Where possible, direct observation normally enables us to achieve a higher degree of certainty than the testimony of a witness can provide us. Likewise, the testimony of a firsthand witness is more reliable than that of a secondhand witness to whom the first has confided his or her experience, and so on. The greater reliability of direct witnesses is the chief reason why hearsay evidence is usually inadmissible in court. In any case, we cannot automatically take witnesses (even "expert" witnesses) at their word, but must use our own judgment to evaluate their credentials. Thus even such indirect knowledge ultimately must rely upon one's own reasoning and objective observation.

An "authority" whom one accepts merely on faith, or merely because one is coerced into acceptance by others, cannot add to one's objective knowledge, because no logical grounds have been established for credibility. Here the term "faith" refers to belief not derived by a process of logic from one's experience of reality. Such belief is clearly different in kind—and therefore conceptually distinct—from knowledge derived by the objective process we have described.