Institutio Oratoria

The means for securing the acquittal of an accused person are strictly limited. His innocence may be established, some superior authority may intervene, force or bribery may be employed, his guilt may be difficult to prove, or there may be collusion between the advocates. You admit that he was guilty; no superior authority intervened, no violence was used and you make no complaint that the jury was bribed, while there was no difficulty about proving his guilt. What conclusion is left to us save that there was collusion?