The following is Jung’s description from the Classics in the History of Psychology. For those who have found hid work hard to followd, I have edited the information without hopefully diluting the meaning. Below is a syllabus of words that have been changed per my own translation. If you have read Jung and see that my translation is in error, please let me know:
Conscious = Dominant Function
Unconscious = Auxiliary Function
Attitude = Extravert/Introvert
Object = External
Subject = Internal
11. The Principal and Auxiliary Functions
In the foregoing descriptions I have no desire to give my readers the impression that such pure types occur at all frequently in actual practice. The are, as it were, only family-portraits, which sum up in a cumulative image the common and therefore typical characters, stressing these disproportionately while the individual features are just as disproportionately effaced. Accurate investigation of the individual case consistently reveals the fact that in conjunction with the most differentiated function, another function of secondary importance, and therefore of inferior differentiation in consciousness, is constantly present, and is a -- relatively determining factor.
For the sake of clarity let us again recapitulate: The products of all the functions can be conscious, but we speak of the dominance of a function only when not merely its application is at the disposal of the will but when at the same time its principle is decisive for the orientation of consciousness. The latter event is true when for instance, thinking is not a mere esprit de l'escalier, or rumination, but when its decisions possess an absolute validity, so that the logical conclusion in a given case holds good, whether as motive or as guarantee of practical action, without the backing of any further evidence. This absolute sovereignty always belongs, empirically, to one function alone, and can belong only to one function, since the equally independent intervention of another function would necessarily yield a different orientation, which would at least partially contradict the first.
But since it is a vital condition for the conscious adaptation-process that constantly clear and unambiguous aims should be in evidence, the presence of a second function of equivalent power is naturally forbidden. This other function therefore, can have only a secondary importance, a fact that is also established empirically. Its secondary importance consists in the fact that in a given case, it is not valid in its own right, as is the primary function as an absolutely reliable and decisive factor. But comes into play more as an auxiliary or complementary function. Naturally only those functions can appear as auxiliary whose nature is not opposed to the leading function. For instance, feeling can never act as the second function by the side of thinking, because its nature stands in too strong a contrast to thinking. Thinking, if it is to be real thinking and true to its own principle, must scrupulously exclude feeling.
This of course, does not exclude the fact that individuals certainly exist in whom thinking and feeling stand upon the same level, whereby both have equal motive power. But in such a case, there is also no question of a different type, but merely of a relatively undeveloped thinking and feeling. Uniform consciousness and unconsciousness of functions is therefore, a distinguishing mark of a primitive mentality.
Experience shows that the secondary function is always one whose nature is different from, though not antagonistic to, the leading function. Thus for example, thinking as a dominant function, can readily pair with intuition or indeed equally well with sensation as an auxiliary but, as already observed, never with feeling. Neither intuition or sensation are antagonistic to thinking. They do not have to be unconditionally excluded since they are not like feeling, of similar nature though of opposite purpose, to thinking. For as a judging function feeling successfully competes with thinking, but are functions of perception, affording welcome assistance to thought. As soon as they reached the same level of differentiation as thinking, they would cause a change of attitude that would contradict the tendency of thinking. For they would convert the judging attitude into a perceiving one whereupon the principle of rationality indispensable to thought would be suppressed in favor of the irrationality of mere perception. Hence the auxiliary function is possible and useful only in so far as it serves the leading function, without making any claim to the autonomy of its own principle.
For all the types appearing in practice, the principle holds good that besides the dominant function there is also a relatively unconscious auxiliary function that is in every respect different from the nature of the main function. From these combinations well-known pictures arise, the practical intellect for instance paired with sensation, the speculative intellect breaking through with intuition, the artistic intuition which selects and presents its images by means of feeling judgment, the philosophical intuition which, in league with a vigorous intellect, translates its vision into the sphere of comprehensible thought, and so forth.
A grouping of the lesser functions also takes place in accordance with the relationship of the dominant functions. Thus for instance, an unconscious intuitive and feeling attitude may correspond with a dominant practical intellect whereby the function of feeling suffers a relatively stronger inhibition than intuition. This peculiarity however, is of interest only for one who is concerned with the practical psychological treatment of such cases. But for such a man it is important to know about it. But the approach to the unconscious and to the most repressed function is disclosed as it were, of itself, and with more adequate protection of the dominant standpoint when the way of development is via the secondary function, thus in the case of a rational type by way of the irrational function. For this lends the dominant standpoint such a range and prospect over what is possible and imminent that consciousness gains an adequate protection against the destructive effect of the auxiliary function. Conversely a perceiving type demands a stronger development of the judging auxiliary function represented in consciousness in order to be sufficiently prepared to receive the impact of the auxiliary. The auxiliary functions are in an archaic, animal state.