Page "Church–Turing thesis" Paragraph 20

Many years later in a letter to Davis ( ca 1965 ), Gödel would confess that " he was, at the time of these lectures, not at all convinced that his concept of recursion comprised all possible recursions ".

By 1963 – 4 Gödel would disavow Herbrand – Gödel recursion and the λ-calculus in favor of the Turing machine as the definition of " algorithm " or " mechanical procedure " or " formal system ".