The source of extraversion or introversion was in the varying levels of excitability of the brain; the driver - Tom Butler-Bowdon

Freud believed human beings to be wholly driven by the stirrings of the unconscious mind, but Adler saw us as social beings who create a style of life in response to the environment and to what we feel we lack.

Neuroscientist V. S. Ramachandran’s work with phantom limbs seems to confirm the brain’s remarkable ability to create a sense of cognitive unity even if the reality (of many selves, and of many layers of consciousness) is more complex.

William James defined psychology as the science of mental life, but it could equally be defined as the science of human nature.