Nisan comments on Reflection in Probabilistic Logic - Less Wrong

63 Post author: Eliezer_Yudkowsky 24 March 2013 04:37PM

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Comment author: Nisan 23 March 2013 08:04:03AM *  3 points [-]

We say that P is coherent if there is a probability measure μ over models of L such that

Annoying pedantic foundational issues: The collection of models is a proper class, but we can instead consider probability measures over the set of consistent valuations , which is a set. And it suffices to consider the product σ-algebra.

Comment author: Benja 23 March 2013 12:34:44PM *  4 points [-]

The collection of models is a proper class

I assumed that this meant "there is some probability space whose set of outcomes is some set of models", rather than "there is a measure over 'the canonical' probability space of all models", whatever that could mean.

And it suffices to consider the product σ-algebra.

This is the smallest σ-algebra such that for every sentence phi of L, {A : phi in A} is measurable, right? If so, it is pleasingly also the Borel-σ-algebra on the Stone space of consistent valuations (aka complete theories) of L. (Assuming L is countable, but I believe the paper does that anyway.)

Comment author: Qiaochu_Yuan 26 March 2013 01:49:08AM 2 points [-]

I assumed that this meant "there is some probability space whose set of outcomes is some set of models", rather than "there is a measure over 'the canonical' probability space of all models", whatever that could mean.

Yes, I asked Paul about this and this is the interpretation he gave.