A search on the word "innermost" in a PDF copy of a book
by Suzanne Gieser on Jung and Pauli yields no definite meaning
for the book's title, The Innermost Kernel (Springer, 2005).
The author does, however, devote a section (pp. 36-41) to the
influence of Schopenhauer on Jung and Pauli, and that section at least
suggests that the historical origin of her title is in Schopenhauer's
reformulation of Kant's "Ding an sich."
The Innermost Kernel , p. 37—
"… an expression of an underlying invisible world,
the one that forms the innermost essence of reality,
the thing-in-itself. This is the will, a blind existence
that forms an omnipresent entity beyond time, space
and individuality." *
* Arthur Schopenhauer, "Über die Vierfache Wurzel
des Satzes vom zureichenden Grunde" (1813),
Kleinere Schriften, SämtlicheWerke III
(Stuttgart, 1962), 805–806.
* See also Mann on Schopenhauer and an "innermost kernel."