 The weaker form of the maximality thesis would be falsified by the actual existence of a physical hypercomputer. Speculation stretches back over at least five decades that there may be real physical processes—and so, potentially, real machine-operations—whose behaviour conforms to functions not computable by any standard Turing machine. See, for example, Scarpellini 1963; Kreisel 1967, 1974, 1982; Pour-El and Richards 1979, 1981; Doyle 1982; Geroch and Hartle 1986; Pitowsky 1990; Stannett 1990; da Costa and Doria 1991, 1994; Hogarth 1994; and Siegelmann and Sontag 1994. At the close of the 20th century Copeland and Sylvan gave an evangelical survey of the emerging field in their 1999. 