This is looking at Wittgenstein's Tractatus and his Philosophical Investigations. How does one stand in the face of this tension? How exactly do both books have an idea of what cannot be spoken of?
The well-known Tractatus statement:
7. What we cannot speak about we must pass over in silence.
is gorunded in the Tractarian thesis of the ineffability of semantics :
3.263. The meanings of primitive signs can be explained by means of elucidations. Elucidations are propositions that contain the primitive signs. So they can only be understood if the meanings of those signs are already known.
4.12. Propositions can represent the whole of reality, but they cannot represent what they must have in common with reality in order to be able to represent it—logical form. In order to be able to represent logical form, we should have to be able to station ourselves with propositions somewhere outside logic, that is to say outside the world.
This train of thoughts is present also in the "second" Wittgenstein; see Philosophical Grammar, §71:
No sign leads us beyond itself, and no argument either.
And see Philosophical Investigations, §120:
When I talk about language (words, sentences, etc.) I must speak the language of every day. Is this language somehow too coarse and material for what we want to say? Then how is another one to be constructed ? — And how strange that we should be able to do anything at all with the one we have!
In giving explanations I already have to use language full-blown (not some sort of preparatory, provisional one); this by itself shews that I can adduce only exterior facts about language.
Yes, but then how can these explanations satisfy us?—Well, your very questions were framed in this language; they had to be expressed in this language, if there was anything to ask!
And your scruples are misunderstandings.
Your questions refer to words; so I have to talk about words.
You say: the point isn't the word, but its meaning, and you think of the meaning as a thing of the same kind as the word, though also different from the word. Here the word, there the meaning. The money, and the cow that you can buy with it. (But contrast: money, and its use.)
In conclusion, there is a deep continuity between Tractatus and PI regrading:
[the] idea of what cannot be spoken of.
Semantics,meaning (i.e. the relation between language and world) "cannot be spoken of".