 The standard test for distinguishing de re from de dicto attributions is referential transparency or opacity. A sentence, or more accurately a position in a sentence, is held to be referentially transparent if terms or phrases in that position that refer to the same object can be freely substituted without altering the truth of the sentence. The (non-belief attributing) sentence “Jill kicked X” is naturally read as referentially transparent in this sense. If “Jill kicked the ball” is true, then so also is any sentence in which “the ball” is replaced by a term or phrase that refers to that same ball, e.g., “Jill kicked Davy’s favorite birthday present”, “Jill kicked the thing we bought at Toys ‘R’ Us on August 26”. Sentences, or positions, are referentially opaque just in case they are not transparent, that is, if the substitution of co-referring terms or phrases could potentially alter their truth value. De dicto belief attribution is held to be referentially opaque in this sense. On the de dicto reading of belief, “Ralph believes that the man in the trenchcoat is a spy” may be true while “Ralph believes that the mayor is a spy” is false. Likewise, “Lois Lane believes that Superman is strong” may be true while “Lois believes that Clark Kent is strong” is false, even if Superman and Clark Kent are, unbeknownst to Lois, one and the same person. (Regarding the Lois example, however, see also §3.4, on Frege’s Puzzle, below.)