Once a person has experienced the problem of the functions, the next step in the process of psychic development is to assimilate the two auxiliary functions. One must not forget that the assimilation of these functions is such a difficult task that people generally spend a very long time at it. Sometimes people actually become a certain type, which was not their original type, for eight or ten years.
To assimilate a function means to live with that function in the foreground. If one does a little cooking or sewing, it does not mean that the sensation function has been assimilated. Assimilation means that the whole adaptation of conscious life, for a while, lies on that one function. Switching over to an auxiliary function takes place when one feels that the present way of living has become lifeless, when one gets more or less constantly bored with oneself and one's activities. Generally it happens that one does not have to come to a theoretical conclusion about which function to switch to. The best way to know how to switch is simply to say: "All right, all this is now completely boring; it does not mean anything to me anymore. Where in my past life is an activity that I feel I could still enjoy? An activity out of which I could still get a kick?" If a person then genuinely picks up that activity, he will see that he has switched over to another function.
However, touching the Inferior function resembles an inner breakdown at a certain crucial point of one's life. It has the advantage, however, of overcoming the tyranny of the dominant function in the ego complex. If someone has really gone through this transformation he can use his thinking function, if that is the appropriate reaction, or he can let intuition or sensation come into operation, but he is no longer possessed by one dominant function.
Jung quotes again and again this old saying of the legendary alchemist and author Maria Prophetissa: "One becomes two, two becomes three, and out of the third comes the one as the fourth."...Out of the third comes not the fourth but the One