What personality classifications have in common

There are many ways to divide people into four personality types, from the classical-sanguine, choleric, melancholic, and phlegmatic-to contemporary systems such as the DISC profile. The Myers-Briggs system divides people into sixteen personality types. I just recently ran across the "enneagram," an ancient system for dividing people into nine categories.

There's one thing advocates of all the aforementioned systems agree on: the number of basic personality types is a perfect square.