Full Idea
The 'Tractatus' concerns the theory of what can be expressed by propositions (and, which comes to the same thing, can be thought), and what cannot be expressed by propositions, but can only be shown; which, I believe, is the main problem of philosophy.
Gist of Idea
The main problem of philosophy is what can and cannot be thought and expressed
Source
report of Ludwig Wittgenstein (Letters to Russell [1919]) by A.C. Grayling - Wittgenstein Ch.2
Book Reference
Grayling,A.C.: 'Wittgenstein' [OUP 2001], p.18
A Reaction
This contains what a I consider the heresy of making thought depend on language, but his main question remains, of the limits of thought. It is dramatised nicely in the 'mysterian' view of the mind-body problem (e.g. Idea 2540).
Related Idea
Idea 2540 Examining mind sees no brain; examining brain sees no mind [McGinn]