A helpful second or third function, according to Jung’s model of typology, that has a co-determining influence on consciousness.
Absolute sovereignty always belongs, empirically, to one function alone, and can belong only to one function, because the equally independent intervention of another function would necessarily produce a different orientation which, partially at least, would contradict the first.
But since it is a vital condition for the conscious process of adaptation always to have clear and unambiguous aims, the presence of a second function of equal power is naturally ruled out. This other function, therefore, can have only a secondary importance. . . .
Its secondary importance is due to the fact that it is not, like the primary function . . . an absolutely reliable and decisive factor, but comes into play more as an auxiliary or complementary function.[“General Description of the Types,” CW 6, par. 667.]
The auxiliary function is always one whose nature differs from, but is not antagonistic to, the superior or primary function: either of the irrational functions (intuition and sensation) can be auxiliary to one of the rational functions (thinking and feeling), and vice versa.
Thus thinking and intuition can readily pair, as can thinking and sensation, since the nature of intuition and sensation is not fundamentally opposed to the thinking function. Similarly, sensation can be bolstered by an auxiliary function of thinking or feeling, feeling is aided by sensation or intuition, and intuition goes well with feeling or thinking.
The resulting combinations [see figure below] present the familiar picture of, for instance, practical thinking allied with sensation, speculative thinking forging ahead with intuition, artistic intuition selecting and presenting its images with the help of feeling-values, philosophical intuition systematizing its vision into comprehensive thought by means of a powerful intellect, and so on.[Ibid., par. 669.]