What do ghosts symbolize in this novel?

The motif of ghosts reapeats a lot in this novel what does it mean?

Here are a few examples:

She smiled slowly and walking through her husband as if he were a ghost shook hands with Tom -- pg. 29

The crowd—it was now a crowd—stepped back involuntarily and when the door had opened wide there was a ghostly pause -- pg. 60

I could have sworn I heard the owl-eyed man break into ghostly laughter. -- pg. 97

They were gone, without a word, snapped out, made accidental, isolated, like ghosts even from our pity. -- pg. 144

Wilson shook his head. His eyes narrowed and his mouth widened slightly with the ghost of a superior ‘Hm!’ -- pg. 169

A new world, material without being real, where poor ghosts, breathing dreams like air, drifted fortuitously about … like that ashen, fantastic figure gliding toward him through the amorphous trees. -- pg. 172