 Mendelson challenged the view that  CT1  is not subject to mathematical treatment by citing cases in which an intuitive concept has literally been replaced by a corresponding mathematically precise one. It is not a mere thesis that the limit concept, for example, is given by the familiar epsilon-delta definition. The definition replaces the intuitive concept (Mendelson 1990). Others have gone further, arguing that computability may be taken as a primitive concept subject to an axiomatic treatment, as Gödel had once suggested (according to Church’s letter to Kleene, 1935d [BE: 1000]). Sieg proposes an axiomatization based on Turing’s analysis of a person who computes (“a computor”). In section 9 of his (1936), Turing argues (argument I) that the Turing machine is a good model of a computor, who, as such, is subject to certain constraints such as that