The origins of Leonard McCoy's iconic "I'm a doctor" catchphrase can most probably be traced back to the pulp detective film, "The Kennel Murder Case," which was released in 1933. The movie follows the investigation of an art collector murdered in a closed room. Along for the ride is one Doctor Doremus, played by Etienne Girardot. At several points throughout the movie, Doremus repeats the phrase, "I'm a doctor, not a detective," and in one instance he says, "I'm a doctor, not a magician" (via MeTV).
To be clear, nobody involved in the making of "Star Trek" has ever commented on this connection, but even "Star Trek" debuted more than 30 years after "The Kennel Murder Case," the similarity between McCoy and Doremus' catchphrases is unlikely to be a pure coincidence. When you add in the fact that both characters are doctors, it's easy to imagine how that catchphrase might have transferred from one character to the other. Perhaps the creator of "Star Trek," Gene Roddenberry, or even one of the show's other writers, saw "The Kennel Murder Case" and lifted Doremus's catchphrase from it, whether consciously or not. Regardless of how it happened, this does appear to be an example of art encouraging art, and a fascinating bit of "Star Trek" history.
