Extensional context
In any of several fields of study that treat the use of signs — for example, in
Take the case of
fell out of a window and Superman caught her. Thus the sentence "Superman caught Lois Lane" is true. Because this sentence is an extensional context, the sentence "Clark Kent caught Lois Lane" is also true. Anybody that Superman caught, Clark Kent caught.In opposition to extensional contexts are intensional contexts (which can involve modal operators and modal logic), where terms cannot be substituted without potentially compromising the truth-value. Suppose that Lois Lane believes that Clark Kent will investigate a news story with her. Thus, the sentence "Lois Lane believes that Clark Kent will investigate a news story with her" is true. However, the statement, "Lois Lane believes that Superman will investigate a news story with her," is false. This is because 'believes' typically induces an intensional context. Lois Lane doesn't believe that Superman is Clark Kent and the propositional attitude "believe" induces an intensional context, so the substitution alters the meaning of the original sentence.
See also
- De dicto and de re
- Extension (semantics)
- Extensional definition
- Extensionalism
- Intensional logic
- Opaque context
- Propositional attitude
- W.V. Quine
Further reading
- Francis Watanabe Dauer, Critical Thinking: An Introduction to Reasoning, Oxford University Press, 1989, p. 392.