The positronic brain is the computational mechanism that controls every robot in the story. It is not described in any detail except that it is highly complex. Without positronic brains, the robots and their stories are impossible. In real life, there is no such device, but the author’s job, as a writer of speculative fiction, is to anticipate such technological developments in the future and then consider the effects it might have on society.

Thus, the positronic brain is a deus ex machina, a “god outside the mechanism” that arbitrarily reaches into human history and changes it. This is a standard literary device when an author needs something to happen that will move a plot forward. It is perfectly legitimate—writers through the ages have invoked arbitrary “what ifs” to launch or redirect plot lines—and it is expected in a work of sci-fi. Positronic brains thus serve as placeholders for a futuristic technology that, in some form or another, is likely to appear one day and overturn much of human culture. The stories in I, Robot look at the implications of such a technology and how humans might address them in the future.

The book itself makes sly reference to its own device.