On Friday, six months after Governor Charlie Baker declared a state of emergency and began issuing shutdown orders in response to the COVID-19 pandemic, the Massachusetts Supreme Judicial Court takes up a threshold question: Were the governor’s commands lawful?

Baker’s March 10 declaration was followed by dozens of emergency orders affecting virtually every aspect of life in the Bay State. His decrees shut down Massachusetts businesses, houses of worship, and schools; banned elective surgery; restricted travel; closed beaches and theaters; prohibited sporting events; and limited weddings and funerals to 10 people. Almost overnight, they plunged Massachusetts into a recession. Unemployment rose to 17.4 percent, the highest rate in America. The state began gradually reopening in late May, but for some businesses the shutdown proved fatal. One-fifth of Massachusetts restaurants, for example, have permanently closed their doors.

The purpose of Baker’s orders was irreproachable: to slow the spread of the coronavirus, which has now killed nearly 190,000 people in the United States and more than 9,000 in Massachusetts. Whether the decrees were the best way to address the pandemic is a question that epidemiologists and other experts will be debating for some time. But the matter before the state’s highest court isn’t whether Baker’s unilateral orders — which can be enforced with fines and imprisonment — were wise or well intended. It is whether he had the legal authority to issue them.

The lawsuit was brought by a group of small-business owners, pastors, and a private school headmaster. They argue that Baker’s orders should be deemed invalid because they were issued under the state’s 1950 Civil Defense Act — a law, they say, that does not apply to the coronavirus pandemic. That statute was enacted by the Legislature to empower governors “to defend Massachusetts from foreign invasions, armed insurrections, and similar catastrophic events,” the plaintiffs contend, and it specifies in detail the types of crises that can trigger its provisions — war or enemy attack; riots or civil disturbance; severe drought; an escape of radiation from a nuclear plant; and “fire, flood, earthquake or other natural causes.” Those, say the plaintiffs, are all “sudden cataclysmic events of limited time, place, and duration.” The law makes no reference to disease because it was never intended to apply to disease.

By contrast, the governor points to the phrase “other natural causes” and insists that the 1950 act gives him all the authority he needs. “Like fires, floods, and earthquakes, COVID-19 is a natural phenomenon,” he observes in a brief prepared by the attorney general’s office. He also points to the law’s preamble, which described the act as “an emergency law, necessary for the immediate preservation of the public health, safety, and convenience.” Far from overstepping his lawful authority, Baker maintains, he is “discharging his constitutional prerogative, as well as his constitutional duty,” by issuing orders under the Civil Defense Act.

If so, why has no governor ever done what Baker has done?

Massachusetts has experienced multiple epidemics in the decades since the Legislature enacted the Civil Defense Act, yet Baker is the first governor to invoke the act to fight the spread of disease. There’s a reason that hasn’t been done before, the challengers argue: A rapidly spreading disease is not a civil defense emergency. It is a public health emergency — and for that, the Legislature enacted the Public Health Act, which has been on the books for more than a century. That is the law intended to govern the state’s response to COVID-19, the plaintiffs assert. “Governor Baker simply cannot substitute the inapposite Civil Defense Act to ignore or suspend the very statute the [Legislature] wrote to protect Massachusetts from pandemics.”

This isn’t merely a wonky dispute over legislative interpretation. Nothing in the Public Health Act authorizes the kind of comprehensive economic shutdown that the governor imposed on Massachusetts. That law delegates considerable power to local health officials, not the governor. If Baker issued orders by relying on a statute that doesn’t actually grant him that power, those orders were, in legalese, “ultra vires” — beyond the scope of his authority. Until and unless lawmakers empower Baker to issue emergency orders to address a pandemic, the plaintiffs argue, he may not do so. So far, lawmakers haven’t acted.

Courts are typically reluctant to overturn emergency measures designed for public protection. But such deference has its limits. Defeating the coronavirus is a crucial public good. Upholding the rule of law is too.

(Jeff Jacoby is a columnist for The Boston Globe).