What is probable cause for a malicious prosecution action?

Probable cause is a low threshold designed to protect a litigant's right to assert arguable legal claims, even those claims unlikely to succeed. In essence, probable cause exists if any reasonable attorney would have believed the claim tenable. Although this standard is lenient, it reflects the important policy of avoiding discouragement of novel or debatable legal claims. Only actions that any reasonable attorney would agree are totally and completely without merit

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may form the basis for a malicious prosecution suit. (Plumley v. Mockett (2008) 164 Cal.App.4th 1031, 1047-1048.)