One fact is evidence of another when it tends in any degree to render the existence of that other probable. The quality by virtue of which it has such an effect may be called its probative force, and evidence may therefore be defined as any fact which possesses such force. Probative force may be of any degree of intensity. When it is great enough to form a rational basis for the inference that the fact so evidenced really exists, the evidence possessing it is said to constitute proof.
It is convenient to be able to distinguish shortly between the fact which is evidence, and the fact of which it is evidence. The former may be termed the evidential fact, the latter the principal fact. Where, as is often the case, there is a chain of evidence, A. being evidence of B., B. of C., C. of D. and so on, each intermediate fact is evidential in respect of all that follow it and principal in respect of all that precede it.
1. Evidence is of various kinds, being in the first place either judicial or extrajudicial. Judicial evidence is that which is produced to the court; it comprises all evidential facts that are actually brought to the personal knowledge and observation of the tribunals. Extrajudicial evidence is that which does not come directly under judicial cognizance, but nevertheless constitutes an intermediate link between judicial evidence and the fact requiring proof. Judicial evidence includes all testimony given by witnesses in court, all documents produced to and read by the court, and all things personally examined by the court for the purposes of proof. Extrajudicial evidence includes all evidential facts which are known to the court only by way of inference from some form of judicial evidence. Testimony is extrajudicial, when it is judicially known only through the relation of a witness who heard it. A confession of guilt, for example, is judicial evidence if made to the court itself, but extrajudicial if made elsewhere and proved to the court by some form of judicial evidence. Similarly a document is judicial evidence if produced, extrajudicial if known to the court only through a copy, or through the report of a witness who has read it. So the locus in quo or the material subject-matter of a suit becomes judicial evidence, when personally viewed by the court, but is extrajudicial when described by witnesses.
It is plain that in every process of proof some form of judicial evidence is an essential element. Extrajudicial evidence may or may not exist. When it is present, it forms an intermediate link or a series of intermediate links in a chain of proof, the terminal links of which are the principal fact at one end and the judicial evidence at the other. Judicial evidence requires production merely; extrajudicial evidence stands itself in need of proof.
2. In the second place evidence is either personal or real. Personal evidence is otherwise termed testimony. It includes all kinds of statements regarded as possessed of probative force in respect of the facts stated. This is by far the most important form of evidence. There are few processes of proof that do not contain it—few facts that are capable of being proved in courts of justice otherwise than by the testimony of those who know them. Testimony is either oral or written, and either judicial or extrajudicial. There is a tendency to restrict the term to the judicial variety, but there is no good reason for this limitation. It is better to include under the head of testimony or personal evidence all statements, verbal or written, judicial or extrajudicial, so far as they are possessed of probative force. Real evidence, on the other hand, includes all the residue of evidential facts. Anything which is believed for any other reason than that some one has said so, is believed on real evidence. This, too, is either judicial or extrajudicial, though here also there is a tendency to restrict the term to the former use.
3. Evidence is either primary or secondary. Other things being equal, the longer any chain of evidence the less its probative force, for with each successive inference the risk of error grows. In the interests of truth, therefore, it is expedient to shorten the process, to cut out as many as possible of the intermediate links of extrajudicial evidence, and to make evidence assume the judicial form at the earliest practicable point. Hence the importance of the distinction between primary and secondary evidence. Primary evidence is evidence viewed in comparison with any available and less immediate instrument of proof. Secondary evidence is that which is compared with any available and more immediate instrument of proof. Primary evidence of the contents of a written document is the production in court of the document itself; secondary evidence is the production of a copy or of oral testimony as to the contents of the original. Primary evidence that A. assaulted B. is the judicial testimony of C. that he saw the assault; secondary evidence is the judicial testimony of D. that C. told him that he saw the assault. That secondary evidence should not be used when primary evidence is available is, in its general form, a mere counsel of prudence; but in particular cases, the most important of which are those just used as illustrations, this counsel has hardened into an obligatory rule of law. Subject to certain exceptions, the courts will receive no evidence of a written document save the document itself, and will listen to no hearsay testimony.
4. Evidence is either direct or circumstantial. This is a distinction important in popular opinion rather than in legal theory. Direct evidence is testimony relating immediately to the principal fact. All other evidence is circumstantial. In the former case the only inference required is one from testimony to the truth of it. In the latter the inference is of a different nature, and is generally not single but composed of successive steps. The testimony of A. that he saw B. commit the offence charged, or the confession of B. that he is guilty, constitutes direct evidence. If we believe the truth of the testimony or confession, the matter is concluded, and no further process of proof or inference is required. On the other hand, the testimony of A. that B. was seen by him leaving the place where the offence was committed, and having the instrument of the offence in his possession, is merely circumstantial evidence; for even if we believe this testimony, it does not follow without a further inference, and therefore a further risk of error, that B. is guilty. Direct evidence is commonly considered to excel the other in probative force. This, however, is not necessarily the case, for witnesses lie, and facts do not. Circumstantial evidence of innocence may well prevail over direct evidence of guilt; and circumstantial evidence of guilt may be indefinitely stronger than direct evidence of innocence.
§ 174. The Valuation of Evidence.
The law of evidence comprises two parts. The first of these consists of rules for the measurement or determination of the probative force of evidence. The second consists of rules determining the modes and conditions of the production of evidence. The first deals with the effect of evidence when produced, the second with the manner in which it is to be produced. The first is concerned with evidence in all its forms, whether judicial or extrajudicial; the second is concerned with judicial evidence alone. The two departments are intimately connected, for it is impossible to formulate rules for the production of evidence without reference and relation to the effect of it when produced. Nevertheless the two are distinct in theory, and for the most part distinguishable in practice. We shall deal with them in their order.
In judicial proceedings, as elsewhere, the accurate measurement of the evidential value of facts is a condition of the discovery of truth. Except in the administration of justice, however, this task is left to common sense and personal discretion. Rules and maxims, when recognised at all, are recognised as proper for the guidance of individual judgment, not for the exclusion of it. But in this, as in every other part of judicial procedure, law has been generated, and, in so far as it extends, has made the estimation of probative force or the weighing of evidence a matter of inflexible rules excluding judicial discretion. These rules constitute the first and most characteristic portion of the law of evidence. They may be conveniently divided into five classes, declaring respectively that certain facts amount to:—