You might understand the verb “ratiocinate” better as “deduce,” a synonym that means “arrive at (a fact or a conclusion) by reasoning; draw as a logical conclusion.” Modern detective stories tend to follow a process of deduction thanks to that wording being used by Sherlock Holmes, but author Sir Arthur Conan Doyle credited his inspiration to earlier works by Edgar Allan Poe. The term “ratiocination” was used by Poe to depict the investigative approach of C. August Dupin, who features in stories including “The Murders in the Rue Morgue,” “The Mystery of Marie Rogêt,” and “The Purloined Letter.” While Poe never actually used the word “detective” in his stories, the ratiocinating by Dupin laid the groundwork for the modern detective genre.