I needed to write some things; I have checked none of my quotations (and give no citations because of it), and am too lazy to italicize where it needs doing etc.; anyone who reads this is advised that they do so only under their own judgement, and I foreswear all responsibility for such actions.
"Only in the context of a proposition has a word really a meaning" Frege tells us; this is the stronger formulation used in the middle of the Grundlagen, after his initial, more cautious warning "Never to ask for the meaning of a word except in the context of a proposition", for fear that what we will confuse with the meaning is an "idea", a subjective Vorstellung which comes to mind when a word is heard but is orthogonal to any question of meaning.
The author of the Tractatus tells us much the same: "Only in the context of a proposition has a name meaning"; only in a proposition can there be a symbol. Outside of the context of a proposition, we have only things which we can confuse with signs: but as a sign is "the perceptible aspect of a symbol", a blot of ink or a noise which is not presently symbolizing is not even a sign. To see the symbol in a sign, we must consider the context of significant use: this means that if we are considering a putative sign in such a way that we can think of it just as we are without its having a context of significant use, then we are not thinking of even a sign: we have only ink or noise, and these have no innate connection to any symbols (for signification is arbitrary).
So, if there is a context in which we are supposed to refer to a sign in a way that is independent of that sign being used, we in fact refer to no sign: we mention only ink or noise, or something else which may (or may not) be arbitrarily connected with a meaning in some further use of it as a symbol. Such a thing cannot have any meaning.
There are many ways to use a symbol: this is demanded by what the author of the Tractatus thinks of as the "bipolarity" of a proposition, its ability to be true or false. If we cannot use the same symbols to say both a true thing and a false thing (with the aid of a sign for negation, or by denying where someone has affirmed) then we lose this bipolarity: the same symbols need to be able to function in both affirmation and denial in the same ways for a given thought to be held as true and held as false. So logic demands that there be ways to modify the force of a proposition, to use Frege's terminology.
Perhaps the same point, perhaps a better one than I just made: "Thaetetus" must be the same symbol in "Thaetetus sits" and in "Thaetetus flies" for the inferences in which these propositions are involved to be intelligible. So there arises the illusion that we can speak of the meaning of "Thaetetus" in these propositions on its own, so that we can after all speak of the meaning of a word outside of the context of a proposition. It seems we have just done so, by putting "Thaetetus" in quotation marks: by doing this we are now talking about the symbol which is combined with other symbols in propositions, but without its being combined in any particular proposition.
I think this must be seeing things wrong.
Rather than saying that in ""Thaetetus" refers to Thaetetus" we refer to a name (or that in the longer quoted expression I just used we refer to a sentence), we might say that we use "Thaeteus" with a modified force: where normally "Thaeteus" symbolizes only in some such proposition as "Thaeteus sits" or "Thaetetus flies", the quotation marks around ""Thaetetus"" cancel the force of the rest of the proposition, for all of the propositions in which "Thaeteus" has a use: thus we do not use quotation marks to refer to something which might exist before any proposition, but to refer to something which exists only in abstraction from propositions. Not: A name has a reference, and can be combined with other words to form a sentences, but: A sentence has names in it, which can be picked out in it and seen in other sentences. The use of quotation marks around an expression thus depend on that expression already having a use in the language, to use them in the way they are ordinarily used in logic and semantics. So this way of using "Thaeteus" is not using it outside of the context of a proposition, but it using it in the context of propositions which are bracketed out: not outside of a context of significant use, but in a different context of significant use which is parasitic on those contexts. "Thaeteus" is not something we can mention which has a meaning by itself, but is something we can mention as having its meaning in this-and-that proposition which we leave unstated (but could state examples of).
This is contrary to the manner in which artificial languages are constructed (as in Carnap), where we distinguish between the introduction of atomic signs and the rules for formation of sentences from the combination of atomic signs (and from other sentences). This gives the appearance that we can have something logical in view before the final proposition-in-a-context-of-significant-use is given, that we can build up one of these out of some things understandable antecedently.
Carnap is one of the inventors of metalogic; he reads into the Tractatus a notion of "a language" which is at home in his own work on metalogic, where a "language" is something we can have in view without presently using it (as its use is external to it: a formal language is in itself a system of manipulable marks, and can be manipulated or not as we please). The Tractatus has no such notion: its author is not teaching us about a formal language, but about a method of rewriting the propositions of ordinary language, which is for its author "the only language I can understand". (Thus we do not need to saddle this author with the view that every language has the same expressive power, that all languages are intertranslatable: his Begriffschrift is not meant to be a language which has the powers of all languages, but to be a style of notation which can serve to rewrite any language. The notation itself does not need to have expressive powers in the way that the languages it is used to rewrite do: anyone who wants to rewrite something in the notation of the Tractatus can make use of whatever sorts of (e.g.) names his own language gives him, and doesn't need a Begriffschrift to supply him with any.)
When working with a formal language which resembled ordinary language, we can mistake a sentence of the formal language (which exists only according to the arbitrary dictates of the formal system in which it is produced) with a sentence of ordinary language which superficially resembles it: we can then imagine that our ordinary language is what this formal language has created, forgetting that the formal language is a free creation of a particular subject and no ordinary language can be this. ("The only language which I understand" cannot be something I produce by a free act, for I must already understand it to formulate any such end for myself.) "Formal language" and "ordinary language" have only a sign in common, and the author of the Tractatus reminds us that this is of no logical import.
(It is probably important that Davidson rejects Tarski's theory of quotation marks in the Warheitsbegriff as an error, and has to replace it to apply Tarski's definition of truth to natural languages; I would have to look at Tarski again to remember what his view actually was.)
This way of thinking goes along with thinking that the rules of "logical syntax" are constitutive of thought, not normative for it: we cannot violate them. Thus there cannot be an impermissible combination of signs, for to speak of "permission" makes no sense here. In not being able to violate these rules, we are not prohibited from thinking anything; outside of the limits of thought there is simply nonsense: not something which we are prohibited from thinking. (I am not happy with any formulation of this I can think of. This is perhaps an important point about the constitutive-normative distinction in these areas, that formulations of either sort of view can be taken as formulations of the other.)
In logic we can never do something we shouldn't do, but only misunderstand what we are doing. We can only mistake something other than logic for logic, as Frege warned us about; but Frege's rule to always sharply distinguish the logical and the psychological does not go far enough, for there are other troubles than psychologism to worry about. (Frege perhaps falls into a confusion of logic and language when he looks for the referents of concept-expressions, as both names and concept-expressions are written in words.) Or perhaps: we all too easily underestimate what it is that is "psychological" as opposed to logical: not just the subjective play of Vorstellungen is opposed to the logical. "Logical" is perhaps not a term that wears the pants, as Austin said of "real".
I have no idea if I'm going anywhere (with this?). I want to say: I am a mass of errors, and can do nothing but err. Is there a context of significant use in which such "philosophy" as I produce has a sense, or is this too nothing but confusion? (That there is not is suggested by the fact that this question strikes me as sophistry: but I feel it forced upon me by my own thinking, which is also tarred as sophistry by this association, and so the question seems to have force: and on in circles I go.)