*In a couple of very well known papers, George Boolos argued that “the axioms of replacement do not follow from the iterative conception”. Was he right? Or can Replacement be justified on (some core version of) the iterative conception? This is the topic of the particularly interesting §3.6 (pp. 90–100) of *Conceptions of Set*, ‘The Status of Replacement’. *

Luca discusses three lines of argument to be found in the literature for the thought that the iterative conception *does* warrant Replacement. I’ll comment on two in this post.

The first he calls *Gödel’s Argument*. Two quotes from Gödel: (i) “From the very idea of the iterative concept of set it follows that if an ordinal number α has been obtained, the operation of power set *P* iterated α times leads to a set *P*^{α}(∅)” And then (ii) “the next step will be to require that any operation producing sets out of sets can be iterated up to any ordinal number.”

In response, Luca makes the following central points:

- Tait and Koellner have argued that elaborating Gödel’s claim (i) requires appeal to Choice. But not so. For we can work with the Scott-Tarski definition of an ordinal, and then, without needing an assumption of Choice, Gödel’s thought will at least warrant adding to Z
^{+}the Axiom of Ordinals — the axiom that there is a level*V*_{α}for every ordinal α. - This theory with the Axiom of Ordinals is rich, much more powerful than Z
^{+}, and in fact buys us the nice results that Boolos claimed for Replacement. However, the Axiom of Ordinals is weaker than full Replacement. - A version of Gödel’s second claim (ii) is needed to get us from the iterative conception to full Replacement, and it isn’t clear why (ii) should be thought of as part of the iterative conception.

On (1), accepting Gödel’s (i), Luca’s discussion seems spot on. On (2) quite a few readers (those familiar with ZFC but who haven’t read Potter’s book) might well have welcomed rather more at this point on the Axiom of Ordinals, on its virtues and mathematical consequences. They might reasonably ask: if the Axiom of Ordinals in fact gives us (a good deal of) what we want, just why — other than conservative adherence to tradition! — should we buy full-power Replacement? On (3), we could I suppose go back to pause over (i) to wonder if the idea of the universe of sets being layered by iterating the *set of* operation *has* to go along with the idea that those iterations should be ordinal-many (for any ordinal we can obtain). But leave that aside: Luca it is surely right that it is one thing to build into the iterative conception the idea that the core *set of* operation should in *some* sense be iterated ‘as far as possible’, it is another thing to require that other operations be iterated as far as possible too.

The second line of argument for Replacement to be discussed is what Luca calls *The Argument from Co-finality*. Thinking in terms of stages of the hierarchy, Shoenfield (in his famous *Handbook* article) suggests that for any collection of stages **S**, there will be one after it “provided that we can imagine a situation in which all of the stages in **S** have been completed”. But then assume that we have a set *x* and for every *y* in *x* there is a stage *S _{y}* correlated somehow or other with

*y*. Then

Suppose … we take each

yinx… and complete the stageS. When we reach the stage at which_{y}xis formed, we will have formed eachyinxand hence completed each stageS._{y}

So if **S** is the collection of stages *S _{y}*, we can imagine a situation in which all of the stages in

**S**have been completed and there will be a stage after it.

Now, if we buy *this* cofinality principle, then Replacement is immediate (since Replacement tells us that the image of *x* under a function *f* will itself be a set; for each *y* in *x* take *S _{y}* to be the stage at which

*f(y)*is formed, and then by Shoenfield’s principle there will be a stage after all those at which we can gather together all the

*f(y)*into a set …).

What are we to make of all this? Luca surely hits the nail on the head! As he neatly notes, Shoenfield’s argument

seems to assume that if a process can be completed, and we replace each stage of the process with a process that can be completed, then the maxi-process consisting of all these processes can itself be completed. But this is just the Axiom of Replacement reformulated in terms of stages and processes.

The supposed defence of Replacement is therefore too close to being circular.

So we haven’t got here an independent argument from the iterative conception to Replacement. Luca concludes, however, on a (surprisingly?) sympathetic note: “the cofinality principle certainly seems to harmonize well with the iterative conception, and can perhaps be seen as one way of spelling out the idea that the cumulative process through which the hierarchy is obtained should be iterated as far as possible.” But equally couldn’t we spin it the other way? — the iterative conception itself doesn’t take us as far as Replacement, and it takes a further independent thought to justify that principle. Which leaves us with the question of what rival further thoughts (equally harmonizing with the basic iterative conception) might be on the cards.

*To be continued on a third line of argument for Replacement, via reflection principles.*

SebastianThis is not related to this post. But I have been meaning to ask you this. I am wondering if could teach an online logic class–some advanced logic, e.g., Godel’s incompleteness, etc. I am sure many people would love it–especially in this strange time! Please consider this.

Rowsety Moid1. It does rather seem

truethat if a process can be completed, and we replace each stage of the process with a process that can be completed, then the thus revised process consisting of all these processes can itself be completed.2. The “reformulation” objection seems to assume that if we have doubts about whether something can be justified, then no reformulation can remove those doubts — and that seems plainly wrong. One formulation may make it hard to see that it’s justified, while a different formulation makes it clear.

Peter SmithThe question that Luca is discussing here isn’t whether Replacement is

correctbut whether it is (near enough)supported by the iterative conception.Boolos famously argues, not against Replacement, but against the idea that Replacement falls out of the iterative conception, and argues that it needs a different defence (either ‘it works’, organizing results we want, or via ‘limitation of size’). And Luca, I think, is similarly arguing that Replacement doesn’t follow from the iterative conception at least by Shoenfield’s route — unless we buy the process principle you state when applied to the transfinite business of stage construction in the iterative hierarchy. That principle might be true; but it seems fair comment to say with Luca that it is a substantive extra principle that isn’t part of what we were initially buying in signing up to the iterative conception

Rowsety MoidI can see I have to put it differently.

Shoenfield has an argument — Incurvati even says it’s “probably the standard argument” — that Replacement “is justified on the iterative conception.” (p 93)

Incurvati argues against that argument, saying it “seems to assume that if a process can be completed, and we replace each stage …”.

Call that “if a process …” P for “process”.

It seems reasonable to me for an argument (Shoenfield’s) to assume (rely on / use) something (P) that’s true. If P is true, and it does seem to be, then that gets Shoenfield’s argument to its conclusion.

Incurvati also says “this is just the Axiom of Replacement reformulated in terms of stages and processes” (p 95), and you say “The supposed defence of Replacement is therefore too close to being circular.”

Wait a minute, though! P doesn’t look very much like Replacement, and it’s not Replacement that makes P seem true. If Shoenfield’s argument relies on P, it isn’t relying on Replacement: it’s relying on something (true) about stages and precesses in order to show that Replacement is justified on the stages and processes involved in the iterative conception.

(Also, P doesn’t automatically give us Replacement; it gives us Replacement only when applied to certain particular stages and processes.)