Common sense is ‘sound and prudent judgment based on a simple perception of the situation or facts.’ Thus, ‘common sense’ (in this view) equates to the knowledge and experience which most people already have, or which the person using the term believes that they do or should have. It is ‘the basic level of practical knowledge and judgment that we all need to help us live in a reasonable and safe way.’
However, identifying particular items of knowledge as ‘common sense’ is difficult. Philosophers may choose to avoid using the phrase when using precise language. Common sense remains a perennial topic in epistemology (the study of knowledge) and many philosophers make wide use of the concept or at least refer to it. Some related concepts include intuitions, pre-theoretic belief, ordinary language, the frame problem, foundational beliefs, good sense, endoxa, axioms, wisdom, folk wisdom, folklore, and public opinion.
Common-sense ideas tend to relate to events within human experience (such as good will), and thus appear commensurate with human scale. Humans lack any common-sense intuition of, for example, the behavior of the universe at subatomic distances (quantum mechanics), or of speeds approaching that of light (special relativity). Often ideas that may be considered to be true by common sense are in fact false. Conversely, certain ideas that are subject to elaborate academic analysis oftentimes yield superior outcomes via the application of common sense.
According to Aristotle, the common sense is an actual power of inner sensation (as opposed to the external five senses) whereby the various objects of the external senses (color for sight, sound for hearing, etc.) are united and judged, such that what one senses by ‘common sense’ is the substance (or existing thing) in which the various attributes inhere (so, for example, a sheep is able to sense a wolf, not just the color of its fur, the sound of its howl, its odor, and other sensible attributes). It was not, unlike later developments, considered to be on the level of rationality, which properly did not exist in the lower animals, but only in man; this irrational character was because animals not possessing rationality nevertheless required the use of the common sense in order to sense, for example, the difference between this or that thing, and not merely the pleasure and pain of various disparate sensations. This also contributes to the understanding held by the Scholastics that when one senses, one senses something, and not just a diversity of sensible phenomena.
Common sense, in this view, differs from later views in that it is concerned with the way one receives sensation, and not with belief, or wisdom held by many; accordingly, it is ‘common,’ not in the sense of being shared among individuals, or being a genus of the different external senses, but inasmuch as it is a principle which governs the activity of the external senses.
John Locke proposed one meaning of ‘common sense’ in his ‘An Essay Concerning Human Understanding.’ This interpretation builds on phenomenological experience. Each of the senses gives input, and then something integrates the sense-data into a single impression. This something Locke sees as the common sense — the sense of things in common between disparate impressions. It therefore allies with ‘fancy,’ and opposes ‘judgement,’ or the capacity to divide like things into separates. The French theologian Jacques-Bénigne Bossuet arguably developed this theory a decade before Locke. Each of the empiricist philosophers approaches the problem of the unification of sense-data in their own way, giving various names to the operation. However, the approaches agree that a sense in the human understanding exists that sees commonality and does the combining: ‘common sense’ has the same meaning.
Appeal to common sense characterizes a general epistemological orientation called epistemological particularism (the belief that one can know something without knowing how one knows that thing). This orientation contrasts with epistemological Methodism (which ask ‘How do we know?’ before ‘What do we know?’). 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. (Particularism allows, however, rejection of an entry on the list for inconsistency with other, seemingly more secure, entries.) Epistemological Methodists, on the other hand, begin with a theory of cognition or justification and then apply it to see which pre-theoretical beliefs survive. Reid and Moore represent paradigmatic particularists, while Descartes and Hume stand as paradigmatic Methodists. Methodist methodology tends toward skepticism, as the rules for acceptable or rational belief tend to be very restrictive (for instance, Descartes demanded the elimination of doubt; and Hume required the construction of acceptable belief entirely from impressions and ideas).
Particularist methodology, on the other hand, tends toward a kind of conservatism, granting perhaps an undue privilege to beliefs in which we happen to have confidence. One interesting question asks whether epistemological thought can mix the methodologies. In such a case, does it not become problematical to attempt logic, metaphysics and epistemology with the absence of original assumptions stemming from common sense? Particularism, applied to ethics and politics, may seem to simply entrench prejudice and other contingent products of social inculcation (compare cultural determinism).