Institutio Oratoria
Quintilian
Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.
We must therefore discover what motives they have for doing our opponent a hurt, and the fact that they were once his enemies will not suffice our purpose: we must find out whether they have ceased to be ill-disposed to him or whether they desire by means of their evidence to effect a reconciliation with him, in order to assure ourselves that they have not been bribed or repented of
For experience shows that they are more likely to repent of their purpose, their promises are less to be relied on, and, if they do keep their promise, their evidence is easier to refute.