Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
In Canto Three of his poem John Shade (the poet in VN’s novel Pale Fire, 1962) mentions a dead school chum who in a blend of jauntiness and gloom points at the puddles in his basement room:
For as we know from dreams it is so hard
To speak to our dear dead! They disregard
Our apprehension, queaziness and shame -
The awful sense that they're not quite the same.