I'm a mathematical physicist. I'm making a big career shift: I've been working on n-categories and fundamental physics, but now I'm visiting the Centre for Quantum Technologies for two years, and I'm thinking about technology and the global ecological crisis. I'll probably keep thinking about the old stuff a bit, too. If you're a scientist or engineer interested in saving the planet, I hope you join the Azimuth Project. Visit it here: http://www.azimuthproject.org/ or else check out my blog: http://johncarlosbaez.wordpress.com/

## First-Order Categorical Logic 5

JB: Okay, let me try to sketch out a more categorical approach to Gödel’s completeness theorem for first-order theories. First, I’ll take it for granted that we can express this result as the model existence theorem: a theory in first-order logic has a model if it is consistent. From this we can easily get the usual formulation: if a sentence holds in all models of a theory, it is provable in that theory.

Filed under Categories, Conversations, Logic

## First-Order Categorical Logic 3

JB: Okay, let’s talk more about how to do first-order classical logic using some category theory. We’ve already got the scaffolding set up: we’re looking at functors

$B \colon \textrm{FinSet} \to \textrm{BoolAlg}.$

You can think of $B(S)$ as a set of predicates whose free variables are chosen from the set S. The fact that B is a functor captures our ability to substitute variables, or in other words rename them.

But now we want to get existential and universal quantifiers into the game. And we do this using a great idea of Lawvere: quantifiers are adjoints to substitution.

Filed under Categories, Conversations, Logic

## Non-Standard Models of Arithmetic 10

JB: So, last time you sketched the proof of the Paris–Harrington theorem. Your description is packed with interesting ideas, which will take me a long time to absorb. Someday I should ask some questions about them. But for now I’d like to revert to an earlier theme: how questions about the universe of sets cast their shadows down on the world of Peano arithmetic.

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Filed under Conversations, Peano Arithmetic

## First-Order Categorical Logic 1

(A conversation between John Baez and Michael Weiss.)

JB: Okay, maybe it’s a good time for me to unleash some of my crazy thoughts about logic. They’ve been refined a lot recently, thanks to all the education I’ve been getting from you and folks on the n-Category Café. So, I can actually start with stuff that’s not crazy at all… although it may seem crazy if you’re not used to it.

I’ll start with some generalities about first-order classical logic. (I don’t want to get into higher-order logic or intuitionistic logic here!) The first idea is this. In the traditional approach, syntax and semantics start out living in different worlds. In categorical logic, we merge those worlds.