The principle of the departure from those rules is clearly fixed by Lord Hardwicke; he lays it down as follows:—­“The first ground judges have gone upon, in departing from strict rules, is absolute strict necessity; 2dly, a presumed necessity.”  Of the first he gives these instances:—­“In the case of writings subscribed by witnesses, if all are dead, the proof of one of their hands is sufficient to establish the deed.  Where an original is lost, a copy may be admitted; if no copy, then a proof by witnesses who have heard the deed:  and yet it is a thing the law abhors, to admit the memory of man for evidence.”  This enlargement through two stages of proof, both of them contrary to the rule of law, and both abhorrent from its principles, are by this great judge accumulated upon one another, and are admitted from necessity, to accommodate human affairs, and to prevent that which courts are by every possible means instituted to prevent,—­A FAILURE OF JUSTICE.  And this necessity is not confined within the strict limits of physical causes, but is more lax, and takes in moral and even presumed and argumentative necessity, a necessity which is in fact nothing more than a great degree of expediency.  The law creates a fictitious necessity against the rules of evidence in favor of the convenience of trade:  an exception which on a similar principle had before been admitted in the Civil Law, as to mercantile causes, in which the books of the party were received to give full effect to an insufficient degree of proof, called, in the nicety of their distinctions, a semiplena probatio.[52]

But to proceed with Lord Hardwicke.  He observes, that “a tradesman’s books” (that is, the acts of the party interested himself) “are admitted as evidence, though no absolute necessity, but by reason of a presumption of necessity only, inferred from the nature of commerce.”  “No rule,” continued Lord Hardwicke, “can be more settled than that testimony is not to be received but upon oath”; but he lays it down, that an oath itself may be dispensed with.  “There is another instance,” says he, “where the lawful oath may be dispensed with,—­where our courts admit evidence for the Crown without oath.”

In the same discussion, the Chief-Baron (Parker) cited cases in which all the rules of evidence had given way.  “There is not a more general rule,” says he, “than that hearsay cannot be admitted, nor husband and wife as witnesses against each other; and yet it is notorious that from necessity they have been allowed,—­not an absolute necessity, but a moral one.”