Institutio Oratoria

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

their previous attitude and changed their purpose. Such precautions are necessary even with witnesses who know that what they propose to say is true; but it is still more necessary with those who promise to give false evidence.