I interpret this dramatic, mysterious passage about Nostromo's frown to be a battle, in which Nostromo wrestles with ultimate skepticism and wins. He has just swum in the Gulf of utter subjectivity, and he is now faced with an insoluble mystery, with the impossibility of knowledge. He is ripe for skepticism. Note that first he loses his "excitement," a word which, explicitly equated with his soul, comprehends all earthly passions and commitments. He is left "inert," in a state where objective reference ceases (he "did not seem to know the gulf"). His stare has an "emptiness." He is then possessed by "an outcast soul" that brings "deep thought," and I submit that the soul in question is Decoud's, bringing Decoud's skepticism with it. The frown, which the text elevates to overwhelming significance, represents the People entertaining the fatal idea of futility. But then he shakes it off. In doing so he again surrenders to the "repose of all visible things," i.e., he again submits to the sustaining illusion of the objective universe.

At that point he is able to move -- he points himself toward the Great Isabel, symbolizing the self -- and he demonstrates that objective knowledge is possible, by only now analyzing the bloodstain. In the empty universe the only objective "thing" may be the certitude of "blood," of conquest, antagonism and suicide, but that certitude is enough to expel the spirit of skepticism and keep Nostromo in the world.