 The step from [3] to [4] runs as follows. Suppose that whether any sentence “has sense” (i.e., on our reading, has a truth-value) depends (in the way just explained) on whether another is true. Then every sentence will have an “indeterminate sense” in the sense that it will lack a truth value with respect to at least one possible world. But an indeterminate sense is no sense at all, for a proposition by its very nature “reaches through the whole logical space” (3.42) (i.e., it is truth-valued with respect to every possible  world).[6]  So if every sentence depended for its “sense” (i.e., truth-valuedness) on the truth of another, no sentence would have a determinate sense, and so no sentence would have a sense. In which case we would be unable to frame senseful propositions (i.e., to “draw up pictures of the world true or false”). 