Conrad makes clear in this paragraph the symbolic meaning of swimming in the Placid Gulf: the struggle with "the paralyzing sense of human littleness" in the formless universe. What the doctor doesn't know is that Nostromo "precipitated" himself into the gulf. He himself sank the lighter which symbolized the larger society, thus symbolically declaring his independence from it, which is the confused condition he is in now. This implies a very different kind of "intrepidity of spirit" than the doctor, seeking to enlist him in a mere intrepid continuation of his "usefulness," imagines.