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An opaque context or referentially opaque context is a linguistic context in which it is not always possible to substitute "co-referential" expressions (expressions referring to the same object) without altering the truth of sentences.[1] The expressions involved are usually grammatically singular terms. So, substitution of co-referential expressions into an opaque context does not always preserve truth. For example, "Lois believes x is a hero" is an opaque context because "Lois believes Superman is a hero" is true while "Lois believes Clark Kent is a hero" is false, even though 'Superman' and 'Clark Kent' are co-referential expressions.


The term is used in philosophical theories of reference, and is to be contrasted with referentially transparent context. In rough outline:

Similar usage of the term applies for artificial languages such as programming languages and logics. The Cicero–Tully example above can be easily adapted. Use the notation as a quotation that mentions a term . Define a predicate which is true for terms six letters long. Then induces an opaque context, or is referentially opaque, because is true while is false. Programming languages often have richer semantics than logics' semantics of truth and falsity, and so an operator such as may fail to be referentially transparent for other reasons as well.

See also


  1. ^ "Definition of "opaque context" | Collins English Dictionary". Retrieved 2015-09-29.
  2. ^ Tully is anglicised historic name from Tullius, source Kate Kearns: Semantics, ISBN 978-0-230-23229-7, p. 145