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.
The propositional attitudes, distinguished by their propositional objects, are not psychologically natural kinds. The causes of what you do and say must supervene on what's between your ears but what propositions you believe depends in important part upon your environment -- upon what situation you find yourself in.
Thus Earth and Twin Earth and their respective inhabitants are atom-for-atom identical. Ralph and Twin Ralph are in the same psychological state. But it so happens that in the Twin Solar System there is no single counterpart to Venus. There are two planets with orbits so interlaced that no earth observer could tell the difference between the Earths' nighttime skies. Of course, the Twin-Earth natives call these planets 'Hesperus' and 'Phosphorus'. Twin-Ralph learned to call Yon-Hesperus , 'Hesperus' in the same way Ralph learned to call Venus 'Hesperus' and he identifies Yon-Phosphorus as the bearer of 'Phosphorus' in the same way Ralph identifies Venus under that name. Ralph and Twin-Ralph endorse the same astronomical sentences but when Twin- Ralph denies (HP), he is saying something true.
Twin-Earth is a situation of the sort Ralph thinks he is in. I remind you that I use these as technical terms. A situation is a place and time in a possible world. The situation you think you are in is not the situation you believe you are in. That is, it is not a situation at which your beliefs would be true. There is no such situation for Ralph. He believes necessary falsehoods. The situation you think you are in is one which, in your psychological state, you would believe nothing but the truth. Twin-Earth is a situation of the sort Ralph thinks he is in, even though, over Yon, he would not believe the same propositions he actually believes.
The Ralphs will have arrived at their beliefs by making inferences from the information given them by their senses. I do not use 'inference' as a technical term. I use it to mean what anyone means -- a change in belief. But I identify inferences in a non-standard way. I count inferences by counting psychological states, not the beliefs they happen to realize. So I would say that Ralph and Twin-Ralph make exactly the same inferences notwithstanding the differences in what they believe. And I measure the rationality of inferences differently than other philosophers.
For reasons we shall explore at another time, post-Fregean philosophers have regarded inference as a formal process. That is, they have supposed that the objects of belief have "forms" which mirror the logical forms of the sentences which express them. They have supposed that rational inferences are ones that conform to the mental correlates of deductive rules. This picture requires saying how the objects of belief can have logical forms (Fodor calls this "Aristotle's Condition") and this is officially a Deep Question for the theory of belief ("What is Narrow Content?"). Like most Deep philosophical Questions philosophers tend to give it wide berth.
I reject Aristotle's condition. I do not think it makes sense to attribute logical form to the objects of belief. I do not think that thinking is deduction. I think the objects of belief are just propositions which I understand to be sets of possible worlds. Set's of possible worlds do not have forms. It does make sense to think of the entailments of propositions, but there is no sense to be made of Ralph by expecting him to believe the entailments of his beliefs. Ralph believes a necessary falsehood; his beliefs entail everything.
To decide if someone is rational you should ignore what he believes and consider the situations he thinks is in.
Suppose Twin-Ralph knows to call a heavenly body 'Hesperus' when he observes it to be the last one seen in the morning sky and he calls Yon-Phosphorus 'Phosphorus' when he observers it as the first thing seen in the evening sky. He has no reason to think that these are sightings of the same object or that the words co-refer. The odds are against it. So Twin-Ralph correctly concludes that Yon-Hesperus ≠ Yon-Phosphorus. Ralph makes the same inference to the conclusion that Venus ≠ Venus but the inference is reasonable given the situation Ralph thinks he is in. If evolution had wired Ralph's brain to avoid believing this falsehood it would have prevented Twin-Ralph from believing the truth about his environment.
Now, imagine the process of discovery that might lead Ralph to change his mind about (HP). Walking on the moors one evening Ralph watches Venus make its appearance in the evening twilight. 'Lo, Phosphorus' he says. Ralph decides to stay out star gazing all night. The time of year is such that when Venus dips towards the horizon it does not disappear but rises again until it is the last thing to be seen in the morning sky. "Lo, Phosphorus', Ralph says, and realizes that, unless he has been the victim of some elaborate hoax or unlikely optical illusion, the things he calls 'Hesperus' and 'Phosphorus' are one and the same. Ralph concludes, reasonably enough, that (HP) is true. Over on Twin-Earth, Twin-Ralph has been watching the same show. The difference is that he has been the victim of a hoax or illusion. He too will conclude (HP) is true; he will be wrong, but his inference is as reasonable as Ralph’s. Now Earth is the sort of situation Twin-Ralph thinks he is in.
What has Ralph discovered? Well, after his discovery, Ralph will believe the proposition Hesperus = Phosphorus. But he already believed that; that is, he already believed that Venus = Venus. As the upshot of his discovery Ralph will have ceased to believe that Hesperus ≠ Phosphorus, that is, he stops believing Venus ≠ Venus. But that hardly seems to do justice to his discovery, since before he had ever seen Venus or heard any of its names he did not believe Venus ≠ Venus. Ralph has come to believe some propositions he didn't before*: the proposition that the sentence (HP) is true; the (Russellian) proposition that the unique thing seen in the morning sky is the unique thing seen in the evening sky; the proposition that 'Hesperus' and 'Phosphorus' co-refer. But it would be a mistake to try to cash out Ralph's change of mind as his coming to believe one of these propositions: to say that Ralph’s discovery was merely linguistic or merely Russellian. After his discovery Ralph believes Venus = Venus in a way that he didn't believe it before. If he didn’t believe it in the way that he does-- that is, in this new psychological state-- he would not believe that HP was true.
This is how I understand Ralph and I am prepared to argue that this how folks understand him too. I take myself to be revealing the tacit workings of Folk psychology. But this is not the way philosophers understand him. Philosophers since Frege have wanted to explain Ralph's change of mind as a matter of his acquiring the belief that Hesperus = Phosphorus and have insisted that there must therefore be a difference between this belief and the belief that Venus = Venus. I think this is a profound mistake and in a future post I'll explain what I think that mistake is.