Decoud is possibly comparing Charles to himself, whose ideal of love has failed to get him "a firm hold of the substance" of Antonia.

This line also emphasizes the novel's theme that "illusion" -- a dream-ideal of some sort -- is the only means by which Man can interact with the "substance" of the physical world. Decoud's suggestion is that some illusions succeed better than others, with the English ideals of unemotional practicality having an advantage.

Note that Decoud calls Charles an "Englishman," although he uses the Spanish "Don Carlos" and "El Rey de Sulaco." (Holroyd, below, receives the English word "King," though in the context of the letter English is presumably French.) The ambiguity over Charles Gould's nationality continues.