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In philosophy of language, a context in which a sub-sentential expression e appears is called extensional if and only if e can be replaced by an expression with the same extension and necessarily preserve truth-value. The extension of a term is the set of objects that that term denotes.
Take the case of Clark Kent, who is secretly Superman. Suppose that Lois Lane fell out of a window and Superman caught her. Thus the statement, "Clark Kent caught Lois Lane," is true because it has an extensional context. The names "Superman" and "Clark Kent" have the same extension, which is to say that they both refer to the same person, i.e., that superhero who is vulnerable to kryptonite. Anybody that Superman caught, Clark Kent caught.
In opposition to extensional contexts are intensional contexts, where synonymous terms cannot be substituted in without potentially compromising the truth-value. Suppose that Lois Lane believes that Clark Kent will investigate a news story with her. The statement, "Lois Lane believes that Superman will investigate a news story with her," is false, even though Superman is Clark Kent. This is because 'believes' is typically an intensional context.
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