5 ideas
Aristotle asserted that women have fewer teeth than men, and the claim went unchallenged for centuries even though anyone could have counted. He never checked, though he could have asked his wife to open her mouth. The case shows that deference to authority and prior assumption can override the simplest available observation.
If you become furious when someone contradicts your opinion, suspect that you have no good reason for it. People rarely rage over things they can demonstrate, such as arithmetic, because they can simply show the answer. Emotional heat therefore marks a belief held on unexamined or unsupported grounds.
To counter your own bias, seek out the opinions of other countries, classes, and parties, for example by reading the newspaper of the side you oppose. Where direct contact is impossible, imagine arguing with someone who holds the opposite prejudice. Seeing that sincere people hold contrary views loosens the illusion that your own view is simply obvious.
Beliefs in the superiority of your own nation, sex, class, or species are held because they flatter self-esteem, not because evidence supports them. Every group reaches the same convenient conclusion about itself, and that alone shows the reasoning is driven by vanity. Any belief that makes you feel superior should be treated with heightened suspicion.
Much superstition, persecution, and mass credulity comes from fear, which pushes people toward comforting dogmas and toward scapegoats. Frightened crowds adopt beliefs that promise safety or assign blame, whether or not those beliefs are true. Mastering fear is therefore a precondition for thinking sanely, both individually and collectively.