Final answer:
Dupin criticizes the police for their lack of creativity and adaptability in investigations, explaining that this narrow view is why they fail to find the truth when faced with a cunning criminal.
Step-by-step explanation:
In sentences 79-85, Dupin uses an analogy to explain why the police cannot find the truth due to their predictable methods of investigation. He suggests that the police only consider their own ideas of ingenuity and fail to measure against the intellect of the individual criminal. Dupin highlights that this narrow approach works only when the criminal's cunning aligns with their expectations. When the intellect of the felon is either more sophisticated or simple than their own, the police are unable to adapt, thus the criminal outwits them. Dupin presents the police as constrained by their routines and lacking innovation, able to extend only the old methods without altering core principles even when extraordinary rewards are at stake.