I'm in NSW, Australia but our witness oath is apparently fairly standard across English-based legal systems. It looks like this generic example from wikipedia:
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.
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.
Taking three basic problems:
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?
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).
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"?
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...
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)?