To avoid the various foolish opinions to which mankind is prone, no superhuman genius is required. If the matter is one that can be settled by observation,make the observation yourself.
Many matters, however, are less easily brought to the test of experience. If, like most of mankind, you have passionate convictions on many such matters,there are ways in which you can make yourself aware of your own bias. If an opinion contrary to your own makes you angry, that is a sign that you are subconsciously aware of having no good reason for thinking as you do.
If someone maintains that two and two are five, or that Iceland is on the equator, you feel pity rather than anger, unless you know so little of arithmetic or geography that his opinion shakes your own contrary conviction. The most savage controversies are those about matters as to which there is no good
evidence either way. Persecution is used in theology, not in arithmetic, because in theology there is only opinion. So wheneveryou find yourself getting angry about a difference of opinion, be on your guard; you will probably find, on examination, that your belief is going beyond what the evidence warrants.
A good way of ridding yourself of certain kinds of dogmatism is to become aware of opinions held in social circles different from your own. If the people and the newspaper seem mad, perverse, and wicked, remind yourself that you seem so to them. In this opinion both parties may be right, but they cannot both be wrong. This reflection should generate a certain caution.
For those who have enough psychological imagination, it is a good plan to imagine an argument with a person having a different bias. This has one advantage, and only one, as compared with actual conversation with opponents; this one advantage is that the method is not subject to the same limitations of time and space.