Michael Shulman is quite right to press the question of how to express the axiom scheme of replacement WITHOUT using classes or universes. There are two traditions on this topic in set theory. One of them states replacement and collection "from the outside" by using classes or universes. The other, found in ZF itself, somehow builds it up "from the inside", without pre-supposing classes, although the large cardinals that result from this hypothesis can themselves be used as universes for weaker set theories. Steve Awodey and Thomas Streicher have referred to the excellent work in Algebraic Set Theory that is collected on Steve's web page at www.phil.cmu.edu/projects/ast/ ASD is a categorical treatment that follows the "sets and classes" tradition. However, Mike acknowledged that in his original posting, and, with all due respect, Steve and Thomas have NOT answered his question there:

I'm not familiar with that particular result, ie "every polynomial functor on Set has a unique initial algebra whose structure map is an identity", mentioned by Paul Levy, but I know other categorical proofs which use set-theoretic ideas like transfinite induction, and so cannot be detached from ZF in an obvious way. On the other hand, there is nothing intrinsically "membership-based" in transfinite induction. The problem seems to be the lack of a categorical analogue of ZF's axiom of replacement, since the sets in V_{\omega+\omega} already form a well-pointed elementary topos with a NNO. I find this especially mysterious because on the surface, replacement merely replaces a set by an isomorphic one (or at most a quotient)!

One categorical analogue of replacement comes from categories of classes in algebraic set theory. That is, we move from a categorical analogue of ZF to an analogue of Godel--Bernays set theory. But it seems natural to wonder whether there could be a categorical analogue of replacement expressible solely as a property of the category Set, without reference to how it sits in a category of classes. Has anyone studied this question?

Although he had expressed himself quite clearly the first time, it seems that he needed to put his question again:

"In consequence, the standard arguments using Replacement that take one outside of V_\lambda(A) for \lambda non-inaccessible, are not reproducible." What I would really like to know is, can one formulate an elementary property of a topos which *does* allow one to reproduce the standard arguments of Replacement?

I agree that it would be very nice to have a formulation of Replacement in category theory that was as simple as, say, the notion of subobject classifier. I proposed such a thing in the final pages of my book, "Practical Foundations of Mathematics" (CUP, 1999), and this posting leads up to my formulation. Colin McLarty gave a non-AST answer to Mike's question, citing a paper by Gerhard Osius and one of his own. I confess that I have not seen either of these, not currently having access to a university library, so please excuse me if they already say what I am about to say. I would, however, ask the algebraic set theorists to acknowledge that there is another point of view on the question, albeit one that has not been worked out so extensively as theirs. Anybody who has attempted to discuss foundations with either set theorists or mainstream mathematicians knows that it is impossible to get them to engage. Colin McLarty has already pointed this out in this thread, saying that

various people including Sol Feferman promote the view that if you use "sets" then you are admitting that you use ZF and not some categorical foundations.

