Not every promise is a pledge to do something. Sometimes it is anillusory promise, where the terms of the contract really bind the promisor to give up nothing, to suffer no detriment. For example, Lydia offers to pay Juliette $10 for mowing Lydia’s lawn. Juliette promises to mow the lawn if she feels like it. May Juliette enforce the contract? No, because Juliette has incurred no legal detriment; her promise is illusory, since by doing nothing she still falls within the literal wording of her promise. The doctrine that such bargains are unenforceable is sometimes referred to as the rule of mutuality of obligation: if one party to a contract has not made a binding obligation, neither is the other party bound. Thus if A contracts to hire B for a year at $6,000 a month, reserving the right to dismiss B at any time (an “option to cancel” clause), and B agrees to work for a year, A has not really promised anything; A is not bound to the agreement, and neither is B.
The illusory promise presents a special problem in agreements for exclusive dealing, outputs, and needs contracts.