Swiss psychologist Carl Jung published his theory of Psychological Types in 1921.
Jung proposed that each person has a preferred Orientation for directing mental energy, either outward, toward people and things – (E) Extraversion, or inward, toward ideas and concepts – (I) Introversion.
He also proposed two methods of gathering information and two ways of making decisions about that information.
Information gathering by means of the five senses (S) Sensing
Information gathering by means of insight (N) Intuiting
Making decisions about that information objectively (T) Thinking
Making decisions about that information subjectively (F) Feeling
Jung believed that we use all of these mental mechanisms, with a relative preference for one of each pair.
Everyone possesses both mechanisms, extraversion as well as introversion, and only the relative predominance of one over the other determines the type.
Carl JungPsychological Types p.4
Outer circumstances and inner disposition frequently favor one mechanism and restrict or hinder the other.
Carl JungPsychological Types p.6
Jung admitted spending ridiculous amounts of time deliberating over the terms used to define the mental mechanisms. He asserted that the names are somewhat arbitrary and could in some cases give the wrong impression. Jung believed that individuals could benefit from understanding these concepts.
Jung likened the mental mechanisms to an internal compass which helps to guide us throughout life.