The document discusses the 90/10 principle, which states that 10% of life is made up of events outside our control, while 90% is determined by our reactions to those events. It provides an example of how reacting negatively to a spilled cup of coffee can ruin the whole day, while reacting positively avoids that outcome. The principle suggests we cannot control what happens but can control our responses, and applying it can reduce stress and improve life by not letting uncontrolled events negatively affect us.