There are hundreds of prejudices, hundreds of forms of prejudice. There is, for instance, the prejudice of conservatism, which manifests itself in a vague fear that if the present order were changed in any particular—if women were given the vote, if socialism were to triumph, if a new filing system were to be installed at the office—all would be lost. But I cannot deal adequately with all the forms of bias which flock to mind. The distinguishing mark of the great thinkers of the ages was their comparative freedom from the prejudices of their time and community. In order to avoid these prejudices one must be constantly and uncompromisingly sounding his own opinions. Eternal vigilance is the price of an open mind. * * * * * Prejudice is not the only danger which lies in wait for the would-be thinker. In his very efforts to get rid of prejudice he is liable to fall into an even greater intellectual sin. This sin is uncertainty. As uncertainty and doubt are nearly synonymous, the reader will probably be surprised at this statement because of the praise I have hitherto accorded to the doubtful attitude. But the doubtful attitude, necessary and praiseworthy as it is, should not be maintained always. We think in order to have opinions. We have opinions in order to guide action; in order to act upon should occasion require. Herbert Spencer, even after his remarks quoted at the beginning of this chapter, which imply the need of extreme caution, adds, “. . . In daily life we are constantly obliged to act out our inferences, trustless as they may be— . . . in the house, in the office, in the street, there hourly arise occasions on which we may not hesitate; seeing that if to act is dangerous, never to act at all is fatal. . . .” There are other reasons why we cannot afford to keep the doubtful attitude. If our lives were interminable, if we had limitless time for thinking, we could afford to remain in doubt indefinitely. But life is fleeting. So if you have examined facts obtainable on such a question as psychic phenomena, have kept your mind open for a certain time, and have decided that communication with the dead is impossible, you are justified in discontinuing to look for evidence on that question. Every hour devoted to examining such evidence would be an hour taken away from thought on some other subject, and the law of diminishing returns is just as applicable in thinking as in economics.