Lack of Personal Knowledge
A witness may not testify to a matter unless evidence is introduced sufficient to support a finding that he has personal knowledge of the matter. Evidence to prove personal knowledge may, but need not, consist of the testimony of the witness himself. This rule is subject to the provisions of rule 703, relating to opinion testimony by expert witnesses.
What, if anything, is left of the common law disqualifications for interest, coverture, infamy, and irreligion? Most of the concerns expressed by these concepts have been relegated to reliance on cross-examination to bring out the infirmities of witness testimony, but some vestiges remain. The "Dead Man's Rule" in some state jurisdictions prevents a party from testifying in a lawsuit when death has stilled the tongue of his opponent. Spouses today have a privilege not to testify against each other. In certain circumstances felons may have their criminal record brought out to impeach their credibility. And witnesses still must swear or affirm that they will testify truthfully: