 Gold’s work has interesting similarities with earlier philosophical work on inductive learning by Hilary Putnam (1963; it is not clear whether Gold was aware of this paper). Putnam gave an informal proof of a sort of incompleteness theorem for inductive regularity-learning devices: no matter what algorithm is used in a machine for inducing regularities from experience, and thus becoming able to predict events, there will always be some possible environmental regularities that will defeat it. (As a simple example, imagine an environment giving an unbroken sequence of presentations all having some property a. If there is a positive integer n such that after n presentations the machine will predict that presentation number n + 1 will also have property a, then the machine will be defeated by an environment consisting of n presentations of a followed by one with the incompatible property b—the future need not always resemble the past. But if on the other hand there is no such n, then an environment consisting of an unending sequence of a presentations will defeat it.)