I quote:

Man [sic] could be compared to someone who lives in the Grand Canyon, but who is so short-sighted that he cannot see more than five yards. Or to someone who lives in a cathedral, but is surrounded by a kind of curtain, like a fortune-teller's cubicle, that goes with him wherever he walks.

The curtain is "everydayness". It is a state of mind rather than an objective reality.