3.13 To a sentence belongs all that belongs to the projection, but not what is projected.
Thus the possibility of what is projected belongs to it, but not it itself.
Its sense is therefore not yet contained in a sentence, but [perhaps?] the possibility of expressing it is.
(“The content of a sentence” means the content of a significant [meaningful, sinnvollen] sentence.)
The form of its sense is contained in a sentence, but not its content.
I’m following Black (p. 100) here in translating Satz as ‘sentence’ rather than ‘proposition.’
You would think that only what is projected (the sense or meaning) is what belongs in common to both a proposition and the sentence that is its projection. But here Wittgenstein says that propositions are really something like potential sentences. Not only do we encounter propositions only in the form of sentences, but propositions exist only as sentences. Because until they are sentences, propositions have no content, no sense. And a proposition without sense is hardly a proposition at all, is it? If a proposition has only form then it certainly has no real existence. It is a logical fiction or hypothetical ‘entity’ used for thinking about logic. Its ‘existence’ is purely logical, not metaphysical at all.
Alternatively, the first sentence of 3.13 might be read as saying that propositions and sentences (tokens) have everything in common except the physical manifestation that is the sentence (token). That (the physical stuff) is what is projected. Thus the possibility of being communicated belongs to a proposition, but not the perceptible properties necessary for communication themselves. It therefore has no use yet (so sense is use?), but only the possibility of being used. This doesn’t sound too implausible, but what would be “the possibility of expressing the sense of a proposition” if this sense were itself the expression of a proposition/thought/sentence? The second half of 3.13 seems incompatible with the reading offered in this paragraph. So we are back with my previous paragraph. What now to make of the first sentence of 3.13, which does sound odd? I think oddness often indicates irony in the Tractatus. To a proposition belongs nothing, in other words, because what is projected exists only in sentences. (Or thoughts ‘embodied’ in some perceptible medium. I don’t see why this has to be physical and not, say, the stuff of a Cartesian mind.)
Black (p. 100) says that what belongs to the projection means “all that is internal to the representing relation, i.e. the logical form that the sentence has in common with the state of affairs it represents (2.18)” and what is projected means the sense. He also says (same page) that: “form of its sense = ‘form of the possible state of affairs presented’ = ‘the logical form’.”