Historically, mankind has employed four methods to acquire knowledge:
Authority. When using authority, something is considered true because of tradition or some person of distinction says it is true. We may believe in the theory of eveolution because our distinguished professors tell us so, or we may believe God exists because our parents say so. This method is very biased and sometimes leads to grave error, however it is indispensible to living our daily lives. We simply must accept a large amount of information on the basis of authority, if for no other reason than we often do not have the time or the expertise to check it out firsthand. For example, I believe, on the basis of physics authorities, that electrons exist, even if I have never seen one. If the Surgeon General says smoking causes cancer, I stop smoking because I have faith in the Surgeon General and do not have time to investigate the matter personally.