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 fleas—up 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."