Full Idea
Kripke's proposal that referential expressions like indexicals, demonstratives, proper names and natural kind terms are de jure rigid designators created a puzzle - it entails 'modal illusions', truths that are in fact necessary appear to be contingent.
Clarification
'de jure' means 'by law' (rather than 'in practice')
Gist of Idea
Rigid designation creates a puzzle - why do some necessary truths appear to be contingent?
Source
report of Saul A. Kripke (Naming and Necessity lectures [1970], p.143-4) by Macià/Garcia-Carpentiro - Introduction to 'Two-Dimensional Semantics' 1
Book Reference
'Two-Dimensional Semantics', ed/tr. Garcia-Carpentero/Macia [OUP 2006], p.1
A Reaction
They are identifying this puzzle as the source of the need for two-dimensional semantics. Kripke notes that rigid designators may have their reference fixed by non-rigid descriptions.