Of course, set theorists who claim this are simply hijacking the English language, and we have to resist them in so doing. One problem on the other side is that most mathematicians couldn't actually quote the ZF axioms that they claim to use, without looking them up. They could, on the other hand, readily give a practical account of - cartesian products, subsets, disjoint sums and quotients of equivalence relations, which are axiomatised in category theory as a "pretopos"; - primitive recursion over N, lists and finite subsets, which, together with the above, make an "arithmetic universe"; and - full powersets, which make an "elementary topos"; because they teach this to first year mathematics and computer science undergraduates. In Section 2.2 of my book I set out what I call "Zermelo type theory", because it differs very little from his set theory. This is what ordinary mathematicians use, and it is also the ("Mitchell--Benabou internal") language of a topos. I would be quite willing to explain to any set theorist how any statement in my book (apart from the last section) can be understood in categorical terms within an elementary topos. (One of the many uses that I would have for a time machine would be to go back 100 years, tap Ernst Zermelo on the shoulder while he was writing "Untersuchungen uber die Grundlagen der Mengenlehre I", and ask him to make those pairs ORDERED, as it would have avoided so much trouble since then! Note that Bourbaki's "Theorie des Ensembles" DOES have ordered pairs.) Some time ago, Saunders Mac Lane found himself in the middle of a dispute with the set theorists because he had claimed that most of mathematics could be expressed in an elementary topos with a natural numbers object. Unfortunately, I have never managed to work out exactly which of his publications was the centre of the controversy - maybe Colin can fill us in here on the bibliographical details. The bit that's missing, as Mike said, is the axiom scheme of replacement. But we are back to the original problem of communication with set theorists in trying to identify what this actually means. They are rather like Proteus in Greek mythology, who could foretell the future, but only to someone who could capture him, which he would avoid by repeatedly changing form - into a lion, serpent, leopard, pig, water or tree (according to Google and Wikipedia). At first, Replacement appears just to provide the image of a set under a function. I have seen several set theory text books that give this impression, but I suspect that their authors have no idea of the logical strength of Replacement. Of course, when you recognise that what mathematical objects DO is what matters, and not what they ARE, you appreciate that the image is simply a quotient by an equivalence relation. I'm sure that set theorists must have realised a long time ago that this could be done within Zermelo set theory. However, no sooner have you rejected that answer than they come up with another, equally fatuous one: Replacement is needed to construct the ordinal omega+omega. Whilst this may actually be true of the kind of intuitionistic ordinals that I called "plump", it is nonsense in a classical setting: a model of omega+omega is provided by the even numbers followed by the odd ones. It's only the set theorists' instistance on representing everything by means of epsilons and nested curly brackets that necessitates Replacement. Giving up on set theorists for a sensible answer, let's return to ordinary mathematics. The usual situation in which Replacement is needed is when you want to form the limit or colimit of some iterated construction. Even then it may be possible to carve the result out from a sufficiently big object that is definable in Zermelo set theory, or in an elementary topos with natural numbers. But there are clear examples in which this cannot be done. One is N-fold iteration of the powerset, starting from N itself, since such a union would itself be a model of Zermelo set theory. An example of this arises in domain theory. Start with the domain X_0 that consists of N+3 points arranged something like this: 0 1 2 3 .... xxxxxxxxxx a b \ / bottom where "xxxxxxxxxx" means that every numeral is above both a and b. Then form the exponentials X_n+1 = X_n ^ X_n. It can be shown that these are algebraic L-domains (in Achim Jung's terminology) whose bases increase in cardinality in the same way as the iterated powersets do. With these examples in mind, we can return to Mike Shulman's original question, or at least some simple example of it: Can we express the transfinite iteration of a functor in purely categorical language? Whether this expresses the full strength of Replacement, I do not know - probably not. However, as I have said, it is impossible to engage set theorists in an intelligent discussion about this, as they seem to be incapable of thinking outside their idiosyncratic representation of mathematical objects as epsilons and curly brackets. Before considering the iteration of functors, we need to have some categorical notion of "ordinal" and "transfinite". In anticipation of anyone else who may feel the need to point this out, let me say first that Andre' Joyal and Ieke Moerdijk studied ordinals as part of the original book on Algebraic Set Theory. However, AST is a "sets and