Peter Wegner and Dina Goldin are at it again. The following is from their Viewpoint article Principles of Problem Solving in the July CACM. Theoretical computer science (TCS) asserted in the 1960s that Turing machines (TMs)—introduced by Turing to help show the limitations of mathematical problem solving—provide a complete model of computer problem solving by negating Turing's claim that TMs could solve only functional, algorithmic problems. The TCS view ignored Turing's assertion that TMs have limited power and that choice machines, which extend TMs to interactive computation, represent a distinct form of computing not modeled by TMs.

The "Choice Machines" from Turing's paper are just what we now call nondeterministic Turing machines. In Endnote 8 of his paper, Turing showed that the choice machines can be simulated by traditional Turing machines, contradicting Wegner and Goldin's claim that Turing asserted his machines have limited power.In the 1960s theorists (such as Martin Davis of New York University) adopted the inaccurate assumptions that "TMs completely express computer problem solving" as a theoretical (mathematical) foundation of the computing discipline. The TCS model is inaccurate because TMs express only closed-box functional transformation of input to output. Computation is not entirely mathematical, since broader models of thinking and research are needed to express all possible scientific and engineering questions. Computational problem solving requires open testing of assertions about engineering problems beyond closed-box mathematical function evaluation.

But more importantly Wegner and Goldin misinterpret the Church-Turing thesis. It doesn't try to explain how computation can happen, just that when computation happens it must happen in a way computable by a Turing machine.

I admit the original single-tape Turing machine does not model interaction as Wegner and Goldin state. Nor does the Turing machine model random-access memory, machines that alter their own programs, multiple processors, nondeterministic, probabilistic or quantum computation. But what that single-tape Turing machine can do is simulate the computational processes of all of the above. Everything computable by these and other seemingly more powerful models can also be computed by the lowly one-tape Turing machine. That is the beauty of the Church-Turing thesis.

The ongoing support for rationalist over empiricist modes of thought (despite repeated questioning by some philosophers) suggests that human thinking is inherently more concerned with the rationality of human desires than with the empirical truth of human reasoning. Our empirical analysis of interactive problem solving continues to be criticized by incorrect rationalist arguments about the strong problem-solving power of TMs, which are accepted as the proper form of valid reasoning, even though they were contradicted in 1936 by Turing himself.

Call me a rationalist then as I continue to hold the belief that no matter how complicated the computational model, we can still use the simple Turing machine to capture its power.We hope you accept that empirical (open) reasoning is often more correct than rationalist (closed) arguments, and that modes of thought about truth and problem solving should promote empiricist over rationalist reasoning, as well as definitive truth over questionable a priori value judgments.

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Posted by Lance to Computational Complexity at 7/14/2006 10:54:00 AM