4.411 It is plausible from the beginning that the introduction of elementary propositions is foundational for the understanding of all other kinds of propositions. Indeed the understanding of all propositions depends palpably [fühlbar – ‘feel-ably’ or sensibly, the Notes on Logic say ‘obviously’ at the corresponding point (p. 100)] on the understanding of the elementary propositions.
‘Plausible’ is suggested by Black (p. 220). Otherwise we have “It is probable from the beginning…” which does not seem to make much sense.
He meant, perhaps, that it feels as though this is the case. Because the previous sentence seems to imply that all this is not true in fact. And indeed it seems not to be true, since we do understand propositions but cannot identify a single elementary proposition. Although perhaps we do not have a philosophical understanding of the nature of any propositions.