4.114 It should delimit the thinkable and therewith the unthinkable.
It should limit the unthinkable from inside, by way of the thinkable.
There is no other way, surely. By thinking only what can be thought we see what cannot be thought. Or: by thinking clearly (or self-consciously?) we see what thinking is and, thus, what it isn’t or cannot be.
Echoes here of Kant’s talk of limiting reason in, e.g., § 59 of Prolegomena. At B xxx of the Critique of Pure Reason he says that he has had to suspend knowledge in order to make room for faith (Ich musste also das Wissen aufheben, um zu Glauben Platz zu bekommen) and the references in Prolegomena to the limits of reason relate to this idea. We might wonder, therefore, whether Wittgenstein is implying that he limits the thinkable in order to make room for some faith in the ineffable.