Front Page Titles (by Subject) Chapter 17: Oath in Evidence - The Principles of Moral and Political Philosophy
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Chapter 17: Oath in Evidence - William Paley, The Principles of Moral and Political Philosophy 
The Principles of Moral and Political Philosophy, Foreword by D.L. Le Mahieu (Indianapolis: Liberty Fund, 2002).
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Oath in Evidence
The witness swears “to speak the truth, the whole truth, and nothing but the truth, touching the matter in question.”
Upon which it may be observed, that the designed concealment of any truth, which relates to the matter in agitation, is as much a violation of the oath, as to testify a positive falsehood; and this, whether the witness be interrogated as to that particular point or not. For when the person to be examined is sworn upon a voir dire, that is, in order to inquire whether he ought to be admitted to give evidence in the cause at all, the form runs thus: “You shall true answer make to all such questions as shall be asked you”: but when he comes to be sworn in chief, he swears “to speak the whole truth,” without restraining it, as before, to the questions that shall be asked: which difference shows, that the law intends, in this latter case, to require of the witness, that he give a complete and unreserved account of what he knows of the subject of the trial, whether the questions proposed to him reach the extent of his knowledge or not. So that if it be inquired of the witness afterwards, why he did not inform the court so and so, it is not a sufficient, though a very common answer, to say, “because it was never asked me.”
I know but one exception to this rule; which is, when a full discovery of the truth tends to accuse the witness himself of some legal crime. The law of England constrains no man to become his own accuser; consequently imposes the oath of testimony with this tacit reservation. But the exception must be confined to legal crimes. A point of honour, of delicacy, or of reputation, may make a witness backward to disclose some circumstance with which he is acquainted; but will in no wise justify his concealment of the truth, unless it could be shown, that the law which imposes the oath, intended to allow this indulgence to such motives. The exception of which we are speaking is also withdrawn by a compact between the magistrate and the witness, when an accomplice is admitted to give evidence against the partners of his crime.
Tenderness to the prisoner, although a specious apology for concealment, is no just excuse: for if this plea be thought sufficient, it takes the administration of penal justice out of the hands of judges and juries, and makes it depend upon the temper of prosecutors and witnesses.
Questions may be asked, which are irrelative to the cause, which affect the witness himself, or some third person; in which, and in all cases where the witness doubts of the pertinency and propriety of the question, he ought to refer his doubts to the court. The answer of the court, in relaxation of the oath, is authority enough to the witness; for the law which imposes the oath, may remit what it will of the obligation: and it belongs to the court to declare what the mind of the law is. Nevertheless, it cannot be said universally, that the answer of the court is conclusive upon the conscience of the witness; for his obligation depends upon what he apprehended, at the time of taking the oath, to be the design of the law in imposing it, and no after-requisition or explanation by the court can carry the obligation beyond that.