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
Understanding the Oath of Allegiance. The US oath of allegiance is a verbal statement by which citizens agree to abide by the laws and constitution of the United States, and agree to give up allegiances to other foreign countries. The US oath of allegiance is spoken out loud at citizenship ceremonies, when citizenship applicants become US citizens.