Under Mitchell's casual aside, this is Conrad explaining his symbol. There are two levels to it. On the one hand, the symbolism is clearly stated: the light stands for private property, for the artifice of ownership and power that holds society together. In this regard Mitchell's "Company" refers broadly to established society as such. Linda, the light-keeper, represents that kind of society. Like other bright light in Nostromo, this is a disillusioning force, since the philosophy of private property implies a reality of perpetual human conflict and the need for law to control it.

On another level, however, one can read "the light is private property" to mean something like, "one's inner soul is private." This is Nostromo's other kind of symbolic light, representing the subjective soul of the individual, also symbolized by the treasure on the Great Isabel that, unknown to Mitchell and Linda, the lighthouse guards.

Together, the lighthouse represents the artifice that enables (or attempts) true communication between subjective souls. One can even step back and say that the lighthouse is the novel itself, a social artifice by which Conrad attempts to communicate his vision of reality.