# Why does Turing-computing (being an inconsistent formalism) has undecidable problems? [closed]

I'd like to apply Church-Turing thesis to Kleene-Rosser paradox:

Since untyped lambda-calculus is an inconsistent formalism AND Turing machines are equal in decisive power to lambda-calculus SO We have the same Kleene-Rosser paradox in Turing machine formalism

THEN Since Turing-complete formalism is inevitably inconsistent then everything should be decidable in it

OTOH We have diagonal reasoning which provides for undecidable halting problem

How come?

## closed as unclear what you're asking by Andrés E. Caicedo, Namaste, Delta-u, user133281, Parcly TaxelJul 20 '18 at 17:01

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The untyped lambda calculus, as understood today, is not itself a system of logical reasoning, and therefore it makes little sense to declare it to be "an inconsistent formalism" in and of itself.

What the untyped lambda calculus is, is just a term rewriting system: A set of strings that we call terms, plus a reduction relation between those terms. This relation has interesting properties that we can use for various purposes -- but those applications are not themselves a part of the calculus.

Church initially developed the calculus with a particular application in mind, namely a certain scheme for expressing all mathematical reasoning as equational manipulations within the calculus, as an alternative to, say, ordinary first-order logic. Unfortunately it turned out that some of the possible manipulations in the calculus do not correspond to valid mathematical reasoning when interpreted in the way Church envisaged -- the Kleene-Rosser paradox is one early instance of this; Curry and others later found simpler ones -- and that sunk the hope of using "can be expressed in this calculus" as a formalization of "is a valid mathematical argument".

(I don't know the exact details of how this was supposed to work. They are not easy to find nowadays -- because they didn't actually work, they don't get a lot of publicity).

So the particular logical system that Church built using the untyped lambda calculus is unsound -- but that doesn't mean that the underlying rewrite system itself is "inconsistent". That might have been a fair way to describe it if, for example, the paradox implied that the reflexive transitive closure of the reduction relation relates everything to everything else. But that is not so; we have impeccable finitistic proofs that different terms in $\beta$-normal form are not $\beta$-equivalent.

When the Church-Turing thesis is applied to the untyped lambda calculus, it is a different application of it, not tainted by Kleene-Rosser's demonstration that Church's attempt to build a general system of logic failed. Here it just means something like

Let $f$ be an effectively computable partial function $\mathbb N \to \mathbb N$. Then there is a lambda term $M_f$ such that

• whenever $f(x)=y$ we have $M_f\, \overline x =_\beta \overline y$, where $\overline x$ means the Church numeral for $x$
• whenever $f(x)$ is undefined, $M_f\, \overline x$ is not $\beta$-equivalent to any term in normal form.

Conversely let $M$ be any untyped lambda term, and define $f_M$ by $$f_M(x)=y \iff M\,\overline x =_\beta \overline y$$ Then $f_M$ is a well-defined partial function $\mathbb N\to\mathbb N$, and is effectively computable.

(or any of a wide variety of other possible coding schemes).

This fact also doesn't mean that a failed attempt to use the lambda calculus as a system of logic casts doubt on the concept of "effectively computable".

• You can easily find the original papers of Alonzo Church, e.g. A set of postulates for the foundation of logic. While that particular attempt failed, it was followed by Church creating the simply typed lambda calculus which has, of course, been extremely successful. The modern descendant of what Church was originally attempting to do would be systems like HOL4. Particularly Isabelle/HOL has been itself extremely successful. – Derek Elkins Jul 23 '18 at 5:21