  It was vital to Turing's work that he justified the definition by showing that it encompassed the most general idea of ‘method’. For if it did not, the Entscheidungproblem remained open: there might be some more powerful type of method than was encompassed by Turing computability. One justification lay in showing that the definition included many processes a mathematician would consider to be natural in computation (Turing 1936, p. 254). Another argument involved a human calculator following written instruction notes. (Turing 1936, p. 253). But in a bolder argument, the one he placed first, he considered an ‘intuitive’ argument appealing to the states of mind of a human computer. (Turing 1936, p. 249). The entry of ‘mind’ into his argument was highly significant, but at this stage it was only a mind following a rule.