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The Church-Turing thesis is given a provable interpretation based on the idea that a computation by an idealized human agent must be a logically definable finite mathematical object. The argument is preserved under a large variation in the expressive power of the underlying logical language, thus providing a possible explanation of why the notion of effective computability is so robust.
Minds and Machines, 2008
We conclude from Goedel's Theorem VII of his seminal 1931 paper that every recursive function f(x_{1}, x_{2}) is representable in the first-order Peano Arithmetic PA by a formula [F(x_{1}, x_{2}, x_{3})] which is algorithmically verifiable, but not algorithmically computable, if we assume that the negation of a universally quantified formula of the first-order predicate calculus is always indicative of the existence of a counter-example under the standard interpretation of PA. We conclude that the standard postulation of the Church-Turing Thesis does not hold if we define a number-theoretic formula as effectively computable if, and only if, it is algorithmically verifiable; and needs to be replaced by a weaker postulation of the Thesis as an equivalence.
I explore the conceptual foundations of Alan Turing's analysis of computability, which still dominates thinking about computability today. I argue that Turing's account represents a last vestige of a famous but unsuccessful program in pure mathematics, viz., Hilbert's formalist program. It is my contention that the plausibility of Turing's account as an analysis of the computational capacities of physical machines rests upon a number of highly problematic assumptions whose plausibility in turn is grounded in the formalist stance towards mathematics. More speciÿcally, the Turing account conates concepts that are crucial for understanding the computational capacities of physical machines. These concepts include the idea of an " operation " or " action " that is " formal, " " mechanical, " " well-deÿned, " and " precisely described, " and the idea of a " symbol " that is " formal, " " uninterpreted, " and " shaped ". When these concepts are disentangled, the intuitive appeal of Turing's account is signiÿcantly undermined. This opens the way for exploring models of hypercomputability that are fundamentally dierent from those currently entertained in the literature.
Logicians are usually philosophically or mathematically minded. Why, then, would they be so interested in problems that belong to computer science, like the explication of the notions of algorithm, effective procedure, and suchlike? The reason for their interest is presumably this. Such problems are interdisciplinary, and modern mathematics, logic and analytic philosophy have much in common, going hand in hand. For instance, the classical decision problem (Entscheidungsproblem) was tremendously popular among logicians. Kurt Gödel, for one, worked on it. Thus I first provide in Section 1 a brief summary of Gödel’s famous incompleteness results. In the summary I use a current technical vernacular. That is, I use terms like ‘algorithm’, ‘effective procedure’, ‘recursive axiomatization’, etc. These terms were not used in the time when Gödel was pursuing his research on (un)decidability, because the study of these modern notions was triggered, inter alia, just by Gödel’s incompleteness r...
Communications of the ACM , 2019
THE CHURCH-TURING THESIS (CTT) underlies tantalizing open questions concerning the fundamental place of computing in the physical universe. For example, is every physical system computable? Is the universe essentially computational in nature? What are the implications for computer science of recent speculation about physical uncomputability? Does CTT place a fundamental logical limit on what can be computed, a computational "barrier" that cannot be broken, no matter how far and in what multitude of ways computers develop? Or could new types of hardware, based perhaps on quantum or relativistic phenomena, lead to radically new computing paradigms that do breach the Church-Turing barrier, in which the uncomputable becomes computable , in an upgraded sense of "com-putable"? Before addressing these questions , we first look back to the 1930s to consider how Alonzo Church and Alan Turing formulated, and sought to justify , their versions of CTT. With this necessary history under our belts, we then turn to today's dramatically more powerful versions of CTT.
Natural Computing, 2007
Turing's notion of human computability is exactly right not only for obtaining a negative solution of Hilbert's Entscheidungsproblem that is conclusive, but also for achieving a precise characterization of formal systems that is needed for the general formulation of the incompleteness theorems. The broad intellectual context reaches back to Leibniz and requires a focus on mechanical procedures; these procedures are to be carried out by human computers without invoking higher cognitive capacities. The question whether there are strictly broader notions of effectiveness has of course been asked for both cognitive and physical processes. I address this question not in any general way, but rather by focusing on aspects of mathematical reasoning that transcend mechanical procedures. Section 1 discusses Go¨del's perspective on mechanical computability as articulated in his [193?], where he drew a dramatic conclusion from the undecidability of certain Diophantine propositions, namely, that mathematicians cannot be replaced by machines. That theme is taken up in the Gibbs Lecture of 1951; Go¨del argues there in greater detail that the human mind infinitely surpasses the powers of any finite machine. An analysis of the argument is presented in Section 2 under the heading Beyond calculation. Section 3 is entitled Beyond discipline and gives Turing's view of intelligent machinery; it is devoted to the seemingly sharp conflict between Go¨del's and Turing's views on mind. Their deeper disagreement really concerns the nature of machines, and I'll end with some brief remarks on (supra-) mechanical devices in Section 4.
