 The formalization of these relations between problems leads to the notions of many-one reducibility and Turing reducibility which provide distinct but complementary analyses of the notions \(A\) is no harder to solve than \(B\) and also the degree of unsolvability (or difficulty) of \(A\) is equal to that of  \(B\).[24]  The latter notion came first historically and was introduced by Turing (1939) and in an equivalent form by Kleene (1943). However it was Post (1944) who both introduced the former notion and also initiated the general study of Turing reducibility. In fact the final sentence of the passage quoted above describes an important technical question about the Turing degrees which would shape the early development of computability theory (i.e., “Post’s problem” given as  Question 3.1  below).