In

Intensional Logic (Stanford Encyclopedia of Philosophy)

equality

in nLab {{Mathematical logic Set theory Concepts in logic Equivalence (mathematics)

logic
Logic is the study of correct reasoning. It includes both Mathematical logic, formal and informal logic. Formal logic is the science of Validity (logic), deductively valid inferences or of logical truths. It is a formal science investigating h ...

, extensionality, or extensional equality, refers to principles that judge objects to be equal if they have the same external properties. It stands in contrast to the concept of intensionality, which is concerned with whether the internal definitions of objects are the same.
Example

Consider the two functions ''f'' and ''g'' mapping from and tonatural number
In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country").
Numbers used for counting are called ''Cardinal n ...

s, defined as follows:
* To find ''f''(''n''), first add 5 to ''n'', then multiply by 2.
* To find ''g''(''n''), first multiply ''n'' by 2, then add 10.
These functions are extensionally equal; given the same input, both functions always produce the same value. But the definitions of the functions are not equal, and in that intensional sense the functions are not the same.
Similarly, in natural language there are many predicates (relations) that are intensionally different but are extensionally identical. For example, suppose that a town has one person named Joe, who is also the oldest person in the town. Then, the two predicates "being called Joe", and "being the oldest person in this town" are intensionally distinct, but extensionally equal for the (current) population of this town.
In mathematics

The extensional definition of function equality, discussed above, is commonly used in mathematics. Sometimes additional information is attached to a function, such as an explicit codomain, in which case two functions must not only agree on all values, but must also have the same codomain, in order to be equal (in contrast, the usual definition of a function in mathematics means that equal functions must have the same domain). A similar extensional definition is usually employed for relations: two relations are said to be equal if they have the same extensions. Inset theory
Set theory is the branch of mathematical logic that studies Set (mathematics), sets, which can be informally described as collections of objects. Although objects of any kind can be collected into a set, set theory, as a branch of mathematics, ...

, the axiom of extensionality states that two sets are equal if and only if they contain the same elements. In mathematics formalized in set theory, it is common to identify relations—and, most importantly, functions—with their extension as stated above, so that it is impossible for two relations or functions with the same extension to be distinguished.
Other mathematical objects are also constructed in such a way that the intuitive notion of "equality" agrees with set-level extensional equality; thus, equal ordered pairs have equal elements, and elements of a set which are related by an equivalence relation belong to the same equivalence class.
Type-theoretical foundations of mathematics are generally ''not'' extensional in this sense, and setoids are commonly used to maintain a difference between intensional equality and a more general equivalence relation (which generally has poor constructibility or decidability properties).
See also

* Duck typing * Identity of indiscernibles * Structural typing * Univalence axiomReferences

Intensional Logic (Stanford Encyclopedia of Philosophy)

equality

in nLab {{Mathematical logic Set theory Concepts in logic Equivalence (mathematics)