Ascertaining the whole truth about a case under legal investigation is difficult and occasionally impossible. The goal is determination of the facts relevant to the matter at hand. This determination is often handicapped by parties,' failure to put the full facts before the court, by constraints of time and expense which preclude determining some marginally relevant facts, and by witnesses' unreliability. Until the 1900s many witnesses, including all women, were excluded from court hearings on the basis of unreliability. Although reforms have abolished these unjustified exclusions, courts persist in regarding certain categories of relevant evidence inadmissible. The most easily explainable category is based on privilege, where there is an overriding public interest in not disclosing the evidence (e.g. national security) or where communications between husband and wife or between client and solicitor are protected. Illegally obtained evidence is also inadmissible. In criminal prosecutions, that evidence is usually obtained by the police. In Scotland, the admission of such evidence is left to the presiding judge's discretion, to protect the public while safeguarding the accused person's rights. Secondary evidence, including secondary hearsay, is also inadmissible. In Scotland and England, however, statutory provisions now enable records relating to trade or business to be received as proof of their contents in criminal cases, if certain requirements are met. Footnotes are provided.