According to Carl Jung
, introversion and extraversion refer to the direction of psychic energy
. If a person’s psychic energy usually flows outwards then he or she is an extravert, while if the energy usually flows inwards, the person is an introvert.
Extraverts feel an increase of perceived energy when interacting with a large group of people, but a decrease of energy when left alone. Conversely, introverts feel an increase of energy when alone, but a decrease of energy when surrounded by a large group of people. Most modern psychologists consider theories of psychic energy to be obsolete. First, it is difficult to operationalize
mental "energy" in ways that can be scientifically measured and tested. Second, more detailed explanations of extraversion and the brain have replaced Jung's rather speculative theories.
Nevertheless, the concept is still in popular usage in the general sense of "feeling energized" in particular situations. Jung’s primary legacy in this area may be the popularizing of the terms introvert and extravert to refer to a particular dimension of personality.