The contrast between Nostromo and Decoud is stated explicitly here. It is precisely "the problem of future conduct" -- i.e., the relation of the individual to society -- that saves Nostromo from the "sense of unreality." If society is an illusory ideal covering the formless darkness, then the perfect man to survive in it is the man with an illusory, idealized self. "Conduct" refers to no state of inner soul whatsoever, but to the physical manifestations expected by a complacent populace. For Nostromo, life consists of nothing else -- or at least, it hasn't so far.