Final answer:
The narrator describes Dupin's ability as 'exceedingly able' and 'thoroughly versed', highlighting his remarkable deductive skills in solving the mystery of the purloined letter.
Step-by-step explanation:
In Edgar Allan Poe's short story, the narrator uses various words to describe Dupin's ability. The two key phrases that stand out in this context are 'exceedingly able' and 'thoroughly versed'. These phrases are used to commend the Parisian police's capabilities and suggest their skill and expertise in their domain.
However, Dupin's abilities are implied to be even more exceptional, given his success where the police have failed. Dupin's strategic thinking and understanding of human nature, as well as his methodical approach to solving the mystery of the purloined letter, demonstrate his superior deductive skills.