The enumeration, given a first-order sentence , of all sentences deducible from in the first-order predicate calculus, and the enumeration, given a non-negative integer n, of the recursively enumerable set Wn, are two well-known examples of effective processes. But are these processes really distinct? Indeed, might there not exist a Gödel numbering of the sentences of first-order logic such that for each n, if n is the number assigned to the sentence , then Wn is the set of numbers assigned to all sentences deducible from ? If this were the case, the first sort of enumeration would just be a particular instance of the second.