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Church–Turing thesis

2. Misunderstandings of the Thesis

❶In Zalta, Edward N.

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1. The Thesis and its History


Barkley Rosser produced proofs , to show that the two calculi are equivalent. Many years later in a letter to Davis c. A hypothesis leading to a natural law? In late Alan Turing 's paper also proving that the Entscheidungsproblem is unsolvable was delivered orally, but had not yet appeared in print.

Actually the work already done by Church and others carries this identification considerably beyond the working hypothesis stage. But to mask this identification under a definition… blinds us to the need of its continual verification.

Rather, he regarded the notion of "effective calculability" as merely a "working hypothesis" that might lead by inductive reasoning to a " natural law " rather than by "a definition or an axiom". Turing adds another definition, Rosser equates all three: Within just a short time, Turing's —37 paper "On Computable Numbers, with an Application to the Entscheidungsproblem" [19] appeared.

In it he stated another notion of "effective computability" with the introduction of his a-machines now known as the Turing machine abstract computational model.

In his review of Turing's paper he made clear that Turing's notion made "the identification with effectiveness in the ordinary not explicitly defined sense evident immediately". In a few years Turing would propose, like Church and Kleene before him, that his formal definition of mechanical computing agent was the correct one. All three definitions are equivalent, so it does not matter which one is used. Kleene proposes Church's Thesis: This left the overt expression of a "thesis" to Kleene.

This heuristic fact [general recursive functions are effectively calculable] The same thesis is implicit in Turing's description of computing machines Every effectively calculable function effectively decidable predicate is general [29] recursive [Kleene's italics]. Since a precise mathematical definition of the term effectively calculable effectively decidable has been wanting, we can take this thesis If we consider the thesis and its converse as definition, then the hypothesis is an hypothesis about the application of the mathematical theory developed from the definition.

For the acceptance of the hypothesis, there are, as we have suggested, quite compelling grounds. Every effectively calculable function effectively decidable predicate is general recursive. The following classes of partial functions are coextensive, i. Turing's thesis that every function which would naturally be regarded as computable is computable under his definition, i. An attempt to understand the notion of "effective computability" better led Robin Gandy Turing's student and friend in to analyze machine computation as opposed to human-computation acted out by a Turing machine.

Gandy's curiosity about, and analysis of, cellular automata including Conway's game of life , parallelism, and crystalline automata, led him to propose four "principles or constraints In the late s Wilfried Sieg analyzed Turing's and Gandy's notions of "effective calculability" with the intent of "sharpening the informal notion, formulating its general features axiomatically, and investigating the axiomatic framework".

These constraints reduce to:. The matter remains in active discussion within the academic community. The thesis can be viewed as nothing but an ordinary mathematical definition. Soare , [42] where it is also argued that Turing's definition of computability is no less likely to be correct than the epsilon-delta definition of a continuous function.

Marvin Minsky expanded the model to two or more tapes and greatly simplified the tapes into "up-down counters", which Melzak and Lambek further evolved into what is now known as the counter machine model. In the late s and early s researchers expanded the counter machine model into the register machine , a close cousin to the modern notion of the computer.

Other models include combinatory logic and Markov algorithms. Gurevich adds the pointer machine model of Kolmogorov and Uspensky , All these contributions involve proofs that the models are computationally equivalent to the Turing machine; such models are said to be Turing complete.

It may also be shown that a function which is computable ['reckonable'] in one of the systems S i , or even in a system of transfinite type, is already computable [reckonable] in S 1. Thus the concept 'computable' ['reckonable'] is in a certain definite sense 'absolute', while practically all other familiar metamathematical concepts e.

Proofs in computability theory often invoke the Church—Turing thesis in an informal way to establish the computability of functions while avoiding the often very long details which would be involved in a rigorous, formal proof.

Dirk van Dalen gives the following example for the sake of illustrating this informal use of the Church—Turing thesis: Each infinite RE set contains an infinite recursive set. Let A be infinite RE. We list the elements of A effectively, n 0 , n 1 , n 2 , n 3 , From this list we extract an increasing sublist: If none of them is equal to k, then k not in B.

Since this test is effective, B is decidable and, by Church's thesis , recursive. But because the computability theorist believes that Turing computability correctly captures what can be computed effectively, and because an effective procedure is spelled out in English for deciding the set B, the computability theorist accepts this as proof that the set is indeed recursive.

The success of the Church—Turing thesis prompted variations of the thesis to be proposed. For example, the physical Church—Turing thesis states: The Church—Turing thesis says nothing about the efficiency with which one model of computation can simulate another.

