An opaque context is a linguistic context in which it is not possible to substitute co-referential terms while guaranteeing the preservation of truth values.

The term is used in philosophical theories of reference, and is to be contrasted with "transparent context". For example: The philosopher Socrates about to take poison hemlock as ordered by the court. ...

• Opacity: "Mary knows that Cicero is a great orator" is "referentially opaque"; although Cicero was also called Tully, we can't simply substitute "Tully" for "Cicero" in this context ("Mary knows that Tully is a great orator"), for Mary might not know that the names 'Tully' and 'Cicero' refer to one and the same thing. Of course, if Mary does know that Cicero is a great orator, then there is a sense in which Mary knows that Tully is a great orator, even if she does not know that 'Tully' and 'Cicero' corefer. It is the sense forced on us by direct theories of reference of proper names, i.e., those that maintain that the meaning of proper names just are their referents.

• Transparency: "Cicero was a Roman orator" is "referentially transparent"; there is no problem substituting "Tully" here: "Tully was a Roman orator". Well, this is an oversimplified account as it may not hold in all circumstances, e.g., when, even though 'Tully' and 'Cicero' are grammatically proper names, their meanings are in part dependent on something other than their referents. This occurs, e.g., when someone has two separate personae that are referred to separately by two distinct proper names, and that, although referring to the same person, do so in significantly different ways that their substitutions into what are usually taken to be transparent contexts results in sentences with different truth values. For example, 'Clark Kent wooed the ladies', or 'Clark Kent was a great hero' are generally taken to be false even though they are true when 'Superman' is substituted for 'Clark Kent' in each.

Cicero at about age 60, from an ancient marble bust Marcus Tullius Cicero (IPA:Classical Latin pronunciation: , usually pronounced in American English or in British English; January 3, 106 BC – December 7, 43 BC) was a Roman statesman, lawyer, political theorist, philosopher, widely considered one of Romes greatest orators...

See also

• Masked man fallacy
• Transitivity of identity