The question of how the relevance or irrelevance of evidence is to be determined has been the subject of a vast amount of discussion in the last 100 to 200 years. There is now a consensus among legal scholars and judges in the U.S. that the relevance or irrelevance of evidence cannot be determined by syllogistic reasoning – if/then logic – alone. There is also general agreement that assessment of relevance or irrelevance involves or requires judgments about probabilities or uncertainties. Beyond that, there is little agreement. Many legal scholars and judges agree that ordinary reasoning, or common sense reasoning, plays an important role. There is less agreement about whether or not judgments of relevance or irrelevance are defensible only if the reasoning that supports such judgments is made fully explicit. However, most trial judges would reject any such requirement and would say that some judgments can and must rest partly on unarticulated and articulable hunches and intuitions. However, there is general (though implicit) agreement that the relevance of at least some types of expert evidence – particularly evidence from the hard sciences – requires particularly rigorous or in any event more arcane reasoning than is usually needed or expected. There is a general agreement that judgments of relevance are largely within the discretion of the trial court – although relevance rulings that lead to the exclusion of evidence are more likely to be reversed on appeal than are relevance rulings that lead to the admission of evidence.
In systems of proof based on the English common law tradition, almost all evidence must be sponsored by a witness, who has sworn or solemnly affirmed to tell the truth. The bulk of the law of evidence regulates the types of evidence that may be sought from witnesses and the manner in which the interrogation of witnesses is conducted such as during direct examination and cross-examination of witnesses. Other types of evidentiary rules specify the standards of persuasion (e.g., proof beyond a reasonable doubt) that a trier of fact – whether judge or jury – must apply when it assesses evidence.
Today all persons are presumed to be qualified to serve as witnesses in trials and other legal proceedings, and all persons are also presumed to have a legal obligation to serve as witnesses if their testimony is sought. However, legal rules sometimes exempt people from the obligation to give evidence and legal rules disqualify people from serving as witnesses under some circumstances.
Hearsay is one of the largest and most complex areas of the law of evidence in common-law jurisdictions. The default rule is that hearsay evidence is inadmissible. Hearsay is an out of court statement offered to prove the truth of the matter asserted. A party is offering a statement to prove the truth of the matter asserted if the party is trying to prove that the assertion made by the declarant (the maker of the pretrial statement) is true. For example, prior to trial Bob says, “Jane went to the store.” If the party offering this statement as evidence at trial is trying to prove that Jane actually went to the store, the statement is being offered to prove the truth of the matter asserted. However, at both common law and under evidence codifications such as the Federal Rules of Evidence, there are dozens of exemptions from and exceptions to the hearsay rule.
Lies, on their own, are not sufficient evidence of a crime. However, lies may indicate that the defendant knows he is guilty, and the prosecution may rely on the fact that the defendant has lied alongside other evidence.
Evidence of an indirect nature which implies the existence of the main fact in question but does not in itself prove it. That is, the existence of the main fact is deduced from the indirect or circumstantial evidence by a process of probable reasoning. The introduction of a defendant’s fingerprints or DNA sample are examples of circumstantial evidence. The fact that a defendant had a motive to commit a crime is circumstantial evidence. However, in an important sense all evidence is merely circumstantial because on no evidence can prove a fact in the absence of one or more inference.
One Wikipedia editor wrote, “Recall, however, that courts of law deal with practical issues, not ontology.” But courts and the law of evidence are very much interested in how best to uncover the truth and they cannot avoid epistemological and ontological premises even if they want to.