This is Nostromo's ideal of reputation again, but now in reference to the "sham" life that must be maintained by keeping the one dark spot secret. Note that the "essence, value, reality" of his life does not consist entirely of his reputation; it also derives from his independent secret; this is new. "That thing" is, symbolically, the knowledge of inner worth, a notion outside the range of "common comprehension," i.e., collective, society-organizing ideals.