Institutio Oratoria

Further there is always a certain tacit prejudice against documentary evidence, since no one can be forced to give such evidence save of his own free will, whereby he shows that he harbours unfriendly feelings towards the person against whom he bears

witness. On the other hand an advocate should be chary of denying that a friend may give true evidence against a friend or an enemy against an enemy, provided they are persons of unimpeachable credit. But the subject admits of copious discussion, from whichever side it be regarded.