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.
For all the types appearing in practice, the principle holds good that besides the conscious main function there is also a relatively unconscious, auxiliary function which 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 [p. 516] 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.
