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 which 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 in con~sdousness. But, in such a case, there is also no question of a differentiated 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 primary function, can readily pair with intuition as auxiliary, or indeed equally well with sensation, but, as already observed, never with feeling.
Neither intuition nor sensation are antagonistic to thinking, i.e. they have not 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, which 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 favour 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.