This story presents a puzzle of mistaken diagnosis (heart attack or murder) in the context of a mistaken identity. Doyle suggests a slippage between difference and identity, but the story eventually realigns itself under traditional, conservative mores.

The contested class status of the nineteenth-century physician is evident in Doyle's portrait of Dr. Lana, who enjoys an aristocratic heritage but relies solely on his professional abilities to succeed. Doyle also interrogates postcolonial identity by setting up an opposition between Ernest and Aloysius Lana. Given Ernest's role as the "bad colonial," postcolonial identity registers as violent, evil, unhealthy, and dissolute. In contrast, Aloysius's good character represents the virtues of the old Spanish aristocracy, and his respectable, successful medical practice follows his medical training in Glasgow.

The moral difference between Ernest and Aloysius disappears at Ernest's death, when his body is mistaken for that of Aloysius, but that difference is emphatically reinstated in the courtroom scene at the end. Likewise, the phrase "the Black Doctor" implies an absolute difference between Aloysius and the villagers' expectations of a British doctor, but in the end he "return[s]"to England. That is, in part thanks to his British medical training, Aloysius now registers as a Briton, culturally, rather than as a "black" foreigner; and it is to Ernest and his Argentine childhood that he is contrasted, not to the village, which now represents his future.

The device of the Doppelgänger here, and its return to popularity in turn-of-the-century sensational texts, implicitly registers the influence of psychoanalysis. Ernest then also represents Aloysius's subconscious--his repressed Other, and his colonial, familial past.