The Broad Conception of Computation

A myth has arisen concerning Turing's article of 1936, namely that Turing set forth a fundamental principle concerning the limits of what can be computed by machine—a myth that has passed into cognitive science and the philosophy of minds to wide and pernicious effect. This supposed principle, sometimes incorrectly termed the Church-Turing thesis, is the claim that the class of functions that can be computed by machines is identical to the class of functions that can be computed by Turing machines. In point of fact, Turing himself nowhere endorses or even states this claim (nor does Church). The author describes a number of notional machines, both analog and digital, that can compute more than a universal Turing machine. These machines are exemplars of the class of nonclassical computing machines. Nothing known at present rules out the possibility that machines in this class will one day be built or that the brain itself is such a machine. These theoretical considerations undercut a number of foundational arguments that are commonly rehearsed in cognitive science and gesture toward a new class of cognitive models.