Carl Gustav Jung argued in his 1921 book Psychological Types that people show consistent patterns in how they direct their psychic energy and how they perceive and judge the world. The theory emerged after his theoretical break from Freud and Adler, as Jung sought a systematic way to explain the diversity of human personality.
The widely used MBTI (Myers-Briggs Type Indicator) was developed later by Isabel Myers and her mother Katharine Briggs, who turned Jung's theory into a practical self-report questionnaire.
Jung's first axis concerns the direction of psychic energy. Extraverts direct their energy outward — toward people, objects, and activity — and are energized by it. Introverts direct their energy inward — toward thoughts, feelings, and images — and recharge through solitude.
A common misunderstanding is treating this as a measure of sociability. It is really about which direction energy flows: introverts can be plenty sociable, but they need solitary time to recover afterward.
Jung proposed two ways of perceiving information — Sensing (concrete data through the five senses) and iNtuition (insight into patterns and possibilities) — and two ways of judging it — Thinking (logic and cause-effect) and Feeling (values and relational impact).
Each of these four functions can operate in an extraverted or introverted mode, theoretically producing eight function-types. MBTI added a Judging–Perceiving axis on top of this, expanding it into the 16-type system used today.
Psychological type theory offers a useful metaphorical framework for understanding personality, but the general academic consensus is that it lacks the statistically validated predictive power of the mainstream Big Five model. Critics also argue that personality is better described as a continuous spectrum than as discrete either-or categories.
Even so, the theory remains widely used as a popular tool for self-understanding and interpersonal insight, because it offers an intuitive vocabulary for the question "why do I react this way in this situation?"