As the MBTI Manual states, the indicator "is designed to implement a theory; therefore the theory must be understood to understand the MBTI".
Fundamental to the Myers-Briggs Type Indicator is the theory of psychological type as originally developed by Carl Jung. Jung proposed the existence of two dichotomous pairs of cognitive functions:
The "rational" (judging) functions: thinking and feeling.
The "irrational" (perceiving) functions: sensing and intuition. Jung believed that for every person each of the functions are expressed primarily in either an introverted or extraverted form. From Jung's original concepts, Briggs and Myers developed their own theory of psychological type, described below, on which the MBTI is based. According to Myers and Briggs, people use all four cognitive functions. However, one function is generally used in a more conscious and confident way. This dominant function is supported by the secondary (auxiliary) function, and to a lesser degree the tertiary function. The fourth and least conscious function is always the opposite of the dominant function. Myers called this inferior function the shadow.
The four functions operate in conjunction with the attitudes (extraversion and introversion). Each function is used in either an extraverted or introverted way. A person whose dominant function is extraverted intuition, for example, uses intuition very differently from someone whose dominant function is introverted intuition. However, many MBTI practitioners hold that the tertiary function is oriented in the same direction as the dominant function. Using the INTP type as an example, the orientation would be as follows: Dominant introverted thinking.
Auxiliary extraverted intuition.
Tertiary introverted sensing.
Inferior extraverted feeling.