On Monday the Supreme Court begins three days of oral arguments concerning possible — actually, probable and various — constitutional infirmities in Obamacare. The justices have received many amicus briefs, one of which merits special attention because of the elegant scholarship and logic with which it addresses an issue that has not been as central to the debate as it should be.

Hitherto, most attention has been given to whether Congress, under its constitutional power to regulate interstate commerce, may coerce individuals into engaging in commerce by buying health insurance. Now the Institute for Justice (IJ), a libertarian public interest law firm, has focused on this fact: The individual mandate is incompatible with centuries of contract law. This is so because a compulsory contract is an oxymoron.

The brief, the primary authors of which are the IJ’s Elizabeth Price Foley and Steve Simpson, says that Obamacare is the first time Congress has used its power to regulate commerce to produce a law “from which there is no escape.” And “coercing commercial transactions” — compelling individuals to sign contracts with insurance companies — “is antithetical to the foundational principle of mutual assent that permeated the common law of contracts at the time of the founding and continues to do so today.”

In 1799, South Carolina’s highest court held: “So cautiously does the law watch over all contracts, that it will not permit any to be binding but such as are made by persons perfectly free, and at full liberty to make or refuse such contracts. . . . Contracts to be binding must not be made under any restraint or fear of their persons, otherwise they are void.” Throughout the life of this nation it has been understood that for a contract to be valid, the parties to it must mutually assent to its terms — without duress.

In addition to duress, contracts are voidable for reasons of fraud upon, or the mistake or incapacity of, a party to the contract. This underscores the centrality of the concept of meaningful consent in contract law. To be meaningful, consent must be informed and must not be coerced. Under Obamacare, the government will compel individuals to enter into contractual relations with insurance companies under threat of penalty.

Also, the Supreme Court in Commerce Clause cases has repeatedly recognized, and Congress has never before ignored, the difference between the regulation and the coercion of commerce. And in its 10th Amendment cases (“The powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people”), the court has specifically forbidden government to compel contracts.

In 1992, the court held unconstitutional a law compelling states to “take title to” radioactive waste. The court said this would be indistinguishable from “a congressionally compelled subsidy from state governments” to those who produced the radioactive waste. Such commandeering of states is, the court held, incompatible with federalism.

The IJ argues: The 10th Amendment forbids Congress from exercising its commerce power to compel states to enter into contractual relations by effectively forcing states to “buy” radioactive waste. Hence “the power to regulate commerce does not include the power to compel a party to take title to goods or services against its will.” And if it is beyond Congress’s power to commandeer the states by compelling them to enter into contracts, it must likewise be beyond Congress’s power to commandeer individuals by requiring them to purchase insurance. Again, the 10th Amendment declares that any powers not given to the federal government are reserved to the states or to the people.

Furthermore, although the Constitution permits Congress to make laws “necessary and proper” for executing its enumerated powers, such as the power to regulate interstate commerce, it cannot, IJ argues, be proper to exercise that regulatory power in ways that eviscerate “the very essence of legally binding contracts.” Under Obamacare, Congress asserted the improper power to compel commercial contracts. It did so on the spurious ground that this power is necessary to solve a problem Congress created when, by forbidding insurance companies to deny coverage to individuals because of preexisting conditions, it produced the problem of “adverse selection” — people not buying insurance until they need medical care.

The IJ correctly says that if the court were to ratify Congress’s disregard for settled contract law, Congress’s “power to compel contractual relations would have no logical stopping point.” Which is why this case is the last exit ramp on the road to unlimited government.

georgewill@washpost.com