From: Matt Oliveri <atm...@gmail.com>
To: Homotopy Type Theory <HomotopyT...@googlegroups.com>
Subject: Re: [HoTT] Impredicative set + function extensionality + proof irrelevance consistent?
Date: Wed, 20 Dec 2017 16:42:16 -0800 (PST) [thread overview]
Message-ID: <318cbeff-e7f2-4c45-b3c4-f392a94dd09d@googlegroups.com> (raw)
In-Reply-To: <20171220114104.GG8054@mathematik.tu-darmstadt.de>
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Hi Thorsten and Thomas,
It still looks to me like you're talking about different things and having
a misunderstanding.
By "propositional extensionality", Thorsten seems to mean the special case
of univalence that applies to hprops. (Which he's simply calling
propositions.) But it sounds like Thomas is counting "propositional
extensionality" as a separate principle from univalence, for a type of
static props. I think the system Thorsten has in mind presents a (pre)topos
as a univalent type system, where hprops are used *instead of* a type of
static props.
But maybe not, and I'm misunderstanding.
On Wednesday, December 20, 2017 at 6:41:14 AM UTC-5, Thomas Streicher wrote:
>
> Hi Thorsten,
>
> > we have already established that my argument was incorrect (for the
> > reasons you state) and I was misinformed about the behaviour of
> > Lean.
>
> I know, I just wanted to spot where the problem precisely is.
>
> > >Another gap in Thorsten's argument is the following. Though Single(a)
> and
> > >Single(a') are isomorphic in order to conclude that they are
> propositionally
> > >equal they would have to be elements of a univalent universe BUT I
> don't see
> > >where such a universe should come from in the general topos case!
>
> > I don???t understand this point. In a type theoretic implementation of a
> topos Single(a) and Single(a???) would be propositionally equal due to
> propositional extensionality. The only additional assumption I need to make
> is that the universe of proposition is strict, e.g. we have that El(A -> B)
> is definitionally equal to EL(A) -> El(B). This seems to be quite natural
> from the point of type theory where universes are usually strict and
> moreover this is true in any univalent category giving rise to a topos.
>
> Well, Single(a) and Single(a') were propsitionally equal if they were
> elements of a univalent universe U but where should this come from if
> you start from an elementary topos in a univalent metatheory.
>
> Thomas
>
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next prev parent reply other threads:[~2017-12-21 0:42 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-12-11 4:22 Kristina Sojakova
2017-12-11 11:42 ` [HoTT] " Jon Sterling
2017-12-11 12:15 ` Kristina Sojakova
2017-12-11 12:43 ` Jon Sterling
2017-12-11 14:28 ` Thomas Streicher
2017-12-11 14:32 ` Kristina Sojakova
2017-12-11 14:23 ` Thorsten Altenkirch
2017-12-12 10:15 ` Andrea Vezzosi
2017-12-12 11:03 ` Thorsten Altenkirch
2017-12-12 12:02 ` Thomas Streicher
2017-12-12 12:21 ` Thorsten Altenkirch
2017-12-12 13:17 ` Jon Sterling
2017-12-12 19:29 ` Thomas Streicher
2017-12-12 19:52 ` Martin Escardo
2017-12-12 23:14 ` Michael Shulman
2017-12-14 12:32 ` Thorsten Altenkirch
2017-12-14 18:52 ` Michael Shulman
2017-12-16 15:21 ` Thorsten Altenkirch
2017-12-17 12:55 ` Michael Shulman
2017-12-17 17:08 ` Ben Sherman
2017-12-17 17:16 ` Thorsten Altenkirch
2017-12-17 22:43 ` Floris van Doorn
2017-12-15 17:00 ` Thomas Streicher
2017-12-17 8:47 ` Thorsten Altenkirch
2017-12-17 10:21 ` Thomas Streicher
2017-12-17 11:39 ` Thorsten Altenkirch
2017-12-18 7:41 ` Matt Oliveri
2017-12-18 10:00 ` Michael Shulman
2017-12-18 11:55 ` Matt Oliveri
2017-12-18 16:24 ` Michael Shulman
2017-12-18 20:08 ` Matt Oliveri
2017-12-18 10:10 ` Thorsten Altenkirch
2017-12-18 11:17 ` Matt Oliveri
2017-12-18 12:09 ` Matt Oliveri
2017-12-18 11:52 ` Thomas Streicher
2017-12-19 11:26 ` Thorsten Altenkirch
2017-12-19 13:52 ` Andrej Bauer
2017-12-19 14:44 ` Thorsten Altenkirch
2017-12-19 15:31 ` Thomas Streicher
2017-12-19 16:10 ` Thorsten Altenkirch
2017-12-19 16:31 ` Thomas Streicher
2017-12-19 16:37 ` Thorsten Altenkirch
2017-12-20 11:00 ` Thomas Streicher
2017-12-20 11:16 ` Thorsten Altenkirch
2017-12-20 11:41 ` Thomas Streicher
2017-12-21 0:42 ` Matt Oliveri [this message]
2017-12-22 11:18 ` Thorsten Altenkirch
2017-12-22 21:20 ` Martín Hötzel Escardó
2017-12-22 21:36 ` Martín Hötzel Escardó
2017-12-23 0:25 ` Matt Oliveri
2017-12-19 16:41 ` Steve Awodey
2017-12-20 0:14 ` Andrej Bauer
2017-12-20 3:55 ` Steve Awodey
[not found] ` <fa8c0c3c-4870-4c06-fd4d-70be992d3ac0@skyskimmer.net>
2017-12-14 13:28 ` Thorsten Altenkirch
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