Full Idea
Aquinas says if a 'de dicto' statement is true, the 'de re' version may be false. The opposite also applies: 'What I am thinking of [17] is essentially prime' is true, but 'The proposition "what I am thinking of is prime" is necessarily true' is false.
Gist of Idea
'De dicto' true and 'de re' false is possible, and so is 'de dicto' false and 'de re' true
Source
Alvin Plantinga (De Re and De Dicto [1969], p.27)
Book Reference
Plantinga,Alvin: 'Essays in the Metaphysics of Modality' [OUP 2003], p.27
A Reaction
In his examples the first is 'de re' (about the number), and the second is 'de dicto' (about that proposition).
Related Idea
Idea 14642 Expressing modality about a statement is 'de dicto'; expressing it of property-possession is 'de re' [Plantinga]