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
A promissory oath is an oath taken, by authority of law, by which the party declares that he will fulfill certain duties therein mentioned, as the oath which an alien takes on becoming naturalized, that he will support the constitution of the United States: the oath which a judge takes that he will perform the duties of his office.