Full Idea
The existence of a general propositional form is proved by the fact that there cannot be a proposition whose form could not have been foreseen (i.e. constructed). The general form of the proposition is: Such and such is the case.
Gist of Idea
There must exist a general form of propositions, which are predictabe. It is: such and such is the case
Source
Michael Morris (Guidebook to Wittgenstein's Tractatus [2008], 4.5)
Book Reference
Wittgenstein,Ludwig: 'Tractatus Logico-Philosophicus (Pears)', ed/tr. Pears,D. /McGuinness,B. [RKP 1961], p.36
A Reaction
[last bit in Ogden translation] LW eventually expresses this symbolically. We could just say a proposition is an assertion. This strikes as either a rather empty claim, or an unfounded one.