Dear Carolyn,
 
I remember Pnin talking himself of "impossible isolation"
I can be wrong, but "isola" in Latin means island, i seem to remember form high school
The sentence falls after Joan shows him a cartoon of a mariner washed on the shore of an island with a sirene
Technically speaking, he is speaking of the mariner on the isle, but it's clear that it refers to his own situation too.
 
I hope that can be of any help to you?
 
Koen