 In conjunction with the other arguments which Church and Turing had already offered in favor of Church’s Thesis (see  Section 1.6),  Propositions  1.1  and  1.2  can thus be taken to show that the Entscheidungsproblem is indeed not decidable in the informal sense described by Hilbert &amp; Ackermann (1928)—i.e., not decidable by a “mechanical procedure using finitely many operations”. As we will see in  Section 3, the desire to develop a general theory of such undecidability results and the relations which they bear to one another was an important motivation for the further development of computability theory starting in the 1940s.