It has been proved for instance that a multi-tape universal Turing machine only suffers a logarithmic slowdown factor in simulating any Turing machine.

A variation of the Church—Turing thesis addresses whether an arbitrary but "reasonable" model of computation can be efficiently simulated. This is called the feasibility thesis , [50] also known as the classical complexity-theoretic Church—Turing thesis or the extended Church—Turing thesis , which is not due to Church or Turing, but rather was realized gradually in the development of complexity theory.

This thesis was originally called computational complexity-theoretic Church—Turing thesis by Ethan Bernstein and Umesh Vazirani The complexity-theoretic Church—Turing thesis, then, posits that all 'reasonable' models of computation yield the same class of problems that can be computed in polynomial time.

Assuming the conjecture that probabilistic polynomial time BPP equals deterministic polynomial time P , the word 'probabilistic' is optional in the complexity-theoretic Church—Turing thesis. A similar thesis, called the invariance thesis , was introduced by Cees F. Slot and Peter van Emde Boas. In other words, there would be efficient quantum algorithms that perform tasks that do not have efficient probabilistic algorithms.

This would not however invalidate the original Church—Turing thesis, since a quantum computer can always be simulated by a Turing machine, but it would invalidate the classical complexity-theoretic Church—Turing thesis for efficiency reasons.

Consequently, the quantum complexity-theoretic Church—Turing thesis states: Eugene Eberbach and Peter Wegner claim that the Church—Turing thesis is sometimes interpreted too broadly, stating "the broader assertion that algorithms precisely capture what can be computed is invalid". Philosophers have interpreted the Church—Turing thesis as having implications for the philosophy of mind.

Jack Copeland states that it is an open empirical question whether there are actual deterministic physical processes that, in the long run, elude simulation by a Turing machine; furthermore, he states that it is an open empirical question whether any such processes are involved in the working of the human brain.

When applied to physics, the thesis has several possible meanings:. There are many other technical possibilities which fall outside or between these three categories, but these serve to illustrate the range of the concept. One can formally define functions that are not computable. A well-known example of such a function is the Busy Beaver function. This function takes an input n and returns the largest number of symbols that a Turing machine with n states can print before halting, when run with no input.

Finding an upper bound on the busy beaver function is equivalent to solving the halting problem , a problem known to be unsolvable by Turing machines. Since the busy beaver function cannot be computed by Turing machines, the Church—Turing thesis states that this function cannot be effectively computed by any method. Several computational models allow for the computation of Church-Turing non-computable functions. These are known as hypercomputers. Mark Burgin argues that super-recursive algorithms such as inductive Turing machines disprove the Church—Turing thesis.

This interpretation of the Church—Turing thesis differs from the interpretation commonly accepted in computability theory, discussed above. The argument that super-recursive algorithms are indeed algorithms in the sense of the Church—Turing thesis has not found broad acceptance within the computability research community. From Wikipedia, the free encyclopedia. For the axiom CT in constructive mathematics, see Church's thesis constructive mathematics.

History of the Church—Turing thesis. This section relies largely or entirely upon a single source. Relevant discussion may be found on the talk page. Please help improve this article by introducing citations to additional sources.

November Learn how and when to remove this template message. Merriam Webster's New Collegiate Dictionary 9th ed. Merriam-Webster's Online Dictionary 11th ed. What is effectively calculable is computable. He calls this "Church's Thesis". Church uses the words "effective calculability" on page ff. Church in Davis Archived from the original PDF on Editor's footnote to Post Finite Combinatory Process.

A significant recent contribution to the area has been made by Kripke Kleene gave an early expression of this now conventional view:. Since our original notion of effective calculability of a function … is a somewhat vague intuitive one, the thesis cannot be proved.

Rejecting the conventional view, Kripke suggests that, on the contrary, the Church-Turing thesis is susceptible to mathematical proof. Furthermore he canvasses the idea that Turing himself sketched an argument that serves to prove the thesis. Put somewhat crudely, the latter theorem states that every valid deduction couched in the language of first-order predicate calculus with identity is provable in the calculus. The first step of the Kripke argument is his claim that error-free, human computation is itself a form of deduction:.

One is given a set of instructions, and the steps in the computation are supposed to follow—follow deductively—from the instructions as given. So a computation is just another mathematical deduction, albeit one of a very specialized form. The execution of this two-line program can be represented as a deduction:.

In the case of Turing-machine programs, Turing developed a detailed logical notation for expressing all such deductions Turing In fact, the successful execution of any string of instructions can be represented deductively in this fashion—Kripke has not drawn attention to a feature special to computation.

