The belief that one can find out something about real things by speculation alone is one of the most long-lived delusions in human thought. It is the spirit of anti-science which is always trying to lead men away from the study of reality to the spinning of fanciful theories out of their own minds. It is the spirit which every one of us (whether he is engaged in scientific investigation or in deciding how to use his vote in an election) must cast out of his mind.