What is the theme in The Hitchhiker's Guide to the Galaxy by Douglas Adams?

Nature and Its meaning is another theme. Rather than being a source of meaning, as is frequently assumed, humanity is presented in this book as a taker of meaning, acting out the roles that are assigned by the animals around us. This is most evident in the interactions with the laboratory mice: scientists believe that they are manipulating the mice's behaviors in order to learn more about nature, but the mice are actually manipulating the scientists' behaviors to learn more about humans.