Quintilian, and J.S. Watson. Institutes of Oratory.Edited by Lee Honeycutt, 2010. Kindle Electronic Edition. Book V Chapter 6.
In a legal case oaths and testimonies are commonly solicited on both sides. They are sometimes refused as well (Quintilian V.6.1). The person who refuses an oath may be able to show his reliability without an oath, in which case the testimony becomes very credible (Quintilian V.6.2). Since the oath is a divine promise it is very serious (Quintilian V.6.3). In general, if people are bound by oaths the conduct of the case is easier as it is rarely violated (Quintilian V.6.5). Quintilian states that oaths were never considered optional in his youth (Quintilian V.6.6).