Gregor's family engages in a fallacy when they assume that he can not understand them just because they can not understand him. While this fallacious reasoning occurs throughout the entire story, it is particularly evident when Kafka writes:
Under what pretext the doctor and the locksmith had been got rid of on that first morning Gregor could not discover, for since what he said was not understood by the others it never struck any of them, not even his sister, that he could understand what they said.
This fallacy is crucial to the story: it results in the Samsa family treating Gregor like an insect as opposed to treating him like a person. The Metamorphosis would have a drastically different message if Grete or Gregor's parents asked and then attempted to answer the question, “Can we communicate with Gregor even though he can not communicate with us?” The fallacious that they can't communicate with him emphasizes the disconnect between Gregor's mind and body. Although Gregor has the body of an insect, at this point in the story he still acts and thinks rationally, something his family fails to do despite appearing (and being) fully human.