What is the burden of proof against a party who is not the party asserting the claim?

Evidence Code section 500 provides that "a party has the burden of proof as to each fact the existence or nonexistence of which is essential to the claim for relief or defense that he is asserting." But this rule applies only "[e]xcept as otherwise provided by law." (Ibid.) Where the evidence necessary to establish a fact essential to the plaintiff's claim lies exclusively within the knowledge of the defendant, the defendant has the burden of going forward with the evidence on the issue even though it is not the party asserting the claim. (Wolf v. Superior Court (2003) 107 Cal.App.4th 25, 35.)