Jung's Map of the Soul: An Introduction
It should be noted that Jung makes a clear distinction between the progression and regression of libido on the one hand and the attitudes of extroversion and introversion on the other.It is easy for the beginner to confuse them. Introverts progress in their own way, adapting to the world in an introverted fashion, while extroverts progress in an extroverted way. The same holds true for regression. For example, an extroverted-thinking type person, who has habitually used thinking to deal with the world and to manipulate people, comes up against a situation in life where that function does not work very effectively and experiences defeat. Relationship problems cannot as rule be solved by extroverted thinking. Here a totally different approach is needed. When this person's superior function is rendered useless, a sense of frustration and defat takes over, for now suddenly other functions are demanded and these are not readily available. So the libido regresses and typically activates the inferior function, in this case the introverted-feeling function. As Jung pointed out, the inferior function is unconscious and carries the slime of the murky depths with it when it comes up into consciousness. An introverted-feeling function is a tool of the ego and is refined, discriminating and rational function that orients one by establishing values. An inferior undifferentiated feeling ffunction surging up from the unconscious, however, provides only a small amount of guidance about values, but rather screams in bright red letters: "This is the most important thing in my whole life! I cannot live without it!" It is highly emotional. The inferior function's lack of adaptive skill is generally all too evident, but the ego is challenged to use emotions and thoughts that come into consciousness in this fashion, and by doing so it begins the task of adapting to the hidden side of the personality, the unconscious.
By contrast, people who get a lot of mileage in the first half of their lives out of their ability to relate well to other people reach a point where this no longer satisfies them. The highly developed extroverted-feeling function does not feed the soul anymore. Other potentials demand to be realized. Perhaps introverted intuitive-thinking projects (studying philosophy or theology) beckon and seem more attractive than another luncheon with friends or one more family gathering over the holidays. The full human life span has many periods of significant transformation.