Institutio Oratoria

If, however, anyone should take this course, he will defend his action by appealing to the blamelessness of his life

as rendering perjury on his part incredible, or by the solemn nature of the oath, with regard to which he will win all the greater credence, if without the least show of eagerness to take the oath he makes it clear that he does not shrink from so solemn a duty. Or again, if the case is such as to make this possible, he will rely on the trivial nature of the point in dispute to win belief, on the ground that he would not incur the risk of the divine displeasure when so little is at stake. Or, finally, he may in addition to the other means which he employs to win his case offer to take an oath as a culminating proof of a clear conscience.