How can one sign a contract before birth? That question can't be answered by President Bill Clinton, who said "we mustn't break the solemn compact between generations," in a 1998 address on Social Security. 

Such a speech constructs Social Security as a contractual mandate in need of protection rather than an insurance and redistribution program. Grand national "contracts" should face significant scrutiny, as they borrow the moral force of a contract without the requirements that define one.

No national contract language appeared in the Social Security Act when Franklin D. Roosevelt signed it in 1935. The argument was about insurance, protection against unemployment, and poverty among the elderly. The original New Deal case for Social Security was that the federal government had a responsibility to create a system of economic security against the hazards of modern life. Contrast that with the contract framing of Social Security. Social Security becomes a question of sacred duty and obligation, not policy. Altering the program, a necessity given the debt, would seem to be a generational betrayal.

But for contracts to be legally enforceable, at a minimum, both parties must be capable of consent. 

Legally, the four requirements of a contract are offer, consideration, acceptance, and an intention to create legal relations. A person not yet born cannot be offered a contract, consider it in any manner or ask for compensation, accept it in any way, or intend legal relations. By any measure, Social Security cannot be a legal contract.

True, some contracts allow you to consent in ways other than signature. John Locke's ideas of Social Contract Theory hold that paying taxes and participating in the democratic process constitute a valid contract with the government. Even this type of contract, however, needs clear parties and justification for the consent. Social Security cannot meet this most basic criterion, as the Social Security contract relies on the participant before they are alive. To highlight this absurdity: I was born in 2005, and Social Security relied on my payments to beneficiaries long before I existed. I supposedly entered a contract decades before I, in any meaningful sense, existed. This is not a minor technicality but a central contradiction. A contract is, at root, an agreement between two parties; Social Security is neither an agreement nor between two parties.

The defender of Social Security may object that this is too literal. "Generational contract" is a mere metaphor. Such a notion creates a dilemma: if the phrase is only metaphorical, Social Security cannot carry the moral duty of an actual contract. Following that claim, younger workers are not morally bound by the choices of prior political actors. But Clinton clearly intended us to be so bound. If the phrase is meant to carry moral force and contractual obligations, then we are entitled to ask the basic questions: Who agreed? When did they agree? What were the terms? And how could people who were not consulted be said to have consented?

The metaphor survives by morphing into a form that is most convenient. When challenged, it is a metaphor. When used politically, it is an obligation — specifically (at least) 30 trillion in unfunded obligations over the next 75 years.