the possibility of replacing an expression alpha by an expression beta with the same reference in such a way that the resulting sentence has the same truth-value. EXAMPLE: given that Beatrix is the eldest daughter of Juliana sentence (i)a is equivalent to (i)b in which the eldest daughter of Juliana has been substituted for Beatrix. Hence the substitution is salva veritate.
(i) a Beatrix lives in The Hague
b The eldest daughter of Juliana lives in The Hague
Substitution salva veritate is not possible in opaque contexts.
(1) Reagan believed that Beatrix lived in The Hague.
(2) Reagan believed that the eldest daughter of Juliana lived in The Hague.
On the usual reading (de dicto) (1) and (2) can be true or false independently of each other; however there is the possibility of a so-called de re reading in which they are still truth-functionally equivalent, if we focus on the referential content of beliefs rather than on what the believer would say.