**Intension** refers to the logical or definitional conditions that specify the set of all *possible* things a word or phrase could describe, while **extension** refers to the set of all *actual* things the word or phrase describes.

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In linguistics, logic, philosophy, and other fields, an **intension** is any property or quality or state of affairs connoted by a word, phrase or other symbol. In case of a word, it is often implied by its definition. The term may also refer to the complete set of meanings or properties that are implied by a concept, although the term *comprehension* is technically more correct for this.

Intension is generally discussed with regard to extension (or *denotation*). For example, the intension of a car is the all-inclusive concept of a car, including, for example, mile-long cars made of chocolate that may not actually exist. But the extension of a car is all actual instances of cars (past, present, and future), which will amount to millions or billions of cars, but probably does not include any mile-long cars made of chocolate.

The meaning of a word can be thought of as the bond between *the idea or thing the word refers to* and *the word itself*. Swiss linguist Ferdinand de Saussure contrasts three concepts:

- the
*signified*—the concept or idea that a sign evokes. - the
*signifier*—the "sound image" or string of letters on a page that one recognizes as a sign. - the
*referent*—the actual thing or set of things a sign refers to.

Intension is analogous to the signified extension to the referent. The intension thus links the signifier to the sign's extension. Without intension of some sort, words can have no meaning.

*Intension* and *intensionality* (the state of having intension) should not be confused with *intention* and *intentionality*, which are pronounced the same and occasionally arise in the same philosophical context. Where this happens, the letter 's' or 't' is sometimes italicized to emphasize the distinction.

In any of several studies that treat the use of *signs*, for example, linguistics, logic, mathematics, semantics, and semiotics, the **extension** of a concept, idea, or sign consists of the things to which it applies, in contrast with its *comprehension* or *intension*, which consists very roughly of the ideas, properties, or corresponding signs that are implied or suggested by the concept in question.

In philosophical semantics or the philosophy of language, the *extension* of a concept or expression is the set of things it extends to, or applies to, if it is the sort of concept or expression that a single object by itself can satisfy. (Concepts and expressions of this sort are *monadic* or "one-place" concepts and expressions.)

So the extension of the word "dog" is the set of all (past, present and future) dogs in the world: the set includes Fido, Rover, Lassie, Rex, and so on. The extension of the phrase "Wikipedia reader" includes each person who has ever read Wikipedia, including you.

In the context of formal logic, the extension of a whole *statement*, as opposed to a word or phrase, is sometimes defined (arguably by convention) as its logical value. So, in that view, the extension of "Lassie is famous" is the logical value *true*, since Lassie *is* famous.

Some concepts and expressions are such that they do not apply to objects individually, but rather serve to relate objects to objects. For example, the words "before" and "after" do not apply to objects individually—it makes no sense to say "Jim is before" or "Jim is after"—but to one thing in relation to another, as in "The wedding is before the reception" and "The reception is after the wedding." Such "relational" or "polyadic" ("many-place") concepts and expressions have, for their extension, the set of all sequences of objects that satisfy the concept or expression in question. So the extension of "before" is the set of all (ordered) pairs of objects such that the first one is before (precedes) the second one.

In mathematics, the *extension* of a mathematical concept is the set that is specified by that concept.

For example, the extension of a function is a set of ordered pairs that pair up the arguments and values of the function; in other words, the function's graph. The extension of an object in abstract algebra, such as a group, is the underlying set of the object. The extension of a set is the set itself. That a set can capture the notion of the extension of anything is the idea behind the axiom of extensionality in axiomatic set theory.

This kind of extension is used so constantly in contemporary mathematics based on set theory that it can be called an implicit assumption. It can mean different things in different cases, and there is no universal definition of the term "extension."

