Answer: it's the last line.
" She had the oddest sense of being herself invisible; unseen; unknown; there being no more marrying, no more having of children now, but only this astonishing and rather solemn progress with the rest of them, up Bond Street, this being Mrs. Dalloway; not even Clarissa any more"
Explanation: Basically, after all that was said and done, it was just her. No one else. Nothing else to prove.
By the way, this was correct in Plato.