The Bicameral Mind Hypothesis is a theory of mind first published in 1976 by Julian Jaynes in his book, "The Origin of Consciousness in the Breakdown of the Bicameral Mind"
The Bicameral Mind proposes that human consciousness is only roughly 3,000 years old. Prior to it's development, the mind of man was divided into two "chambers" (Bicameral, meaning "two chambered"). The right hemisphere of the brain, referred to as the "god portion" and the left hemisphere as the "human portion". Neither chamber possessed consciousness as modern humans comprehend it. Instead, humans were similar to robots or automaton, having no concept of self-awareness or introspection. In the Bicameral Mind, the god portion communicated to the human portion via auditory hallucinations featuring command language, i.e "hearing voices", and the human portion merely obeyed the commands without question. In time, these auditory hallucinations were viewed as being the voice of the gods (or God), or the voice of ancestors, depending on cultural conventions.
According to the Bicameral Mind hypothesis, somewhere around 2000 BCE, the Bicameral Mind began to collapse, possibly due to change and new social stresses. As an adaptation to the new environmental and cultural change, consciousness, i.e. unicameral mind, evolved to take it's place.
The Bicameral Mind Hypothesis features heavily in HBO's series, West World. In the show it is revealed that one of the creators of the A.I., or "hosts", designed them to hear their programming as an inner voice, creating a Bicameral Mind.