"I have often been asked, almost accusingly, why I speak of four functions and not of more or fewer. That there are exactly four was a result I arrived at on purely empirical grounds. But as the following consideration will show, these four together produce a kind of totality. Sensation establishes what is actually present, thinking enables us to recognize its meaning, feeling tells us its value, and intuition points to possibilities as to whence it came and whither it is going in a given situation..somewhat like the four points of the compass..provid[ing] a system of comparison and orientation..." [Italics mine]

From Psychological Types (1976 Edition: paragraph 958-59) by C. G. Jung (1931)