The instructions do not need to be ones that a computer can carry out. Nachum Dershowitz and Yuri Gurevich and independently Wilfried Sieg have also argued that the Church-Turing thesis is susceptible to mathematical proof. In their Dershowitz and Gurevich offer. Dershowitz and Gurevich According to Turing, his thesis is not susceptible to mathematical proof. He did not consider either argument I or argument II to be a mathematical demonstration of his thesis: The statement is … one which one does not attempt to prove.

Propaganda is more appropriate to it than proof, for its status is something between a theorem and a definition. Are rhubarb and tomatoes vegetables or fruits? Is coal vegetable or mineral? What about coal gas, marrow, fossilised trees, streptococci, viruses? Has the lettuce I ate at lunch yet become animal? Turing in Copeland b: This myth has passed into the philosophy of mind, theoretical psychology, cognitive science, computer science, Artificial Intelligence, Artificial Life, and elsewhere—generally to pernicious effect.

Turing showed that his very simple machine … can specify the steps required for the solution of any problem that can be solved by instructions, explicitly stated rules, or procedures. Richard Gregory writing in his Turing had proven—and this is probably his greatest contribution—that his Universal Turing machine can compute any function that any computer, with any architecture, can compute That is, it can display any systematic pattern of responses to the environment whatsoever.

These various quotations are typical of writing on the foundations of computer science and computational theories of mind. In reality Turing proved that his universal machine can compute any function that any Turing machine can compute; and he put forward, and advanced philosophical arguments in support of, the thesis that effective methods are to be identified with methods that the universal Turing machine is able to carry out.

The Church-Turing thesis is a thesis about the extent of effective methods, and therein lies its mathematical importance. Putting this another way, the thesis concerns what a human being can achieve when working by rote, with paper and pencil ignoring contingencies such as boredom, death, or insufficiency of paper. Essentially, then, the Church-Turing thesis says that no human computer, or machine that mimics a human computer, can out-compute the universal Turing machine.

This loosening of established terminology is unfortunate, since it can easily lead to misunderstandings and confusion. Some examples from the literature of this loosening are:. If there is a well defined procedure for manipulating symbols, then a Turing machine can be designed to do the procedure.

Geroch and Hartle The behaviour of any discrete physical system evolving according to local mechanical laws is recursive. I can now state the physical version of the Church-Turing principle: Turing and Church were talking about effective methods, not finitely realizable physical systems. A machine m will be said to be able to generate a certain function e. Mutatis mutandis for functions that, like addition, demand more than one argument. All functions that can be generated by machines working in accordance with a finite program of instructions are computable by effective methods.

It is worth noting the existence in the literature of another practice with the potential to mislead the unwary. Although, unlike the terminological practices complained about above, this one is in itself perfectly acceptable.

Thus a function is said to be computable if and only if there is an effective method for obtaining its values. All functions that can be generated by machines working in accordance with a finite program of instructions are computable. Boolos and Jeffrey However, to a casual reader of the technical literature, this statement and others like it may appear to say more than they in fact do. That a function is uncomputable , in this sense, by any past, present, or future real machine, does not entail that the function in question cannot be generated by some real machine past, present, or future.

No possible computing machine can generate a function that the universal Turing machine cannot. But the question of the truth or falsity of the maximality thesis itself remains open. Although the terminological decision, if accepted, does prevent one from describing any machine putatively falsifying the maximality thesis as computing the function that it generates. For example, statements like the following are to be found:.

The stronger-weaker terminology is intended to reflect the fact that the stronger form entails the weaker, but not vice versa. The stronger form of the maximality thesis is known to be false. Although a single example suffices to show that the thesis is false, two examples are given here. An ETM is exactly like a standard Turing machine except that, whereas a standard Turing machine stores only a single discrete symbol on each non-blank square of its tape e.

The method of storing real numbers on the tape is left unspecified in this purely logical model. As previously explained, Turing established the existence of real numbers that cannot be computed by standard Turing machines Turing Abramson also proved that ETMs are able to generate functions not capable of being computed by any standard Turing machine. Therefore, ETMs form counterexamples to the stronger form of the maximality thesis. Accelerating Turing machines ATMs are exactly like standard Turing machines except that their speed of operation accelerates as the computation proceeds Stewart ; Copeland a,b, a; Copeland and Shagrir This enables ATMs to generate functions that cannot be computed by any standard Turing machine.

One example of such a function is the halting function h. The ATM then proceeds to simulate the actions of the n th Turing machine. 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.

At the close of the 20 th century Copeland and Sylvan gave an evangelical survey of the emerging field in their To summarize the situation with respect to the weaker form of the maximality thesis: At the present time, it remains unknown whether hypercomputation is permitted or excluded by the contingencies of the actual universe.

