Academian comments on Second-Order Logic: The Controversy - Less Wrong

24 Post author: Eliezer_Yudkowsky 04 January 2013 07:51PM

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Comment author: Qiaochu_Yuan 06 January 2013 04:55:48AM 1 point [-]

That was imprecise, but I was trying to comment on this part of the dialogue using the language that it had established:

Argh! No, damn it, I live in the set theory that really does have all the subsets, with no mysteriously missing subsets or mysterious extra numbers, or denumerable collections of all possible reals that could like totally map onto the integers if the mapping that did it hadn't gone missing in the Australian outback -

I was also commenting on this part:

Screw set theory. I live in the physical universe where when you run a Turing machine, and keep watching forever in the physical universe, you never experience a time where that Turing machine outputs a proof of the inconsistency of Peano Arithmetic.

The point I was trying to make, and maybe I did not use sensible words to make it, is that This Guy (I don't know what his name is - who writes a dialogue with unnamed participants, by the way?) doesn't actually know that, for two reasons: first, Peano arithmetic might actually be inconsistent, and second, even if it were consistent, there might be some mysterious force preventing us from discovering this fact.

I just don't understand yet what you mean by living in a model in the sense of logic and model theory, because a model is a static thing.

Models being static is a matter of interpretation. It is easy to write down a first-order theory of discrete dynamical systems (sets equipped with an endomap, interpreted as a successor map which describes the state of a dynamical system at time t + 1 given its state at time t). If time is discretized, our own universe could be such a thing, and even if it isn't, cellular automata are such things. Are these "static" or "dynamic"?

Comment author: Academian 06 January 2013 07:17:22AM *  1 point [-]

That was imprecise, but I was trying to comment on this part of the dialogue using the language that it had established

Ah, I was asking you because I thought using that language meant you'd made sense of it ;) The language of us "living in a (model of) set theory" is something I've heard before (not just from you and Eliezer), which made me think I was missing something. Us living in a dynamical system makes sense, and a dynamical system can contain a model of set theory, so at least we can "live with" models of set theory... we interact with (parts of) models of set theory when we play with collections of physical objects.

Models being static is a matter of interpretation.

Of course, time has been a fourth dimension for ages ;) My point is that set theory doesn't seem to have a reasonable dynamical interpretation that we could live in, and I think I've concluded it's confusing to talk like that. I can only make sense of "living with" or "believing in" models.

Comment author: Eliezer_Yudkowsky 06 January 2013 01:55:56PM 4 points [-]

Set theory doesn't have a dynamical interpretation because it's not causal, but finite causal systems have first-order descriptions and infinite causal systems have second-order descriptions. Not everything logical is causal; everything causal is logical.