This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong; it cannot perjure itself; it cannot be wholly absent. Only its interpretation can err.
Oath definition, a solemn appeal to a deity, or to some revered person or thing, to witness one's determination to speak the truth, to keep a promise, etc.: to testify upon oath. See more. Dictionary.com
Pauper's oath. A Pauper's oath is a sworn statement or oath by a person that he or she is completely destitute or a pauper, i.e. without any money or property. A person without the ability to pay court costs has the option to swear a pauper's oath in order to file a lawsuit without paying filing fees. Prisoners filing legal actions often use a pauper's oath because persons in prison are often completely without money or any means of acquiring any.