This is a fascinating variation on the theme of inherent subjectivity. It suggests
that knowledge in other people is an idea that we have, which exists for us only
insofar as we have knowledge of it. If a person is sufficiently removed from the
sphere of our experience, they become the ultimate "comparative stranger":
they are assumed to be actually incapable of thought. Compare Captain Mitchell's
opinion of "Smith" in Chapter 1-2 ,
and also the alcalde of the mining village in Part Three.
In Nostromo Conrad posits that this alienating mechanism is a fundamental
cause of war.