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Frege's Mistake

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":

Lo, Hesperus. Lo, Phosphorus.

Here is Ralph. Ralph is a competent speaker of English. In particular, Ralph understands the words 'Hesperus' and 'Phosphorus' and knows what they refer to. When you are walking on the moors with him and he says "That's Hesperus!' or "There is Phosphorus!', you can be sure it will be Venus he's pointing at. Nevertheless, Ralph denies:

(HP)    'Hesperus = Phosphorus'

Though, of course, he would never deny:

(HH)    'Hesperus = Hesperus'

If we take Ralph at his word in denying (HP) we must conclude that Ralph believes that Hesperus ≠ Phosphorus in other words, that he believes Venus ≠  Venus. And given his endorsement of (HH) we may conclude that he also believes that Venus = Venus.

Ralph might puzzle us if we thought that psychological explanation must make sense of people in terms of what propositions they believe. But, as we have already observed, this is a mistake.