[Agda] If all functions (N->N)->N are continuous, then 0=1.

Altenkirch Thorsten psztxa at exmail.nottingham.ac.uk
Thu Nov 28 09:28:24 CET 2013

I am puzzled again!

Clearly, this formulation of continuity is inconsistent with functional
extensionality because it makes it possible to observe an intensional
property of a function - its modulus of continuity. What is surprising is
that you manage to derive a contradiction w.o. using ext! So the wrong
formulation of continuity doesn't even work in vanilla intensional Type


On 27/11/2013 21:56, "Martin Escardo" <m.escardo at cs.bham.ac.uk> wrote:

>   If all functions (N->N)->N are continuous, then 0=1.

That is a bit of a lie. What you are saying is if for all functions we
know why they are continuous, then 0=1.

I mean "is continuous" does sound like a proposition, doesn't it?

>Think of functions a : N -> N as sequences of natural numbers:
>    a 0, a 1, a 2, a 3, ..., a n, ...
>The "continuity axiom" for functions
>     f : (N -> N) -> N,
>that map sequences a : N -> N to numbers f(a), going back to Brouwer
>in his intuitionistic mathematics in the early 20th century, says that
>the value f(a) can depend only on a finite prefix of the infinite
>argument a.
>This makes sense computationally: there are no crystal balls in
>computational processes, able to see and grasp the infinite input at
>once. The input of f is computed bit-by-bit, in fact, often only when
>f queries it.
>After finitely many queries to its argument a : N -> N, the function f
>is supposed to deliver its answer, if it's ever going to produce an
>When this finiteness condition holds, we say that f is continuous
>(never mind the reason for the terminology "continuous" in this message).
>The link
>   http://www.cs.bham.ac.uk/~mhe/continuity-false/continuity-false.html
>writes down Brouwer's formulation of continuity in Agda notation, and
>proves that:
>   If all functions (N->N)->N are continuous, then 0=1.
>What is going on? There is some discussion in the above link.
>You may wish to read Andrej Bauer's related discussion
>The ideas of http://homotopytypetheory.org/book/ are relevant in this
>respect too. To solve the apparent paradox, think about how one can
>constructively prove existence without disclosing witnesses. Doubly
>negated existence is too weak. One needs a stronger, constructive
>notion of existence that deliberately doesn't exhibit witness, which
>sometimes, but not always, can get to be disclosed. I attended a nice
>talk by Thierry Coquand yesterday, in which he showed how the "axiom
>of description" explains when and how the secret can be constructively
>disclosed. In the HoTT book, this is explained as the axiom of unique
>NB. Only the purest form of intensional Martin-Loef Type Theory is
>used in the above Agda proof, and no universes are needed. No HoTT
>axioms in particular.
>Best, Martin
>Agda mailing list
>Agda at lists.chalmers.se

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