In "Everything That Rises Must Converge," to where does Julian withdraw?

On the bus, Julian's
mother is engaged in a conversation with another woman, and she tells this woman that Julian
wants to be a writer but that he is selling typewriters for now, until he's able to get his
writing off the ground. Julian finds their conversation inane, and he resents being examined by
this other woman, who said earlier that the black people on the bus the other day were
"'thick as fleasup front and all through.'" He finds her repulsive, and so he gives
her a dirty look when she looks at him. In order to escape the conversation of his mother and
the woman, he

. . . with[drew] into the inner compartment
of his mind where he spent most of his time. This was the kind of mental bubble in which he
established himself when he could not bear to be a part of what was going on around
him.

In the inner sanctum of his brain, Julian feels safe
from whatever is going on outside himself, though he can still see out and judge those around
him. This is where he goes, mentally, to be "free of the general idiocy of his
fellows." He apparently goes to this mental space often when around his mother; she cannot
follow him there but he can still "see her with absolute
clarity."