What does the witness oath mean in legal situations?
<p>I'm in NSW, Australia but our witness oath is apparently fairly standard across English-based legal systems. It looks like this <a href="https://en.wikipedia.org/wiki/Sworn_testimony" rel="nofollow">generic example from wikipedia</a>:</p>

<blockquote>
  <p>Oath: I swear that the evidence that I shall give, shall be the truth, the whole truth and nothing but the truth, so help me God. Affirmation: I solemnly affirm that the evidence that I shall give, shall be the truth, the whole truth and nothing but the truth.</p>
</blockquote>

<p>It seems to me that anyone willing to say that with a straight face is not a credible witness for a range of reasons. From the ontological and epistemological questions about the nature of truth to the simply mechanical objection that "the whole truth" cannot be told, I can't help feel that I'm missing a key element of the legal view of this.</p>

<p>Taking three basic problems:</p>

<ol>
<li><p>define "truth" - is it just anything the witness believes to be true? Is there any standard? Or any penalty for not meeting the standard, if a standard exists?</p></li>
<li><p>human memory and perception are limited. Are those caveats implicit in the process? The oath doesn't say "to the best of my knowledge and as accurately as I can recall" it says "truth" unconditionally. Using the cliche "do you know what colour this pen is?" question, the only truthful answer must be "no", since colour perception is both limited and biased, as well as the actual colour being a matter of opinion (viz, reasonable people can disagree).</p></li>
<li><p>What does "whole truth" mean? It's obviously insane to require every witness to begin their testimony at the first instant of the big bang. But to permit otherwise means leaving something out. How and where does the law draw the line between "only tell us what you want to" and "tell the whole truth"?</p></li>
</ol>

<p>How do legal systems resolve those questions? Are the terms just taken to be legal jargon and redefined in non-obvious ways? I can't find those re-definitions, so presumably it's all so obvious that only an idiot would need it made explicit. Count me as that idiot...</p>

<p>Edit: to make it explicit, I'd like links or references that lay these "obvious" things out. Examples would be handy. This is law, presumably this stuff has been litigated and there's scholarship as well as case law? Is there even obscure case law saying that a witness who, for example, refuses to speculate and answers only with facts is obstructing the court (or not)?</p>

<p>I'm sorry to deflate what is clearly a <em>very</em> philosophically interesting question, but the law is straightforward here.</p>

<ol>
<li>The truth, essentially, is the set of facts that you believe to be true.</li>
<li>Yes.</li>
<li>It means that you will not lie by omission, and that you will provide the relevant facts. No, you don't need to recount history since the first instant of the big bang.</li>
</ol>

<p>Only <em>expert</em> witnesses may answer by giving their opinion or evaluation. This is presumed accurate by their experience and where it is not accurate, the other side may present opposing expert witnesses to contest their conclusion or evaluation.</p>

<p>Laypeople are permitted to answer only with their recollection of facts. Lawyers may not ask them what their opinion is, although by your definition every question is about opinion, since perception and memory is limited. But the question "What colour was Mr Smith's house?" and "What architectural style informed the facade of Mr Smith's house?" require different amounts of expertise and opinion.</p>

<p>"Truth" isn't jargon, or even technical language here. I generally aim to be truthful, and so when someone asks me what time it is, I don't feel compelled to answer to the nano/picosecond. Would you call me a liar? Am I lying by omission?</p>

<p>Similarly, if someone asks me what colour a car is, I don't feel compelled to say "I can't possibly know, because my perception may differ from yours.</p>

<p>If you honestly feel that when someone asks you to be truthful about something, then you must either be lying, or lying by omission if you don't start your answer with the first instant of the big bang, your problem is not one of law.</p>

<p><em>Bronston v. United States</em>, 409 U.S. 352 (1973) interpreted the federal perjury statute (<a href="https://www.law.cornell.edu/uscode/text/18/1621" rel="noreferrer">18 USC §1621</a>) in the US.</p>
<p>In this case, the following exchange happened in a bankruptcy hearing:</p>
<blockquote>
<p>&quot;Q. Do you have any bank accounts in Swiss banks, Mr. Bronston?&quot;</p>
<p>&quot;A. No, sir.&quot;</p>
<p>&quot;Q. Have you ever?&quot;</p>
<p>&quot;A. The company had an account there for about six months, in Zurich.&quot;</p>
</blockquote>
<p>Questioning went on to other matters.</p>
<p>Each of Mr. Bronston's answers was true. However, Mr. Bronston previously had a personal bank account in a Swiss bank that he did not mention in the second answer. You might consider that he had not told the &quot;whole truth&quot;.</p>
<p>The court held that the federal perjury statute does not prohibit an answer &quot;that is literally true, but unresponsive, even assuming the witness intends to mislead his questioner by the answer, and even assuming the answer is arguably 'false by negative implication'&quot;.</p>
<p>The most pertinent portion of the opinion, with respect to your question is this:</p>
<blockquote>
<p>Under the pressures and tensions of interrogation, it is not uncommon for the most earnest witnesses to give answers that are not entirely responsive. Sometimes the witness does not understand the question, or may, in an excess of caution or apprehension, read too much or too little into it. It should come as no surprise that a participant in a bankruptcy proceeding may have something to conceal and consciously tries to do so, or that a debtor may be embarrassed at his plight and yield information reluctantly. It is the responsibility of the lawyer to probe; testimonial interrogation, and cross-examination in particular, is a probing, prying, pressing form of inquiry. If a witness evades, it is the lawyer's responsibility to recognize the evasion and to bring the witness back to the mark, to flush out the whole truth with the tools of adversary examination.</p>
</blockquote>
<p>It is up to the questioner to realize the misunderstanding or idiosyncratic way in which a witness has interpreted the question and rephrase so as to flush out the whole truth that they seek.</p>
<p>As a practical matter, if you were to answer in the way that you suggest, the examiner would rephrase in order to get your testimony about how you believe something happened to the best of your recollection, or the particular color experience you are having when observing the pen, etc.</p>

<p>It also means, I believe, that if you are found to have lied or misled the court 'under oath' then you can be charged with perjury. Thus being put under oath is a real incentive to not lie or obscure things that the lawyer or judge or jury might reasonably be entitled to know in order to pursue their respective tasks in good faith.</p>

witnesses