Full Idea
Definite descriptions used with referential intentions (usually in subject position) are normally rigid, ..but in predicate position they are normally not rigid, because there is no referential intention.
Gist of Idea
Definite descriptions are usually rigid in subject, but not in predicate, position
Source
Mark Sainsbury (The Essence of Reference [2006], 18.5)
Book Reference
'Oxford Handbook of Philosophy of Language', ed/tr. Lepore,E/Smith,B [OUP 2008], p.419
A Reaction
'The man in the blue suit is the President' seems to fit, but 'The President is the head of state' doesn't. Seems roughly right, but language is always too complex for philosophers.