In Alvarado v. Anderson (1959) 175 Cal. App. 2d 166, plaintiff was injured in a diving accident. Defendant sought to introduce testimony that he had been warned not to attempt the dive. The trial court excluded the evidence on the ground that there was no evidence showing that plaintiff had heard the warning. The ruling was upheld on appeal. The court explained that the testimony: "was not material unless the jury could have determined that the plaintiff must have heard this warning. This latter fact was foundational to the materiality of the question and the trial court was entitled to require the presentation of substantial evidence upon the fact of knowledge before permitting the question to be answered. Whether foundational evidence is of such substantiality as will support a finding of a foundational fact is a matter which lies within the legal discretion of the trial judge and his decision will not be overruled on appeal unless it constitutes an abuse of that discretion." (175 Cal. App. 2d at p. 179.)