Primary function:

The psychological function that is most differentiated. (Compare inferior function.)

In Jung’s model of typology, the primary or Superior Function is the one we automatically use because it comes most naturally.

Experience shows that it is practically impossible, owing to adverse circumstances in general, for anyone to develop all his psychological functions simultaneously. The demands of society compel a man to apply himself first and foremost to the differentiation of the function with which he is best equipped by nature, or which will secure him the greatest social success. Very frequently, indeed as a general rule, a man identifies more or less completely with the most favored and hence the most developed function. It is this that gives rise to the various psychological types.[“Definitions,” CW 6, par. 763.]

In deciding which of the four functions-thinking, feeling, sensation or intuition-is primary, one must closely observe which function is more or less completely under conscious control, and which functions have a haphazard or random character. The superior function (which can manifest in either an introverted or an extraverted way) is always more highly developed than the others, which possess infantile and primitive traits.
The superior function is always an expression of the conscious personality, of its aims, will, and general performance, whereas the less differentiated functions fall into the category of things that simply “happen” to one.[“General Description of the Types,” ibid., par. 575.]

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