In legal proceedings, generally, facts that rely upon an individual's memory or knowledge are most reliably proven by having the person give testimony in court: he appears in person before a judge at a time and place known to other interested persons, swears that his testimony will be true, states his testimony so that all can hear it, and can be cross-examined by opposing parties.
Such a procedure, although maximizing fairness and the likelihood of obtaining the truth, is expensive, troublesome, and time-consuming.
Traditionally, that has required an affidavit: the person must put his testimony into written form and then sign the document in front of an official, such as a notary public or clerk, swearing to the official that the contents of the document are true.
Such an affidavit has several advantages over simple signed testimony: In recent years, however, to provide for even greater economy of time and money, courts have increasingly allowed persons to omit the step of swearing before a notary public or official.
The federal courts and a few states have general statutes allowing a sworn declaration in any matter where an affidavit can be used.