"Parties to an action against the estate an executor or administrator, upon a claim or demand against the estate of deceased persons," can not be witnesses. (Blood vs. Fairbanks, 50 Cal., 420.) 

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In the very early history of the common law parties interested in actions and proceedings were prohibited from giving testimony during the trial of said actions, or proceedings. The theory of this original disqualification was that persons interested were likely to bear false witness. Long experience however, has demonstrated that this rule worked greater hardship than good. Liberal rules for cross-examination have made it possible to disclose in the presence of the court whether or not interested witnesses were actually falsifying. However, the original common law rule is still in force in the majority of the states of the United States in actions where the adverse party is deceased. As was said by Brickell, chief justice, in the case of Louis vs. Easton (50 Ala., 471), in discussing this same question, "If death has closed the lips of one party, the policy of the law is to close the lips of the other."