Harmony can be described as the natural resting point between any two opposites. For example, the uncontainable and the contained.
Duality seems to be an intrinsic part of human nature. We gravitate towards categories that seem "whole"; like right or wrong, but find that the nuance between opposites is where truth, if there can be such a thing, lies. We find harmony in between. There are examples of this desired middle path in our bodies: between our two eyes lay the third eye, the place of a deeper sight, and while our sex organs come in pairs - ovaries and testicles - they become one in our penises and vaginas, those beautiful inner and outer shapes.
The thresholds between inner and outer, between dark and light, are rich places for noticing.