Our body operates in 3D via polarity. This means that movement happens in time and space through the relationship of opposites.
Tense, release. Open, close. Up, down. Left, right. Hard, soft. Negative, positive. Male, female. Etc.
In traditional Chinese philosophy, these are referred to as “yin and yang”. In modern times, we often say “male-female”, “masculine-feminine” in the West.
This wasn’t a theory invented from ego, it was based on nature.