The Bulletin of Symbolic Logic, 2002
One of the first analyses of the notion of computability, and certainly the most influential, is due to Turing. Alan M. Turing, from "On Computable Numbers, with an Application to the Entscheidungsproblem, " 1936 The "computable"numbers may be describedbriefly as the real numbers whose expressionsas a decimalare calculableby [mite means. ... Accordingto my definition, a number is computable, if its decimal can be written down bya machine. p. 116
Church's Thesis after 70 years, 2006
Mathematical Problems from Applied Logic II
This paper begins by briefly indicating the principal, non-standard motivations of the author for his decades of work in Computability Theory (CT), a.k.a. Recursive Function Theory. Then it discusses its proposed, general directions beyond those from pure mathematics for CT. These directions are as follows. 1. Apply CT to basic sciences, for example, biology, psychology, physics, chemistry, and economics. 2. Apply the resultant insights from 1 to philosophy and, more generally, apply CT to areas of philosophy in addition to the philosophy and foundations of mathematics. 3. Apply CT for insights into engineering and other professional fields. Lastly, this paper provides a progress report on the above non-pure mathematical directions for CT, including examples for biology, cognitive science and learning theory, philosophy of science, physics, applied machine learning, and computational complexity. Interweaved with the report are occasional remarks about the future.
Theoretical Computer Science, 2006
The recently initiated approach called computability logic is a formal theory of interactive computation. It understands computational problems as games played by a machine against the environment, and uses logical formalism to describe valid principles of computability, with formulas representing computational problems and logical operators standing for operations on computational problems. The concept of computability that lies under this approach is a nontrivial generalization of Church-Turing computability from simple, two-step (question/answer, input/output) problems to problems of arbitrary degrees of interactivity. Restricting this concept to predicates, which are understood as computational problems of zero degree of interactivity, yields exactly classical truth. This makes computability logic a generalization and refinement of classical logic. The foundational paper "Introduction to computability logic" [Annals of Pure and Applied Logic 123 (2003), pp. 1-99] was focused on semantics rather than syntax, and certain axiomatizability assertions in it were only stated as conjectures. The present contribution contains a verification of one of such conjectures: a soundness and completeness proof for the deductive system CL3 which axiomatizes the most basic first-order fragment of computability logic called the finite-depth, elementary-base fragment. CL3 is a conservative extention of classical predicate calculus in the language which, along with all of the (appropriately generalized) logical operators of classical logic, contains propositional connectives and quantifiers representing the so called choice operations. The atoms of this language are interpreted as elementary problems, i.e. predicates in the standard sense. Among the potential application areas for CL3 are the theory of interactive computation, constructive applied theories, knowledgebase systems, systems for resource-bound planning and action. This paper is self-contained as it reintroduces all relevant definitions as well as main motivations. It is meant for a wide audience and does not assume that the reader has specialized knowledge in any particular subarea of logic or computer science.
Church’s and Turing’s theses assert dogmatically that an informal notion of effective calculability is captured adequately by a particular mathematical concept of computabilty. I present analyses of calculability that are embedded in a rich historical and philosophical context, lead to precise concepts, and dispense with theses. To investigate effective calculability is to analyze processes that can in principle be carried out by calculators. This is a philosophical lesson we owe to Turing. Drawing on that lesson and recasting work of Gandy, I formulate boundedness and locality conditions for two types of calculators, namely, human computing agents and mechanical computing devices (or discrete machines). The distinctive feature of the latter is that they can carry out parallel computations. Representing human and machine computations by discrete dynamical systems, the boundedness and locality conditions can be captured through axioms for Turing computors and Gandy machines; models o...
2013
In this paper a distinction is made between Turing's approach to computability, on the one hand, and his approach to mathematical reasoning and intelligence, on the other hand. Unlike Church's approach to computability, which is top-down being based on the axiomatic method, Turing's approach to computability is bottom-up, being based on an analysis of the actions of a human computer. It is argued that, for this reason, Turing's approach to computability is convincing. On the other hand, his approach to mathematical reasoning and intelligence is not equally convincing, because it is based on the assumption that intelligent processes are basically mechanical processes, which however from time to time may require some decision by an external operator, based on intuition. This contrasts with the fact that intelligent processes can be better accounted for in rational terms, specifically, in terms of non-deductive inferences, rather than in term of inscrutable intuition.
