Every Myers-Briggs type traces back to one book: Psychological Types, published in German in 1921 by the Swiss psychiatrist Carl Gustav Jung. Jung proposed that people differ in two basic ways. They differ in the direction their attention habitually flows, outward or inward, and in which mental functions they rely on most. Isabel Briggs Myers and Katharine Cook Briggs later used these ideas to build the MBTI. To understand the four letters, it helps to start with what Jung actually said.
Where Jung’s Typology Came From
Jung worked closely with Sigmund Freud for several years before the two fell out around 1912–1913. Part of what Jung was trying to explain was why gifted thinkers could look at the same patients and build such different theories. He contrasted Freud, whose theory focused on a person’s relationship to outside objects, with Alfred Adler, whose theory centred on the individual’s own drive for power and superiority. Jung concluded that each theorist’s temperament shaped their psychology. He wanted a framework that could account for both outlooks.
He presented early ideas on types in 1913 and developed them at length in Psychological Types. The English translation appeared in 1923. In the book, Jung drew on a long history of attempts to classify human character, from ancient temperament theories to the philosopher William James’s “tender-minded” and “tough-minded” thinkers.
The Two Attitudes: Extraversion and Introversion
Jung called extraversion and introversion attitudes. He meant habitual directions of psychic energy, not levels of sociability.
- Extraversion: interest flows outward toward the object, meaning people, things and events. The extravert is oriented by what is out there.
- Introversion: interest flows inward toward the subject, meaning one’s own impressions, reactions and inner images. The introvert is oriented by how things register inside.
This is broader than the everyday sense of “outgoing” or “quiet.” A Jungian introvert can be socially skilled. What marks them out is that their inner response to the world carries more weight than the world itself. Our article on what extraversion means in personality psychology follows how the term changed after Jung.
The Four Functions
Jung described four basic functions, or ways consciousness deals with experience:
| Function | What it does |
|---|---|
| Sensation | Registers what is actually there through the senses |
| Intuition | Perceives possibilities, connections and where things might lead |
| Thinking | Judges by logical connection and impersonal criteria |
| Feeling | Judges by value: whether something is acceptable, worthwhile or important |
A summary often attributed to Jung puts it this way: sensation tells you that something exists, thinking tells you what it is, feeling tells you what it’s worth, and intuition tells you where it came from and where it’s going.
Rational and irrational functions
Jung grouped thinking and feeling as rational functions because both weigh and evaluate. He grouped sensation and intuition as irrational functions. By that he didn’t mean unreasonable. He meant that they simply take in information without passing judgment on it. The MBTI later relabelled these as “judging” and “perceiving” functions. Our pieces on Thinking vs Feeling and Sensing vs Intuition cover each pair in modern terms.
Dominant, Auxiliary and Inferior Functions
Jung didn’t think people used one function exclusively. In his account:
- The dominant (or superior) function is the most developed and conscious one, and it sets the overall tone of the personality.
- An auxiliary function supports it. It comes from the other pair: a dominant judging function is backed by a perceiving function, and the reverse.
- The inferior function is the opposite of the dominant one within the same pair. For a thinking type, it’s feeling. It stays relatively undeveloped and unconscious. Jung believed it could erupt in crude or exaggerated ways under strain, and that integrating it was part of lifelong psychological growth.
Combining the two attitudes with the four functions gives Jung’s eight types, such as the extraverted thinking type or the introverted intuitive type. We describe each one in the 8 personality types according to Carl Jung.
From Eight Types to Sixteen: How the MBTI Built on Jung
Katharine Cook Briggs had been developing her own typology before she discovered Jung’s book in the 1920s. She and her daughter Isabel then spent decades turning his ideas into a questionnaire. Their main changes were these:
| Jung’s typology | The MBTI | |
|---|---|---|
| Basic units | 2 attitudes × 4 functions | 4 preference pairs: E/I, S/N, T/F, J/P |
| Number of types | 8 core types (with auxiliaries implied) | 16 named four-letter types |
| Auxiliary function | Described, but not built into the type name | Built into every type through the J/P letter |
| How you find your type | Clinical observation and self-reflection | A standardised self-report questionnaire |
| Main purpose | Understanding the psyche and its one-sidedness | Practical self-understanding at work and in relationships |
The J/P letter is the key addition. It indicates whether a person’s preferred judging function (T or F) or perceiving function (S or N) is the one they show to the outer world. Combined with E/I, that tells you which function is dominant. Our article on Judging vs Perceiving explains how this works.
What Jung Would and Wouldn’t Recognise in Modern Typing
Jung was cautious about his own system. He stressed that pure types are rare, and that everyone has both attitudes and all four functions available. He described typology mainly as a way to orient yourself and understand differences, not as a box to put people in. He is often quoted as saying that a person who was purely extraverted or purely introverted would not be able to function.
Modern online typing often goes further than Jung did. It assigns fixed four-letter labels, ranks eight functions in a “stack,” and links types to careers or compatibility. Some of these ideas are later developments by type theorists, not things Jung wrote. See our explainer on MBTI cognitive functions for how the function stacks were added.
How Well Does Jung’s Typology Hold Up?
Jung’s typology came from clinical observation and his own reflection, not from statistical testing. Academic personality psychology has largely moved to trait models such as the Big Five, which measure people along continuous dimensions.
Extraversion–introversion is the part of Jung’s model that has lasted best. It survives, in modified form, as one of the Big Five traits. The sensation/intuition and thinking/feeling distinctions show some overlap with Big Five traits such as openness and agreeableness, but they don’t map onto them cleanly. Many people still find Jung’s categories a rich vocabulary for thinking about themselves. Just read them as interpretive ideas, not measured facts.
Common Questions
Which MBTI type was Carl Jung? Jung described himself as an introverted thinking type, with intuition as his next strongest function. People who translate that into MBTI letters usually suggest INTP. Some argue for INTJ. Since Jung never took the MBTI, any four-letter label for him is a guess.
Which type is rarest according to Jung? Jung didn’t publish statistics on how common each type was. Popular claims that a type such as INFJ is “the rarest” come from later MBTI survey data, not from Jung. See what are the 5 rarest MBTI types for those figures.
What was Jung’s “No. 1” personality? That phrase comes from his autobiography, not from his typology. It refers to the everyday, outward-facing side of his own character as a boy. We explain it in what was Jung’s No. 1 personality.
Is the MBTI the same as Jung’s theory? No. The MBTI is a practical adaptation that keeps Jung’s attitudes and functions but adds the J/P letter, fixes 16 types and uses a questionnaire. For an overview of the result, see Myers-Briggs personality types explained.