California Code, Evidence Code - EVID § 403

(1) The relevance of the proffered evidence depends on the existence of the preliminary fact;

(2) The preliminary fact is the personal knowledge of a witness concerning the subject matter of his testimony;

(3) The preliminary fact is the authenticity of a writing; or

(4) The proffered evidence is of a statement or other conduct of a particular person and the preliminary fact is whether that person made the statement or so conducted himself.

(1) May, and on request shall, instruct the jury to determine whether the preliminary fact exists and to disregard the proffered evidence unless the jury finds that the preliminary fact does exist.

(2) Shall instruct the jury to disregard the proffered evidence if the court subsequently determines that a jury could not reasonably find that the preliminary fact exists.