ABSTRACT
Referential opacity, the phenomenon that substitutivity of identity and other logical principles relating to singular terms break down in certain contexts, might seem to be a rather incidental feature of our language. Apparently, little attention was paid to it by ancient and medieval logicians and philosophers. The Megaric logician Eubulides of Miletus (fourth century B.C.), who is best know for inventing the paradox of the Liar, is reported to have been concerned about Electra, who knows and does not know her veiled brother. 1 And in the fourteenth century a group of Oxford logicians, notable William Heytesbury and his pupil Billingham, were concerned with what we would now call the difference between the notional and relational sense of certain verbs which relate to mental activity, like ‘knows that’ and ‘promises that’. 2
