 In addition to the goal of widening the scope of Gödel’s Incompleteness Theorems, another motivation for work on recursive functions during the 1930s was the study of so-called undecidable (or unsolvable) problems. The original example of such a problem was that of determining whether a given formula \(\varphi\) of first-order logic is valid—i.e., true in all of its models. This was first described as the Entscheidungsproblem (or decision problem) for first-order logic by Hilbert &amp; Ackermann in their textbook Grundzüge der theoretischen Logik (1928):[18] 