 According to the Church-Turing thesis (CTT), any function that is intuitively computable is computable by some TM (i.e., Turing-computable). Alternatively, CTT may be formulated as the thesis that any function “naturally regarded as computable” (Turing 1936–7, 135) is Turing-computable. The phrases “intuitively computable” and “naturally regarded as computable” are somewhat ambiguous. When they are disambiguated, CTT takes different forms.