of juridical discipline they add to the readiness and sagacity of those who are called to plead or to judge.  But as human affairs and human actions are not of a metaphysical nature, but the subject is concrete, complex, and moral, they cannot be subjected (without exceptions which reduce it almost to nothing) to any certain rule.  Their rules with regard to competence were many and strict, and our lawyers have mentioned it to their reproach.  “The Civilians,” it has been observed, “differ in nothing more than admitting evidence; for they reject histriones, &c., and whole tribes of people."[44] But this extreme rigor as to competency, rejected by our law, is not found to extend to the genus of evidence, but only to a particular species,—­personal witnesses.  Indeed, after all their efforts to fix these things by positive and inflexible maxims, the best Roman lawyers, in their best ages, were obliged to confess that every case of evidence rather formed its own rule than that any rule could be adapted to every case.  The best opinions, however, seem to have reduced the admissibility of witnesses to a few heads.  “For if,” said Callistratus, in a passage preserved to us in the Digest, “the testimony is free from suspicion, either on account of the quality of the person, namely, that he is in a reputable situation, or for cause, that is to say, that the testimony given is not for reward nor favor nor for enmity, such a witness is admissible.”  This first description goes to competence, between which and credit Lord Hardwicke justly says the discrimination is very nice.  The other part of the text shows their anxiety to reduce credibility itself to a fixed rule.  It proceeds, therefore,—­“His Sacred Majesty, Hadrian, issued a rescript to Vivius Varus, Lieutenant of Cilicia, to this effect, that he who sits in judgment is the most capable of determining what credit is to be given to witnesses.”  The words of the letter of rescript are as follow:—­“You ought best to know what credit is to be given to witnesses,—­who, and of what dignity, and of what estimation they are,—­whether they seem to deliver their evidence with simplicity and candor, whether they seem to bring a formed and premeditated discourse, or whether on the spot they give probable matter in answer to the questions that are put to them.”  And there remains a rescript of the same prince to Valerius Verus, on the bringing out the credit of witnesses.  This appears to go more to the general principles of evidence.  It is in these words:—­“What evidence, and in what measure or degree, shall amount to proof in each case can be defined in no manner whatsoever that is sufficiently certain.  For, though not always, yet frequently, the truth of the affair may appear without any matter of public record.  In some cases the number of the witnesses, in others their dignity and authority, is to be weighed; in others, concurring public fame tends to confirm the credit of the evidence in question.  This alone I am able, and in a few words, to give you as my determination:  that you ought not too readily to bind yourself to try the cause upon any one description of evidence; but you are to estimate by your own discretion what you ought to credit, or what appears to you not to be established by proof sufficient."[45]