1. Witnesses must be competent.  That is, in general, they must be able to understand the nature and solemnity of an oath.  This will usually exclude children below a certain age, insane persons and persons drunk at the time of offering testimony.

2. Witnesses must testify of their own knowledge.  Usually they are barred from telling what they simply believe to be the fact or what they have learned from hearsay.

3. Evidence must go to prove the material allegations of the pleadings.  It must be confined to the question at issue.  It is to be observed that the evidence must not only go to prove the matter alleged, but it must be the material not the superfluous matter.  What is material and what superfluous will depend upon the case.  Thus if it is alleged that a suit of clothes was obtained by the defendant at a certain time, his obtaining the clothes is the material fact and the time may be superfluous or immaterial.  But if a note is in controversy its date is material as establishing its identity.

4. "The evidence must be the best of which the case is susceptible." Thus, in case of a written instrument the best evidence is the instrument itself; the next best, a copy of it; the next, oral statement of its contents.  And a copy will not be accepted if the original can be produced.

5. The burden of proof lies on the affirmative.  In civil cases the party affirming is usually the plaintiff.  In criminal cases it is the state.  Harmonizing with this principle is the constitutional provision that in criminal cases the accused shall not be required to give evidence against himself.

These are the principal rules of evidence, but they have many applications.  Learned volumes have been written elaborating them.

Grand Jury.—­A grand jury may be defined as a body of men returned at stated periods from the citizens of the county, before a court of competent jurisdiction, chosen by lot, and sworn to inquire of public offenses committed or triable in the county.

The number of grand jurors was formerly twenty-three.  By statute many of the states have fixed upon a smaller number, Oregon having only seven.  A common number is fifteen.  Some states have no grand jury.  In some others the grand jury is summoned only when requested by the court.

The United States constitution and most of the State constitutions declare that no person shall be held to answer for a criminal offense, except a minor one, “unless on the presentment or indictment of a grand jury.”  This is to save people from the vexation and expense of arrest and trial unless there is reasonable presumption of their guilt.  On the other hand, a grand jury should aid in bringing to justice persons who indulge in practices subversive of public peace, but which individuals are disinclined to prosecute, such as gambling.  Incidentally the grand jury examines into the condition of the county jail and poor-house.