“Men are apt to mistake the strength of their feeling for the strength of their argument. The heated mind resents the chill touch and relentless scrutiny of logic.”
In other words, when you debate or discuss something, don’t let your passion for your opinion cloud your mind. You can hold an opinion, certainly, but if you will be a critical thinker, then you must stay cool in the discussion and willing to listen. At the very least, you must allow logic and the facts to speak, even if they start to paint a picture that is opposite to your own opinion.