O'Sullivan & Hilliard's The Law of Contract (2018 8 ed). p 122.
5.99 There are three plausible alternative candidates. First, a requirement of form might be introduced, so that, for example, only promises made in writing were enforceable. But this would be unworkable in modern society, where many thousands of oral transactions are made every day. Second, more emphasis could be given to the requirement that the promisor intended to create legal relations. This is a much more realistic alternative, reflecting the position in many civilian jurisdictions that even a gratuitous promise is enforceable if it was made seriously with the intention of attracting legal effect. It has the advantage of flexibility, allowing a court to explore whether a particular promise was seriously made and whether on the facts it should or should not be enforced. But with flexibility comes uncertainty: the parties could not be certain until the matter was litigated whether or not their particular promise was enforceable. Of course, rules might grow up to encourage certainty, whereby a promisor might be deemed to have intended to create legal relations in certain circumstances—but the chances are those rules would not be so very different from the rules of consideration. A third possibility is to choose reliance as the relevant filter and to allow any promise to be enforced, but only once it has been relied on by the promisee. For all its apparent fairness, this approach has significant disadvantages. It would make it almost impossible for promisors to order their affairs safe in the knowledge that their promise either was or was not enforceable, since enforceability would not ‘kick in’ until some time after the promise was made, when the other party relied on it for the first time. Promisors would be obliged to keep track of the actions of the other party—not a very attractive
prospect commercially. Furthermore, ‘reliance’ is a slippery concept (as we have seen in our discussion of estoppel): it is hard to imagine first instance courts reaching and applying a consistent, generally acceptable definition of reliance.