Hope you don't mind me sharing...
1. Let us assume a simple Verbal Behaviorism model, such that if S thinks that p, S utters aloud "that p". We shall call this 'thinking-out-loud' and assume S only thinks-out-loud when he thinks.
2. We shall assume that there are 'inner' utterances of the sentence along with the overt utterance, such that thinking-out-loud is the culmination of a process which begins with inner speech. We can revise this model later to account for non-thought-out-loud thoughts, but we will stick to the simple notion for now that all thoughts are thought out loud.
3. We can say: When S thinks that p, S has a tokening of the inner-speech type 'that p' and then has a tokening of the outer-speech type 'that p'. We can assume that Mentalese is in English and that S utters sentences in German.
4. It seems that we can say: When S thinks-out-loud that p, S tokens an inner and an outer sentence of the same type. S tokens two tokens of the •that p• type. One token-class is of the inner (English) kind, one token-class of the outer (German) kind.
5. Let us imagine S is presented with some rain. S thinks: "It rains". He thinks-out-loud: "Es regnet." His thinking-out-loud and his thinking are both •It rains•.
6. It seems reasonable to think that "Es regnet" (in German) is true iff it rains. We can understand more generally: •It rains• is true iff it rains. Because 'Es regnet', his thought-out-loud, and his thought 'It rains' are both tokens of the same type, one being in Mentalese (English) and one in German, both should have the same truth-conditions.
7. Presumably our simple correspondence theory, ' "P" is true iff P is the case', tells us that the left-hand bit 'maps' onto the right-hand bit. We can crudely say that the left-hand bit 'pictures' or 'models' the right-hand bit. So 'It rains' is true iff it maps onto how things are in the world.
8. Some sentences have a 'sense' or are 'significant' because they are about the world. Presumably the motto: "Intentionality is the mark of the mental", is pretty good for our purposes. We can say that even if not all sentences are intentional, or have sense or significance, or whatever, all mental sentences are intentional. All thoughts-out-loud, being overt expressions of thoughts, will be intentional as well.
9. Mental sentences, being about things, will be 'encoding sentences'. Such encoding sentences not only may have appropriate contexts of utterance as ordinary sentences like "Hello Jones" do, but have something more. It has an encoding function which maps onto the world.
10. When S sees rain, in an important sense his seeing of rain makes him respond by saying to himself: "It rains." This makes him, on our model, think-out-loud: "Es regnet." His thinking is not only caused by the world, however. It is also importantly about the world and not a mere reaction to sensory stimulation.
11. We might say: 'His stimulus-meaning of "Es regnet" is rain. When he encounters rain, he responds by thinking "Es regnet" to himself.' But his thinking is about the world. His thought: "It rains", which is of the •It rains• type, encodes information about the world. If S told us: "It rains", we would not only say: "S must be having such-and-such sensory stimulations" or "S must be in such-and-such a state," but we also would say: "S must be seeing rain" or "S is reporting that it rains," for it seems that we take S to be offering a reliable report of how things are in the world and not merely what his current sensory states are. S seems to be telling us that it is raining.
12. Intuitively, S is an agent. S has acts of belief. When he thinks-out-loud: "Es regnet", he is asserting something he takes to be the case. What is he asserting? Presumably more than merely what he is uttering. It seems he is asserting that what he said is true. What makes the sentence true? We might say: It is true if the proposition it encodes obtains in the world. If it rains, then 'Es regnet' is true. If 'Es regnet' is true, and if an 'Es regnet' (in German) is a •It rains•, then any •It rains• is true when 'Es regnet' is true.
13. It seems that on this model some sentences are intentional or about the world, by virtue of stating that thus-and-so is the case. They 'picture' or 'encode' or 'present' something as being the case, which then maps onto the world. If it maps, it is true. If it fails to map, it is false.
14. The thought 'It rains' encodes or implies that it rains. Thus the thought encodes a proposition. If the proposition obtains in the world, i.e. if it is the case that it rains, then sentences of the type •It rains• are true. ' "P" is true iff P' seems to tell us that language encodes how the world is. A bit of encoding language is a proposition.
16. It seems most reasonable to have propositions along with types. Propositions are the sort of things that types which are intentional encode. "Es regnet" and "It rains" both play the same role, the •It rains• role. But in addition, both sentences express the proposition that it rains. Any •It rains• encodes the proposition that it rains.