Russell's paradox, discovered by Bertrand Russell, specified as "the set of all sets that are not members of themselves," was an interesting case of a specification of a set (a supposed intension) that could not be satisfied—it could have no extension—because the intension specified in the definition of that set led to a contradiction. The result of the discovery of Russell's paradox was to show that the so-called naive set theory of Gottlob Frege required revision because Frege had thought that any specifiable condition (intension) should be able to define a set (extension), but this assumption was shown by Russell to be false. This required a revision of the axioms of set theory so that they would not permit such contradictory membership conditions (such contradictory intensions) to be specified within the system. Russell and Whitehead's solution (in their work *Principia Mathematica*) was to set up a theory of types, in which membership was restricted to a given type, and there were different levels (or types) of membership. Other set theories have coped with the problem in different ways.

In computer science, some database textbooks use the term *intension* to refer to the schema of a database, and *extension* to refer to particular instances of a database. The distinction, however, is the same: intension is the logical specification of something, whereas extension is the set of objects or other things that satisfy the conditions of the logical specification given in the intension.

There is an ongoing controversy in metaphysics about whether or not there are, in addition to actual, existing things, non-actual or nonexistent things. If there are—if, for instance, there are possible but non-actual dogs (dogs of some non-actual but possible species, perhaps) or nonexistent beings (like Sherlock Holmes, perhaps), then these things might also figure in the extensions of various concepts and expressions. If not, only existing, actual things can be in the extension of a concept or expression. Note that "actual" may not mean the same as "existing." Perhaps there exist things that are merely possible, but not actual. (Maybe they exist in other universes, and these universes are other "possible worlds"—possible alternatives to the actual world.) Perhaps some actual things are nonexistent. (Sherlock Holmes seems to be an *actual* example of a fictional character; one might think there are many other characters Arthur Conan Doyle *might* have invented, though he *actually* invented Holmes.)

A similar problem arises for objects that no longer exist. The extension of the term "Socrates," for example, seems to be a (currently) non-existent object. Bertrand Russell dealt with this problem by means of his theory of definite descriptions. He used as an example the statement, "The present king of France is bald." But there is no present king of France. How then shall we deal with such statements, that purport to be about someone or something, but that purported someone or something does not, in fact, exist? Are such statements true? False? Meaningless?

Russell, proposed that when we say "the present King of France is bald," we are making three separate assertions:

- there is an x such that x is the King of France
- there is no y, y not equal x, such that y is the King of France (i.e., x is the only King of France)
- x is bald.

Since assertion 1 is plainly false, and our statement is the conjunction of all three assertions, our statement is false.

Free logic is another attempt to avoid some of these problems.

Some fundamental formulations in the field of general semantics rely heavily on a valuation of extension over intension.^{[1]}

- ↑ Alfred Korzybski Theoretical and Practical Implications, European Society for General Semantics, 2001, Retrieved June 22, 2007.

- Cochrane, Roberta, Leo Mark.
*Automating relational database support for objects defined by context-free grammars : the intension-extension framework*. College Park, Md.: University of Maryland, 1989. - Fox, Chris, and Shalom Lappin.
*Foundations of Intensional Semantics*. Blackwell Publishing Limited, 2005. ISBN 0631233768 ISBN 9780631233763 - Munitz, Milton Karl.
*Logic and ontology*. New York: New York University Press, 1973. ISBN 0814753639 ISBN 9780814753637 - Manzano, Maria.
*Extensions of First-Order Logic*. Cambridge University Press, 1996. ISBN 0521354358 ISBN 9780521354356 - Saussure, Ferdinand De, Charles Bally, Albert Sechehaye, and Albert Reidæinger.
*Course in General Linguistics*. London: Owen, 1964, cop. 1960. ISBN 0070165246 ISBN 9780070165243 - Whitehead, Alfred North, and Bertrand Russell.
*Principia Mathematica*. Cambridge: Cambridge University Press, 1925-27.

All links retrieved March 4, 2018.

- Stanford Encyclopedia of Philosophy
- The Internet Encyclopedia of Philosophy
- Paideia Project Online
- Project Gutenberg

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