When setting out to determine someone’s type, don’t think of the functions as eight individual, separate functions, but as four function axes.
One of Jung’s influences was the Greek philosopher Heraclitus (535-475 BCE), who believed in the unity of opposites. He is credited with saying, as one translation puts it: “The road up and the road down are the same road.”
We see this idea reflected in the dichotomies of Jung’s typology. Here, Se is the opposite of Ni, and Ne is the opposite of Si, but since they are interdependent opposites, they do not operate independently of each other. They are, rather, two opposing ends on the same axis, or two different directions on the same road. Ni is not an individual entity that is simply completely different from Se. Rather, Ni exists by virtue of Se and vice versa: As the theory of function axes would have it, each has no meaning apart from the other.According to Heraclitus, any pair of opposites exists by virtue of each other: Without white, we cannot comprehend black. Without warmth, we cannot understand cold. Blackness and cold would be without meaning. No matter what direction on the road is preferred (white or black; warm or cold; up or down; Se or Ni), there are some characteristics of the road itself that will manifest in a person’s psyche.
In Jungian typology, these roads are Se/Ni, Ne/Si, Te/Fi, and Fe/Ti.
They are the four function axes.