Early in the 20th Century, the Swiss pioneer psychologist C. G. Jung developed the model that he called “psychological type” (now often called “personality type” or just “type”), based on his clinical work with his clients. The model is based upon a few simple theoretical principles that Jung drew from his observations:

• There are two key mental tasks that are crucial to our survival, and which account for most of our normal mental activity: taking-in / processing information and making decisions.
• For each of these tasks there are two very different approaches available to us. Information gathering (“Perception”) can focus on the concrete information gathered by the senses (Jung labeled this approach “Sensation”) or the focus can be on the meaning, implications, extrapolations, ideas, etc. that come from those bits of concrete information (called “Intuition”). Decision-making (“Judgment”) can be accomplished using objective logical analysis (“Thinking”) or can be based on evaluation of the impact (“Feeling”).
• Each of the psychological functions of Sensation, Intuition, Thinking, and Feeling can be used in two very distinct ways. They can either have an internal (“Introverted”) orientation or an external (“Extraverted”) one.
• Every human being, early in life, develops a preference for and a reliance on some of these ways of perceiving and judging over other approaches. It is these preferences which lead both to the infinite diversity of individual personalities and to the common personality themes that we all share—and which are described by the model we call psychological type.