To avoid the various foolish opinions to which mankind is prone, no superhuman brain is required. A few simple rules will keep you away, not from all errors, but from silly errors. If the matter is one that can be settled by observation, make the observation yourself. Aristotle could have avoided the mistake of thinking that women had fewer teeth than men, by the simple device of asking Mrs. Aristotle to keep her mouth open while he counted. Thinking that you know when in fact you do not, is a bad mistake, to which we are all prone. I believe myself that hedgehogs eat black beetles, because I have been told that they do; but if I was writing a book on the habits of hedgehogs, I should not commit myself until I had seen one enjoying this diet. Aristotle, however, was less cautions. Ancient and medieval writers knew all about unicorns and salamanders; not one of them thought necessary to avoid dogmatic statements about them because they had never seen one of them. Many matters, however, are less easily brought to the test of experience. If, like most of mankind, you have strong convictions on many such matters, there are ways in which you can learn 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 says 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 won contrary conviction. The most savage controversies are those about the kind of opinion s to which there is no good evidence, either way. Persecution is used in theology, not in arithmetic, because in arithmetic there is knowledge, but in theology there is only opinion. So whenever you find yourself getting angry about a difference of opinion, be on the alert; 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.