The history of the MBTI begins with Carl Jung’s groundbreaking work in the early 20th century, particularly his 1921 book "Psychological Types". In this work, Jung introduced the idea that people have innate preferences in how they perceive the world and make decisions. These preferences were organized into two attitudes—introversion and extroversion—and four functions: thinking, feeling, sensing, and intuition. While Jung’s theory provided the conceptual framework, it was not yet a structured system designed for practical application.

In the 1940s, Katharine Cook Briggs, an avid reader and amateur psychologist, became deeply interested in Jung’s theories. She recognized their potential for helping people understand themselves and others. Her daughter, Isabel Briggs Myers, shared her interest and decided to develop a tool that could apply Jung’s ideas in everyday life. Their goal was to make personality theory accessible and practical for individuals and organizations.

During World War II, Briggs and Myers began formalizing their ideas, believing that understanding personality types could help individuals find roles where they would be most effective and satisfied. This was particularly important during the war effort, as people were being placed into jobs based on their perceived strengths. Myers refined the original framework by introducing additional categories, such as Judging vs. Perceiving, and developed a questionnaire to assess personality type.

The resulting tool became the Myers-Briggs Type Indicator (MBTI), first published in the 1940s. Over the next several decades, Isabel Briggs Myers continued to refine and validate the instrument, seeking to ensure its scientific reliability and practical utility. The MBTI gained traction in the 1960s and 1970s, particularly in the fields of education, counseling, and workplace dynamics.

By the late 20th century, the MBTI had become one of the most widely used personality assessments in the world, offering a structured approach to understanding Jung’s theories. Its legacy continues to grow, with millions of people using the tool to explore their cognitive preferences, improve communication, and build stronger relationships.

How does MBTI work?

MBTI is the system that measures one aspect of a person's individuation in the form of cognitive functions; these serve to understand how an individual engages with the external and internal world. This is what is characterised as intraversion and extraversion in MBTI.

Extraversion is when an individual interacts with the external world. This can take two forms, the first is the perception meaning the person does this evaluating the information presented to them, rather experiencing it and analysing it internally; this is called a perceiving function. Then we have judgment, which is the second form of how a person acts, where one makes judgements and selects information; a judging function.
In a way one might see it as a sword and a shield, where one doesn’t directly engage with the information but absorbs and deflects and the other parries and slices through it.
The same principle can be applied to the realm of intraversion. The difference between this one is that unlike extraversion, it pushes inwards and engages with internal information instead of external information.

Codes 

ISFJ      Si - Fe - Ti - Ne

ISTJ      Si - Te - Fi - Ne

ESFJ     Fe - Si - Ne - Ti

ESTJ     Te - Si - Ne - Fi

ISFP     Fi - Se - Ni - Te

ISTP     Ti - Se - Ni - Fe

ESFP    Se - Fi - Te - Ni 

ESTP    Se - Ti - Fe - Ni

INFJ     Ni - Fe - Ti - Se

INFP    Fi  - Ne - Si - Te

ENFJ    Fe - Ni - Se - Ti

ENFP   Ne - Fi - Te - Si

INTJ     Ni - Te - Fi - Se 

INTP    Ti - Ne - Si - Fe

ENTJ    Te - Ni - Se - Fi

ENTP   Ne - Ti - Si - Fe