In popular literature the word "balance" usually means "you must balance work and rest, reason and feeling, masculine and feminine". Sounds nice. Explains nothing. Because balance is not a point you can step into and stay at. Balance is a regime — a mode the system enters and keeps falling out of, then returning to.
The engineering name for that is feedback.
Homeostasis: engineering inside the body
Claude Bernard, in 1865, stated it clearly: the internal environment of the organism stays constant despite external change. Body temperature — 37 °C ± 0.5. Blood pH — 7.35–7.45. Glucose concentration — 4–6 mmol/L.
Those numbers hold not because the body "found balance and stood in it". They hold because dozens of feedback loops continuously measure deviations and compensate. Temperature drops — shivering, vasoconstriction. Rises — sweating, vasodilation. Glucose up — insulin. Glucose down — glucagon.
A system with a regulator, rather than "opposites in harmony". There is a setpoint, there are sensors, there are effectors. When sensors fail (diabetes), the regulator collapses, and no "inner wisdom" remains in reserve — you need insulin from outside and external control.
Cybernetics: same for any system
Norbert Wiener (MIT, Cybernetics, 1948) showed that any system with negative feedback behaves the same way: measure deviation, apply correction, return to setpoint. Thermostat. Heartbeat. Market price. Political opposition stabilising power. One mathematics.
A useful conclusion: "balance" is the dynamics of return, rather than stasis. Frequency of returns, amplitude of deviations, response delay — these describe the state of the system.
Feedback too fast — the system oscillates (visible in a swaying crane). Too slow — drifts (visible in a neglected political regime). The interesting range lives narrowly between those extremes.
What it means for a human
Two things.
First: "balance" means "return", not "middle". A system that never deviates is dead — it has nothing to stabilise. Life, by definition, is a sequence of deviations with returns. Stability is measured by the ability to damp oscillations, not by their absence.
Second: different systems run at different regulation speeds. Body temperature — seconds. Body weight — months. Psychological state — from minutes to years. Mixing up the speeds is the source of most errors. Trying to "fix" depression in a day works about as well as heating a room with a thermometer.
The honest edge
In biology, homeostasis is a measurable engineering property. In psychology, "emotional balance" has neurological correlates (prefrontal cortex modulates the limbic system), but a precise cybernetic model is only beginning to emerge. In social systems, it is still murkier.
The general principle is the same everywhere. Feedback with the right time constant. Where it exists — stability. Where it doesn't — catastrophe.
"Balance of opposites" is a poetic way of saying "stable regime with correctly tuned feedback loops". The poetic version is easier to remember. The engineering version works.