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
pledge (plural pledges) A solemn promise to do something. Something given by a person who is borrowing money etc to the person he has borrowed it from, to be kept until the money etc is returned. A person who has taken a pledge of allegiance to a college fraternity, but is not yet formally approved.