It is, therefore, an open empirical question whether or not the weaker form of the maximality thesis is true. As previously mentioned, this convergence of analyses is generally considered very strong evidence for the Church-Turing thesis, because of the diversity of the analyses. However, this convergence is sometimes taken to be evidence for the maximality thesis.

Allen Newell, for example, cites the convergence as showing that. Yet the analyses Newell is discussing are of the concept of an effective method, not of the concept of a machine-generatable function. The equivalence of the analyses bears only on the question of the extent of what is humanly computable, not on the question of whether the functions generatable by machines could extend beyond the functions generatable by human computers even human computers who work forever and have access to unlimited quantities of paper and pencils.

The error of confusing the Church-Turing thesis properly so called with one or another form of the maximality thesis has led to some remarkable claims in the foundations of psychology. For example, one frequently encounters the view that psychology must be capable of being expressed ultimately in terms of the Turing machine e. To one who makes this error, conceptual space will seem to contain no room for mechanical models of the mind that are not equivalent to Turing machines.

Yet it is certainly possible that psychology will find the need to employ models of human cognition transcending Turing machines. A similar confusion is found in Artificial Life. Christopher Langton, the leading pioneer of A-Life, said the following when writing about foundational matters:.

Turing proved that no such machine can be specified. However, Turing certainly did not prove that no such machine can be specified. It is also worth mentioning that, although the Halting Problem is very commonly attributed to Turing as Langton does here , Turing did not in fact formulate it. Another example is the simulation thesis. For example, the entry on Turing in the Blackwell Companion to the Philosophy of Mind contains the following claims:.

Sam Guttenplan writing in his Can the operations of the brain be simulated on a digital computer? Is there some description of the brain such that under that description you could do a computational simulation of the operations of the brain.

Any process that can be given a mathematical description or that is scientifically describable or scientifically explicable can be simulated by a Turing machine. Paul and Patricia Churchland and Philip Johnson-Laird also assert versions of the simulation thesis, with a wave towards Church and Turing by way of justification:.

Assuming, with some safety, that what the mind-brain does is computable, then it can in principle be simulated by a computer. Churchland and Churchland If you assume that [consciousness] is scientifically explicable … [and] [g]ranted that the [Church-Turing] thesis is correct, then the final dichotomy rests on … functionalism.

If you believe [functionalism] to be false … then … you hold that consciousness could be modelled in a computer program in the same way that, say, the weather can be modelled … If you accept functionalism, however, then you should believe that consciousness is a computational process. But Turing had no result entailing what the Churchlands say.

In fact, he had a result entailing that there are patterns of responses that no standard Turing machine is able to generate. One example of such a pattern is provided by the function h , described earlier. In reality the Church-Turing thesis does not entail that the brain or the mind, or consciousness can be modelled by a Turing machine program, not even in conjunction with the belief that the brain or mind, or consciousness is scientifically explicable, or rule-governed, or scientifically describable, or characterizable as a set of steps Copeland c.

The simulation thesis is much stronger than the Church-Turing thesis: This is equally so if the simulation thesis is taken narrowly, as concerning processes that conform to the physics of the real world. If, on the other hand, the thesis is taken as ranging over all processes, including merely possible or notional processes, then the thesis is known to be false, for exactly the same reasons that the stronger form of the maximality thesis is false. Any device or organ whose internal processes can be described completely by means of what Church called effectively calculable functions can be simulated exactly by a Turing machine providing that the input into the device or organ is itself computable by Turing machine.

But any device or organ whose mathematical description involves functions that are not effectively calculable cannot be so simulated.


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Computability and Complexity the Church-Turing Thesis: types of evidence • large sets of Turing-Computable functions many examples no counter-examples • equivalent to other formalisms for algorithms Church’s l calculus and others • intuitive - any detailed algorithm for manual calculation can be implemented by a Turing Machine.

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Quantum Computation and Extended Church-Turing Thesis Extended Church-Turing Thesis The extended Church-Turing thesis is a foundational principle in computer science.

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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. The Church-Turing thesis in a quantum world Ashley Montanaro Centre for Quantum Information and Foundations, Department of Applied Mathematics and Theoretical Physics.

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In computability theory, the Church–Turing thesis (also known as computability thesis,[1] the Turing–Church thesis,[2] the Church–Turing conjecture, Church's thesis, Church's conjecture, and Turing's thesis) is a hypothesis about the nature of computable functions. This PDF version matches the latest version of this entry. To view the PDF, you must Log In or Become a Member. You can also read more about the Friends of the SEP Society.