Monygham has indeed changed over the course of the novel, since earlier he said
that the self was "the last thing a man ought to be sure of."
The reference hints at the deeper meaning to this line, in which Monygham has
come to respect not merely his own self, but the self, i.e., the
value of the irreplaceable subjective human soul, which Nostromo posits
as humanity's only true treasure.