“The principle of Velayat e-Faqih is neither intuitively obvious nor rationally necessary,” Mr. Kadivar wrote. “It is neither a requirement of religion nor a necessity for denomination. It is neither a part of Shiite general principles nor a component of detailed observances. It is, by near consensus of the Shiite Ulama, nothing more than a jurisprudential minor hypothesis.”

Or, as Mr. Kadivar simplified it for me in an interview in the back of his van, “There are two interpretations of Islam. The aggressive Islam of Ahmadinejad, or the mercy Islam of Mousavi.”

Why is this significant? Take a look at the color Mr. Mousavi’s supporters have chosen for their movement: Green is the color of Islam, meaning the demonstrators are taking on the regime on its own terms. Part of that challenge is to Iran’s republican pretensions, mocked by voter turnout that the regime itself admits exceeded 100% in some 50 districts.