The vending machine already did this

A vending machine is a promise with no person behind it: put in 3, press B4, receive the crisps. It doesn't like you, doesn't check your name, doesn't decide whether today is a good day to honor the deal. The rule is the machine.

Compare that to a promise held by a person — a landlord returning a deposit, a marketplace releasing a payment once the package arrives. Those promises are real too, but they depend on someone choosing to keep them, and on somewhere to complain if they don't.

A contract on a shared ledger is the first arrangement: the vending machine, for money and rules, sitting inside the book from Chapter I.