Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.theory
Subject: Re: How the requirements that Professor Sipser agreed to are exactly met --- WDH
Date: Wed, 14 May 2025 17:11:29 -0700
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Ben Bacarisse <ben@bsb.me.uk> writes:
> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>> olcott <polcott333@gmail.com> writes:
>>> On 5/14/2025 3:51 PM, dbush wrote:
>>>> On 5/14/2025 11:45 AM, olcott wrote:
>>>>> On 5/14/2025 6:20 AM, Richard Damon wrote:
>>>>>>
>>>>>> And since the DD that HHH is simulating WILL HALT when fully
>>>>>> simulated (an action that HHH doesn't do) 
>>>>>
>>>>> *NOT IN THE ACTUAL SPEC*
>>>>> <MIT Professor Sipser agreed to ONLY these verbatim words 10/13/2022>
>>>>>      If simulating halt decider H correctly simulates its
>>>>>      input D until H correctly determines that its simulated D
>>>>>      would never stop running unless aborted then
>>>>>
>>>> That Sipser didn't agree what you think the above means:
>>>> 
>>>
>>> If that was actually true then you could provide an
>>> alternative meaning for the exact words stated above.
>>>
>>> I keep challenging you to provide this alternative
>>> meaning and you dodge because you know that you are
>>> lying about there being any alternative meaning
>>> FOR THE EXACT WORDS LISTED ABOVE.
>>
>> No alternative meaning is needed, just a correct interpretation of the
>> words (which appear to be incomplete).
>>
>> The quoted sentence is cut off, something that I suspect you didn't
>> notice.  Here's the full quotation from a previous article:
>>
>>>> <Sipser approved abstract>
>>>> MIT Professor Michael Sipser has agreed that the following verbatim
>>>> paragraph is correct (he has not agreed to anything else in this
>>>> paper):
>>>>
>>>> If simulating halt decider H correctly simulates its input D until H
>>>> correctly determines that its simulated D would never stop running
>>>> unless aborted then H can abort its simulation of D and correctly
>>>> report that D specifies a non-halting sequence of configurations.
>>>> </Sipser approved abstract>
>>
>> **If** H correctly simulates its input in the manner you claim,
>> **then** H can correctly report the halting status of D.  (That's a
>> paraphrase that probably doesn't capture the full meaning; the full
>> **quotation is above.)
>>
>> To put it another way, If H correctly simulated its input in
>> the manner you claim, then H could correctly report the halting
>> status of D.
>>
>> I'm not surprised that Sipser would agree to that.  The problem is
>> that it's a conditional statement whose premise is impossible.
>>
>> If an equilateral triangle had four sides, then each of its four
>> vertices would be 90 degrees.  That doesn't actually mean that
>> there exists an equilateral triangle with four 90-degree vertices,
>> and in fact no such triangle exists.  Similarly, *if* a general
>> halt decider existed, then there are a lot of things we could say
>> about it -- but no general halt decider can exist.
>
> It's certainly true that it can be taken to be a purely hypothetical
> remark, but I suspect PO played a more subtle trick on the professor.
> Nothing in the quote that PO presented states that D is special in any
> particular way.  If D is simply "its input" then the statement is simply
> true of some inputs to H: those that can be determined to be non-halting
> can be reported as such.  There will be many such inputs if the
> simulator is clever at detecting certain patterns.
>
> But D and H are the names that Sipser uses in his book in his HP proof.
> Was it made clear that D is not just the name of the input (as the first
> few words of the quote might suggest) but is in fact one very specific
> input constructed from H itself?  I don't know, and I doubt we ever
> will.  If that was made clear, then PO should be saying that Sipser
> agreed to more than just the quoted words and in that case I'd wager his
> agreement was based on the hypothetical nature of the quote.
>
>> I'm not quite 100% confident in my reasoning here.  I invite any
>> actual experts in computational theory (not you, PO) to criticize
>> what I've written.
>
> I am no expert, though I did teach this material in a CS degree course
> for many years.
>
> Richard (H) has reminded us about avoiding straw man arguments and to
> instead address the strongest, clearest version of the point of view we
> reject.  PO is playing with this idea by constructing straw men to be
> agreed with!.  He spent years honing a sentence to express his position
> until is was vague enough that someone important would agree with it.
>
> PO should, however, have asked Sipser to agree with an honest and clear
> statement of his (incorrect) views:
>
> (a) That false (non-halting) is the correct result for some inputs that
> represent halting computations.
>
> (b) This is because, in the special case of a putative halt decider that
> uses simulation, it is correct to report on what /would/ happen were the
> simulation /not/ aborted.
>
> (c) In the specific case where D is constructed from a simulating H in
> the manner typically found in Halting Theorem proofs, H would then be
> able to correctly report a result because H can detect that D halts only
> because H has to stop simulating to report at all.
>
> No one would agree to any of this, so PO must hide these points in some
> sort of waffle or other.  As I say, it took years to move from being
> relatively clear to being vague enough to trick the professor.  All of
> (a), (b) and (c) can be found, in varying degrees of clarity, in PO's
> posts of the last 20 years.

Fair enough, but what I was trying to do in this instance was
to focus on the single statement that PO says Sipser agreed to.
PO complains, correctly or not, that nobody understands or
ackowledges the statement.  I suggest that perhaps it's actually
a true statement *in isolation* (very roughly if a working halt
detector exists then it works as a halt detector), even though it
does not support PO's wider claims.  I've seen a lot of time and
bandwidth expended on this one statement (that PO recently hasn't
even been quoting correctly).

I do not expect to make any progress in helping PO to see the light.
I'm just curious about this one statement and the reaction to it.
I am neither sufficiently qualified nor sufficiently motivated to
analyze the rest of PO's claims.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */