Psychological state expressed by a verb that may take a subordinate clause beginning with that as its complement. Verbs such as believe, hope, fear, desire, intend, and know all express propositional attitudes. The linguistic contexts created by their use are typically referentially opaque (see intentionality) in the sense that the substitution of co-referential expressions within them may change the truth value (true or false) of the containing sentence. Thus, to use Bertrand Russell's example, though it is true that Peter believes that Walter Scott was a Scotsman, it may be false that he believes that the author of Waverley (who is Scott) was a Scotsman.
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