense is off-limits in philosophy? Why should we care whether a belief is a matter of common sense or not? 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 we characterize them? Supposing that there are no beliefs we are willing to hold come what may, are there some we ought to hold more stubbornly at least?