6 ideas
Jaynes argues that most of what we credit to consciousness happens without it: perception, learning, skilled performance, reasoning, and even problem solving mostly run automatically, and introspection has no access to how they are done. Consciousness is far smaller than it feels, because we cannot be aware of the moments we are not aware. So a civilization could build cities, farm, and wage war without anyone introspecting.
In the bicameral mind, the right hemisphere put together experience and stored admonitions into a solution for new or stressful situations. It delivered that solution to the left hemisphere as a heard verbal command, an auditory hallucination taken to be the voice of a god, king, or ancestor. The person obeyed without deliberating, because there was no inner self to weigh the order against.
Jaynes says consciousness is an analog 'mind-space' built by language through metaphor. We describe the mind with words taken from physical space and action, such as 'grasp', 'see the point', and 'in the back of my mind'. That space then supports specific operations: spatialization of time, excerption, an analog 'I' and metaphor 'me', narratization of life into a story, and conciliation of events into coherent wholes.
Jaynes treats changes in vocabulary as a record of changes in mentality. In the Iliad, heroes act because gods speak to them, and words like thumos and noos name bodily sensations rather than mental states. By the Odyssey and later Greek texts, those same words have come to name an inner space where a person deliberates. The lens: when a culture has no language for introspection, ask whether it had introspection at all.
Jaynes places the breakdown of bicamerality around the second millennium BC and ties it to specific pressures. Writing weakened the authority of heard voices, and populations grew too large and complex for gods to control. Catastrophes, migrations, and trade brought together peoples whose gods gave conflicting commands. Deception, planning, and dealing with strangers rewarded anyone who could model other minds and their own. Under those conditions consciousness spread as a learned cultural solution, not a biological mutation.
Jaynes reads certain modern phenomena as remnants of the old mentality. These include oracles, prophecy, possession, glossolalia, hypnosis, and the command hallucinations of schizophrenia. In each, conscious self-direction steps aside and the person follows an external-seeming authority. He also reads the ancient laments that the gods have gone silent, and the rise of divination and prayer, as attempts to reach voices that no longer spoke unprompted.