classes" formulation. I too studied (intuitionistic and categorical) ordinals at the same time (the mid 1990s), but in an intrinsic (no classes) way. My first account of this, @article{TaylorP:intso, author = {Taylor, Paul}, title = {Intuitionistic Sets and Ordinals}, journal = {Journal of Symbolic Logic}, volume = 61, year = 1996, pages = {705--744}} developed set theory as (well founded, extensional) epsilon-structure on a carrier, in the same way as we develop group theory as algebraic structure on a carrier. It proved Mostowski's theorem - that any well founded structure has an extensional quotient - WITHOUT using Replacement. The set theory textbooks say that this is necessary, because of their epsilontic representation. It then developed set theory a la Zermelo, and various kinds of intuitionistic ordinals, providing a parallel approach to that in AST, with analogous results. (Intuitionistically, there are several different kinds of ordinals.) Later I studied "well founded coalegebras" and showed that this notion of INDUCTION implies RECURSION, in the form of coalgebra-to-algebra homomorphisms. This is in Section 6.3 of my book, and also in an unpublished paper that you can find on my web site at www.PaulTaylor.EU/ordinals Two years ago, a certain categorist tried to pass this work off as his own, at the Nova Scotia category theory meeting and in the proceedings of "Computer Science Logic". In doing so, he also demonstrated his ignorance of both set theory and category theory as foundations. I hereby warn him that, should he utter a single word on this subject in my hearing, I will identify him and publish full details of the events. My work on this topic was based on a different paper by Gerhard Osius from the one that Colin cited, @article{OsiusG:catstc, author = {Osius, Gerhard}, title = {Categorical Set Theory: a Characterisation of the Category of Sets}, journal = {Journal of Pure and Applied Algebra}, volume = 4, year = 1974, pages = {79--119}, review = {MR 51/643}} which is also summarised in Section 9.2 of "Topos Theory" by Peter Johnstone. It has also been taken up by Capretta, Uustalu and Vene, but in the context of functional programming, not set theory. If there are more recent developments based on the tradition of Osius then I would be pleased to hear about them. Briefly, we express an ordinal number alpha as a coalgebra parse: alpha ---> D(alpha) in the category of posets, where D is the covariant functor that yields the poset of lower subsets. The reason for the name "parse" is given in my book; in this case, its set-theoretic translation is parse (alpha) = { beta | beta epsilon alpha } Now we are in a position to define transfinite iteration of a functor T. This has to be INDEXED (or FIBRED if you prefer), ie it is applied, not to a single object of the category, but to an alpha-indexed family of objects. The display map X--->alpha whose beta'th fibre is the beta-fold iteration of T on the empty set is characterised by the fact that it forms a pullback [beta:alpha, X[beta]] ---> [U:D(alpha), colim {TX[gamma] | gamma in U] | | | | | | |------+ | | | | | | | V parse V alpha ------------------------------> D(alpha) Considered in the category of all discrete fibrations over posets, those for which this is a pullback form reflective subcategory. More precisely, this subcategory is closed under limits, but it only has a reflector if the axiom scheme of replacement holds (say, with the covariant powerset for T). I don't actually think that the ordinals (ie totally ordered sets) as such are needed in this approach. I think one could present it using proofs (of well formedness of terms in a type theory) instead. Then each type theory has a version of Replacement that proves that the given theory is consistent. I got a brief and tiny glimpse of the awesome power of the axiom scheme of replacement in the later stages of writing my book, but that was ten years ago. It appeared to say something like this: if you have some theory that is provably consistent, ie it has a free internal model, then it has a model that is a submodel of the universe in which you are working. Or, "any script can be played out in real life". Please forgive me if I cannot now say this very clearly - I found the idea rather frightening at the time, and it has long since been "swapped out" of my skull. Replacement was conceived by Dimitri Mirimanoff in 1917, and incorporated into Zermelo's axioms by Abraham Fraenkel in 1922. This was well after the fuss over Russell's paradox (1900) had died down, but well before Godel and Turing provided a new generation of paradoxes in the 1930s. So it may be that set theorists had let their guard down in the intervening decades. On Good Friday 1998, I (thought that I had) found a proof that ZF is inconsistent, but the system was resurrected on the third day. I published this "proof" as the final exercise of my book and on "categories" on 1 April 1999; this posting has since being doing the rounds, and I got a reply a couple of years ago. I am certainly not going to spend my time looking for contradictions in set theory, but I find it amazing that anyone should be so confident that Replacement is safe to work with. I also think that it would do Mathematics a great deal of good if someone were to find an inconsistency in ZFC. Paul Taylor