It has been clear since the discovery of Twin Earth that there must be two different determinants of the propositional attitudes. Our Twin Earth doppelgangers are atom for atom identical with us. Any psychological theory— any theory about how sensory inputs get translated into behavioral upshots— true of us must be true of them. And yet, because of our different environments, we and our twins have different propositional attitudes. Apparently, these differences in what we think and think about, must be irrelevant to psychology. How can we reconcile this with our folk psychological conviction that people do what they do because of what they think?
Suppose that we one day have to hand The One True Psychological Theory of Human Beings. Like any theory it will have a proprietary vocabulary– a way of carving up the world into psychological natural kinds– and a finite set of laws. While we do not know what those laws are, we can be sure they will entail any number of nomolological regularities of the form:
(L) ("x)( Ψ1x at t1 É Ψ2x at t2)
where the ‘Ψ’s name individuals’ total psychological states; everything that can be said about them in the terms of the Theory.
The lesson of Twin Earth is that individuals might share the same total state but believe different things. What propositional attitudes one has depends upon extra-psychological facts about the world in which one realizes that state and about one’s location in time and space in that world. I’m going to call these locations in possible worlds situations (with applogies to Perry and Barwise).
I write:
in σ in Ψ: x believes p
to say that, in a certain situation σ, someone in state Ψ would believe p (among, presumably, many other things). In Putnam’s story the situations and states are such that:
In σe in Ψ: x believes rain is H2O.
In σt in Ψ: x believes rain is XYZ.
It might be tempting to gloss all this by treating psychological states as functions from situations onto attitudes, but this would be a mistake. Functions are identical if they yield the same values for arguments but psychological states can be different in psychologically potent respects even though the differences don't show up as differences in what subjects think or think about. (see. "Now,Me")
If we were perfect epistemic engines, our psychological states would be realized by true beliefs in every situation in which we found ourselves. Alas we know that isn’t so and the situation dependence of the attitudes reveals another source of error. A psychological state realized by true beliefs in one situation might be realized by false beliefs in another.
Here is Oedipus insisting, “My Mom is not my wife”. The proposition he believes is false; indeed, necessarily false by some ways of reckoning. But Oedipus is not irrational or unfathomable, only badly situated. Surely things could be arranged, behind the scenes, over on Twin Earth, so that Twin Oedipus is married to a woman who is not his mother, without it making any difference to what is in Twin Oedipus’s head.
Twin Earth, so constructed, is what Dan Dennett, would call one of Oedipus’s “notional worlds”. That is, worlds or , better, (since Twin Earth is in our possible world) , situations in which Oedipus’s psychological state would be realized by true beliefs.
Dennett noticed that, for the purposes of explaining behavior, what matters is not what propositions subjects actually believe, but what they would believe in their notional situations. This makes sense from an evolutionary point of view. Presumably, true belief has survival value. But evolution can operate only on what is inside our heads and what we believe and whether it is 
true is largely determined by what’s outside. However clever we may be, we may still, like Oedipus, end up badly situated. On the other hand, it would be a poor psychological design that allowed us to get into psychological states that could not be realized by true beliefs in any situation. Thus we might expect that the psychological states of well adapted organisms would be realizable by true beliefs in at least some situations. Moreover we should expect those organisms to behave and think in ways that were appropriate to those notional situations. As I will put it, your notional situations are the sorts of situations you think are in.
As folk psychologists we are adept at understanding one another in this way. If we are told that someone believes the proposition that Jocasta ≠ Jocasta, we have no way of telling what he might do. On the other hand if we know that someone thinks they are in a situation of the sort Twin Oedipus is actually in, we understand his psychological state, know how he might have acquired it and how he is likely to respond to what happens next.
What happens next in the Oedipus story is that a group of messengers arrive telling tales. Listening to them, Oedipus and Twin Oedipus will change their common psychological state until each them comes to think he is in a situation in which it is immensely probable that his mother is his wife. At that point, in a transition described by laws like (L), the penny drops. Oedipus and Twin Oedipus move into a new psychological state which is realizable by true beliefs only at situations in which they have committed incest. Oedipus’s actual situation has become his, and Twin Oedipus’s notional one. And both behave appropriately to that situation; they go ape, as anyone would if he thought he was in a situation in which he had married their mother.
In the course of the story, Oedipus and Twin Oedipus acquire new beliefs given what they see and hear. Assessed in terms of propositional content, they learn very different things. The messengers hither are telling the awful truth about Jocasta; over yon, they are telling ugly lies about Jocasta’s twin. Still, the Oedipi’s sensory inputs will be psychologically indistinguishable and will have identical effects on their psychological states and notional situations. For the purposes of explaining and predicting behavior, the most productive way to characterize empirical uptake is not in terms of what propositions it is evidence for or what propositions it might lead a subject to believe, but rather in terms of its effects on the subject’s notional situations. By this accounting we can say that in our story both Oedipi make the same inferences based upon the same information. Of course, they end up with different beliefs. In Twin Oedipus’s case: necessarily false beliefs; but that is beside the psychological point.
Likewise it is natural to want to describe Oedipus' change of mind through the course of this story as an increase in knowledge. By the end of the story he knows something about Jocasta he did not know before. But, as we shall see in later posts, this sort of epistemic gain cannot be understood in terms of learning new propositions, but by coming to believe propositions already believed, by way of different, more informed, psychological states. Knowledge is a matter of information, not justification.
Of course, mistakes about identity have had a special philosophical luminance ever since Frege wrote about The Morning and The Evening Star. In the next post on this theme, we will put two dimensional psychology to work at sorting out sense and reference.
