I've been thinking recently: in a lot of MBTI and Socionics tests, whatever form, they often start out with measuring E/I, followed by S/N, T/F and finally J/P. Jung himself started with differentiating humans between E/I types. I myself don't believe in E/I, these are just aspects of functions (S/N/T/F).

I don't know the reasons yet, but I have a hunch that E/I is not the primary divider of people, but J/P, or better: rationality/irrationality. Does this make sense? Is there a primary divider of cognitive styles? Why or why not?