My colleague Michael Potter organized a one-day workshop today focusing on issues arising out of Dan Isaacson’s paper, “Arithmetical truth and hidden higher-order concepts”, which appeared 20 years ago.

It’s quite difficult to tease out the full content of Dan’s claims about the status of first-order PA in his paper, but a core claim — I call it Isaacson’s Thesis — is surely this:

If we are to give a proof for any true sentence of the language of first-order PA which is independent of PA, then we will need to appeal to ideas that go beyond those that are required in understanding PA (and more generally, but more vaguely, beyond those that are required to understand the structure of the natural numbers and thus elementary pure arithmetic).

Dan wants to say more (though that’s where things get a bit murkier), but the core Thesis is reasonably clear and interesting in itself — and if not even that is true, then Dan’s stronger claims won’t be true. In my talk “Ancestral Arithmetic and Isaacson’s Thesis” (NB the link is to an unrevised version!), I suggested that we can construct a formalized arithmetic PA* with a primitive ancestral operator which in fact is within the conceptual grasp of someone who understands PA. If PA* enabled us to prove new theorems in the language of PA that would be a strike against the Thesis. But I outlined a proof that PA* is in fact conservative over PA, a result not just in harmony with the Thesis but giving it positive support. (The talk went pretty well, but I made a bit of a hash of the questions, but what’s new?)

The other talks were by Ben Colburn (on of our PhD students) and Hannes Leitgeb, who talked more about what we might reasonably mean by saying with Dan that PA is sound and complete for “arithmetical truth”. And Luca Incurvati (another of our students) and Dan Isaacson himself talked about analogues for set theory of Dan’s thesis for arithmetic. In a way, the day ended too soon, as the discussions after the final paper by Hannes were just seeming to get to the heart of issues about arithmetic when we broke up. But a very productive and enjoyable day all the same.