In a recent post I told the story of Ralph, 'Hesperus' and 'Phosphorus'. We saw there why Ralph might assert
('h≠p') 'Hesperus ≠ Phosphorus'
and how astronomical observation might lead him to change his mind. Remarkably, at no point in telling that story did we find any reason to deny that
(Bh=p) Ralph believes Hesperus = Phosphorus.
It was not that we refused to take Ralph at his word. Ex hypothesi, Ralph is a competent speaker of English and we took him to be sincere when he asserted ('h≠p'). We had to conclude that:
(Bh≠p) Ralph believes Hesperus ≠ Phosphorus.
The principle at work here is an important one. I call it (following Kripke) "The Disquotation Principle":