The document discusses the 90/10 principle, which states that 10% of life is made up of external events outside of our control, while 90% is determined by our reactions to what happens. It gives an example of how reacting negatively to a daughter spilling coffee can ruin the whole day, while responding positively avoids stress. The principle suggests we cannot control what happens but can control our reactions, and choosing to respond well to challenges like insults or traffic keeps us from being stressed or losing relationships. Applying this principle of not letting external events affect our inner state can significantly improve our lives.