Minds and Machines, 1993
The Church-Turing thesis makes a bold claim about the theoretical limits to computation. It is based upon independent analyses of the general notion of an effective procedure proposed by Alan Turing and Alonzo Church in the 1930's. As originally construed, the thesis applied only to the number theoretic functions; it amounted to the claim that there were no number theoretic functions which couldn't be computed by a Turing machine but could be computed by means of some other kind of effective procedure. Since that time, however, other interpretations of the thesis have appeared in the literature. In this paper I identify three domains of application which have been claimed for the thesis: (1) the number theoretic functions; (2) all functions; (3) mental and/or physical phenomena. Subsequently, I provide an analysis of our intuitive concept of a procedure which, unlike Turing's, is based upon ordinary, everyday procedures such as recipes, directions and methods; I call them "mundane procedures." I argue that mundane procedures can be said to be effective in the same sense in which Turing machine procedures can be said to be effective. I also argue that mundane procedures differ from Turing machine procedures in a fundamental way, viz., the former, but not the latter, generate causal processes. I apply my analysis to all three of the above mentioned interpretations of the Church-Turing thesis, arguing that the thesis is (i) clearly false under interpretation (3), (ii) false in at least some possible worlds (perhaps even in the actual world) under interpretation (2), and (iii) very much open to question under interpretation (1).
SOME REMARKS ON THE LOGIC AND EPISTEMOLOGY OF COMPUTATION
The paper focuses on some logical and epistemological aspects of the notion of computation. The first part questions the Church-Turing thesis as a fundamental principle concerning the limits of computation and some of its consequences for Philosophy of Mind and Cognitive Science. The second part discusses one of the main presumptions of the traditional conception of computability, namely, its reliance on the absolute character of classical logic which is taken as an underlying framework.
Chapman & Hall/CRC applied algorithms and data structures series, 1998
NeuroQuantology, 2007
There are various equivalent formulations of the Church-Turing thesis. A common one is that every effective computation can be carried out by a Turing machine. The Church-Turing thesis is often misunderstood, particularly in recent writing in the philosophy of mind.
Interactive Computation
Springer eBooks, 2020
The paradoxes discovered in Cantor's set theory sometime around 1900 began a crisis that shook the foundations of mathematics. In order to reconstruct mathematics, freed from all paradoxes, Hilbert introduced a promising program with formal systems as the central idea. Though the program was unexpectedly brought to a close in 1931 by Gödel's famous theorems, it bequeathed burning questions: "What is computing? What is computable? What is an algorithm? Can every problem be algorithmically solved?" This led to Computability Theory, which was born in the mid-1930s, when these questions were resolved by the seminal works of Church, Gödel, Kleene, Post, and Turing. In addition to contributing to some of the greatest advances of twentieth-century mathematics, their ideas laid the foundations for the practical development of a universal computer in the 1940s as well as the discovery of a number of algorithmically unsolvable problems in different areas of science. New questions, such as "Are unsolvable problems equally difficult? If not, how can we compare their difficulty?" initiated new research topics of Computability Theory, which in turn delivered many important concepts and theorems. The application of these is central to the multidisciplinary research of Computability Theory. Aims Monographs in Theoretical Computer Science usually strive to present as much of the subject as possible. To achieve this, they present the subject in a definitiontheorem-proof style and, when appropriate, merge and intertwine different related themes, such as computability, computational complexity, automata theory, and formal-language theory. This approach, however, often blurs historical circumstances, reasons, and the motivation that led to important goals, concepts, methods, and theorems of the subject. vii viii Preface My aim is to compensate for this. Since the fundamental ideas of theoretical computer science were either motivated by historical circumstances in the field or developed by pure logical reasoning, I describe Computability Theory, a part of Theoretical Computer Science, from this point of view. Specifically, I describe the difficulties that arose in mathematical logic, the attempts to recover from them, and how these attempts led to the birth of Computability Theory and later influenced it. Although some of these attempts fell short of their primary goals, they put forward crucial questions about computation and led to the fundamental concepts of Computability Theory. These in turn logically led to still new questions, and so on. By describing this evolution I want to give the reader a deeper understanding of the foundations of this beautiful theory. The challenge in writing this book was therefore to keep it accessible by describing the historical and logical development while at the same time introducing as many modern topics as needed to start the research. Thus, I will be happy if the book makes good reading before one tackles more advanced literature on Computability Theory.
The Church-Turing Thesis confuses numerical computations with symbolic computations. In particular, any model of computability in which equality is not definable, such as the lambda-models underpinning higher-order programming languages, is not equivalent to the Turing model. However, a modern combinatory calculus, the SF-calculus, can define equality of its closed normal forms, and so yields a model of computability that is equivalent to the Turing model. This has profound implications for programming language design.
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