Imagine walking into class late and your teacher announces it in front of everyone. How do you feel? Embarrassed? Defensive? Do you resolve to work harder — or do you feel a flash of resentment? Criticism almost always produces the second response, not the first. Carnegie's first principle is direct: do not criticise, condemn, or complain. This is not soft advice. It is backed by psychology. B. F. Skinner, one of the most respected psychologists of the twentieth century, proved through experiments that animals rewarded for good behaviour learn faster than animals punished for bad behaviour. The same applies to people. When you criticise someone, here is what actually happens: the person gets defensive. Their pride is wounded. They start building arguments against you in their head. Even if you are completely right, they are now spending their energy resisting you rather than improving. Abraham Lincoln understood this the hard way. As a young man, he wrote sharp, mocking letters about political opponents. One of those letters led a man to challenge him to a duel. Lincoln nearly died — over words written in anger. From that day, he almost never criticised anyone for anything. During the Civil War, one of his generals lost a golden opportunity by refusing to follow Lincoln's clear orders. Lincoln was furious. He sat down and wrote a devastating letter — then put it in a drawer and never sent it. He had learned that sending it would accomplish nothing except make the general defensive and damage their working relationship. The letter was found among Lincoln's papers after his death. It was never sent. How do you change people's behaviour, then? Carnegie's answer: try to understand why they do what they do. Everyone has a reason that makes complete sense to them. Try to see it. Instead of condemning Chidi for arriving late, ask what is making it hard for him to arrive on time. Instead of criticising Aisha for being distracted, consider what might be weighing on her. Any fool can criticise, condemn, and complain — and most fools do. It takes real character to try to understand.