Sense and Substance in Wittgenstein"s Tractatus

In the early pages of his Tractatus, Wittgenstein says that
the substance of the world consists of unalterable, simple
objects (Gegenstände) (2.021, 2.027). Substance is connected
to the Sinn of a proposition in the following cryptic
way:
"If the world had no substance, then whether a proposition
had sense [Sinn] would depend on whether another
proposition was true.� (2.0211)�
"In that case we could not sketch any picture of the
world (true or false).� (2.0212)
How can the sense of a proposition depend on the existence
of simple, unalterable objects? No connection between
Sinn and substance is evident on the basis of these
short remarks. Furthermore, the remarks sound strange
because elsewhere in the Tractatus Wittgenstein makes it
clear that we can understand the sense of a proposition
without knowing anything about how things actually stand
in the world (see e.g. 4.024).