Common sense, based on a strict construction of the term, consists of what people in common would agree on : that which they "sense" as their common natural understanding. Some people (such as the authors of Merriam-Webster Online) use the phrase to refer to beliefs or propositions that — in their opinion — most people would consider prudent and of sound judgment, without reliance on esoteric knowledge or study or research, but based upon what they see as knowledge held by people "in common". 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. Another meaning to the phrase is good sense and sound judgment in practical matters.

Whatever definition one uses, identifying particular items of knowledge as "common sense" becomes difficult. Philosophers may choose to avoid using the phrase when using precise language. But common sense remains a perennial topic in epistemology 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, and axioms.

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 [see Quantum mechanics], or of speeds approaching that of light [see Special relativity]. Often ideas that may be considered to be true by common sense are in fact false.

Noun

common sense (uncountable)

1. (obsolete) An internal sense, formerly believed to be the sense by which information from the other five senses is understood and interpreted.
   • 1621, Robert Burton, The Anatomy of Melancholy, Book I, New York 2001, p. 159:

     This common sense is the judge or moderator of the rest, by whom we discern all differences of objects; for by mine eye I do not know that I see, or by mine ear that I hear, but by my common sense [...].

2. Ordinary sensible understanding; one's basic intelligence which allows for plain understanding and without which good decisions or judgments cannot be made.
   • 1776, Horace Walpole, Letter to Sir Horace Mann:

     To act with common sense, according to the moment, is the best wisdom I know; and the best philosophy, to do one's duties, take the world as it comes, submit respectfully to one's lot, bless the goodness that has given us so much happiness with it, whatever it is, and despise affectation.

• (ordinary sensible understanding): good sense, mother wit, native wit

Common sense, roughly speaking, is what people in common would agree: that which they "sense" in common as their shared natural understanding. Some use the phrase to refer to beliefs or propositions that in their opinion they consider would in most people's experience be prudent and of sound judgment, without dependence upon esoteric knowledge or study or research, but based upon what is believed to be knowledge held by people "in common".