It was noted that for rigid terms the de re/de dicto distinction collapses. Indeed, if \(f\) and \(g\) are rigid, \([\lambda xy\ x=y](f, g)\), \({\square}[\lambda xy\ x=y](f, g)\) and \([\lambda xy\ {\square}x=y](f, g)\) are all equivalent. This is a problem that sets a limit on what can be handled by the Carnap-style logic as presented so far. Two well-known areas of difficulty are mathematics and proper names, especially in an epistemic setting.