If we go through with this idea of the || a private experience which we don't know we can't talk of a certain private experience either, because

this expression is taken from the case in which what we don't know is something like say, whether if he says he sees red he sees red & not perhaps blue green or yellow.
In which it alludes to a certain class of experiences which we know though we don't know which one of its members he has. Rather the private experiences || impressions which we imagine as the background to the foreground of our actions dissolve¤ into a mist which we wished to refer to talk about & imagined to be back of our action ¤ || Rather the private experiences which we imagined as an unknown x a, y, z etc. behind our actions, lo¤se their individualities & dissolve into a mist & into nothing.

(2015–) Wittgenstein Source Bergen Nachlass Edition (WS-BNE). Edited by the Wittgenstein Archives at the University of Bergen under the direction of Alois Pichler. In: Wittgenstein Source, curated by Alois Pichler (2009–) and Joseph Wang-Kathrein (2020–). (N) Bergen: WAB.