Phil. of maths

Philosophy of mathematics — a reading list

A few people recently have quite independently asked me to recommend some introductory reading on the philosophy of mathematics. I have in fact previously posted here a short list in the ‘Five Books’ style. But here’s a more expansive draft list of suggestions.

Let’s begin with an entry-level book first published twenty years ago but not yet superseded or really improved on:

  1. Stewart Shapiro, Thinking About Mathematics (OUP, 2000). After introductory chapters setting out some key problems and sketching some history, there is a group of chapters on what Shapiro calls ‘The  Big Three’, meaning the three programmatic ideas that shaped so much philosophical thinking about mathematics for the first half of the twentieth century — i.e. varieties of logicism, formalism, and intuitionism. Then there follows a group of chapters on ‘The Contemporary Scene’, on varieties of realism, fictionalism, and structuralism. This might be said to be a rather conservative menu — but then I think this is just what is needed for a very first introduction to the area, and Shapiro writes with very admirable clarity.

By comparison, Mark Colyvan’s An Introduction to the Philosophy of Mathematics (CUP, 2012) is far too rushed to be useful. And I would say much the same of Øystein Linnebo’s Philosophy of Mathematics (Princeton UP, 2017). David Bostock’s Philosophy of Mathematics: An Introduction (Wiley-Blackwell, 2009) is more accessible, but — apart from a chapter on predicativism — covers similar ground to the earlier parts of Shapiro’s book, but has little about more recent debates.

A second entry-level book, narrower in focus, that can also be warmly recommended is

  1. Marcus Giaquinto, The Search for Certainty (OUP, 2002). Modern philosophy of mathematics is still in part shaped by debates starting well over a century ago, springing from the work of Frege and Russell, from Hilbert’s alternative response to the  “crisis in foundations”, and from the impact of Gödel’s work on the logicist and Hibertian programmes. Giaquinto explores this with enviable clarity: this is really exemplary exposition and critical assessment. A terrific book.

Then, before moving on, I have to mention that most accessible of modern classics:

  1. Imre Lakatos, Proofs and Refutations (originally published in 1963-64, and then in expanded book form by CUP, 1976). Textbooks tend to present developed chunks of mathematics in a take-it-or-leave-it spirit, the current polished surface hiding away the earlier rough versions, the conceptual developments, the false starts. Proofs and Refutations makes for a wonderful counterbalance. A classic exploration in dialogue form of the way that mathematical concepts are refined, and mathematical knowledge grows. We may wonder how far the morals that Lakatos draws can be generalized; but this remains a fascinating read (I’ve not known a good student who didn’t enjoy it).

Mathematical Structuralism, Essay 3

The third essay in The Pre-history of Mathematical Structuralism is by José Ferreirós and Erich H. Reck, on ‘Dedekind’s Mathematical Structuralism: From Galois Theory to Numbers, Sets, and Functions’. The title promises something rather more exciting than we get. Why? Let’s work backwards by quoting from (some of) their concluding summary. They write:

From … Dirichlet and Riemann, Dedekind inherited a conceptual way of doing mathematics. This involves replacing complicated calculations by more transparent deductions from basic concepts.

“Replacing”? That seems rather misleading to me: isn’t it more a matter of a change of focus to new, more abstract, general questions, rather than a replacement of methods for tackling the old questions that required “complicated calculations”?

Both Dedekind’s mainstream work in mathematics, such as his celebrated ideal theory, and his more foundational writings reflect that influence. Thus, he distilled out as central the concepts of group, field, continuity, infinity, and simple infinity. A related and constant aspect in his work is the attempt to characterize whole systems of objects through global properties.

From early on, Dedekind also pursued the program of the arithmetization of analysis … . A decisive triumph came in 1858, with Dedekind’s reductive treatment of the real numbers. From the 1870s on, he added a reduction of the natural numbers to a general theory of sets and mappings. This led to an early form of logicism, since he conceived of set theory as a central part of logic … [H]is attempt to execute a logicist program
had a decisive effect on the rise of axiomatic set theory in the 20th century.

Its conceptualist and set-theoretic aspects are central ingredients in Dedekind’s mathematical structuralism. But we emphasized another characteristic aspect that goes beyond both. This is the method of studying systems or structures with respect to their interrelations with other kinds of structures, and in particular, corresponding morphisms.

But again this seems slightly misleading to me. It isn’t that Dedekind and contemporaries “distilled out” the concepts of a group and of a field, for example, for the fun of it and then — as an optional further move — considered interrelations between structures. Weren’t the abstracting moves and their interrelations and their applications by relating them back to more concrete structures all tied together as a package from the start, as for example in the very case that Ferreirós and Reck consider:

A historically significant example, particularly for Dedekind, was Galois theory. As reconceived by him, in Galois theory we associate equations with certain field extensions, and we then study how to obtain those extensions in terms of the associated Galois group (introduced as a group of morphisms from the field to itself, i.e., automorphisms). …

Just a word more about this in a moment. Finally,

As we saw, Dedekind connected his mathematical or methodological structuralism with a structuralist conception of mathematical objects, i.e., a form of philosophical structuralism.

Now, in bald outline, all this is familiar enough (though of course in part because of the earlier work of writers like Reck and Ferreirós in helping to highlight for philosophers Dedekind’s importance in the development of mathematics). So does this essay add much to the familiar picture?

Not really, it seems to me. For it proceeds at too armwavingly general a level of description. There’s too much of the mathematical equivalent of name-dropping: ideas and results are mentioned, but with not enough content given to be usefully instructive.

Take the nice case of Galois theory again. If you are familiar with modern basic Galois theory from one of the standard textbooks like Ian Stewart’s or D.J.H. Garling’s, you won’t pick up much idea of just how Dedekind’s work related to the modern conception (well, I didn’t anyway). And if you aren’t already familiar with Galois theory, then you won’t really understand anything more about it from Reck and Ferreirós few paragraphs, other than it is something to do tackling questions about the roots of equations by using more abstract results about groups and fields — which isn’t exactly very helpful. By my lights, it would have been much more illuminating if our authors had devoted the whole essay to Dedekind’s work on Galois theory as a case study of what can be achieved by the “structuralist” turn.

Mathematical Structuralism, Essays 1 & 2

Here are some quick comments on the first two of the essays in The Pre-history of Mathematical Structuralism. Let me start, though, with a remark about the angle I’m coming from.

I have been wondering about getting back to work on my stalled project, Category Theory: A Gentle Introduction. And what I’d like to do is write some short preliminary chapters around and about the familiar pre-categorial idea that (lots of) mathematics is about abstract structures and their inter-relationships. What does that idea really come to? I sense that there is some disconnect between, on the one hand, what this amounts to in the nitty-gritty of ordinary mathematical practice and, on the other hand, some of the arm-wavingly generalities of philosophers with axes to grind. I’m interested then in seeing discussions of varieties of structuralism which are, perhaps, more grounded in the varieties of mathematical practice. Hence I hope that looking at some of the historical developments in maths that have led to where we are might prove to be illuminating. Let’s read on …

The book opens with an introductory essay by the editors, Erich Reck and Georg Schiemer. The first part of this essay is in effect a summary version of the same authors’ useful entry on ‘Structuralism in the Philosophy of Mathematics’ in the Stanford Encyclopedia of Philosophy. In this book, then, they touch again more briskly on the familiar varieties of structuralism as a philosophical position; perhaps too briskly? — a reader relatively new to the debates will find their longer SEP version significantly more helpful. But they also press a distinction between structuralism as a philosophical story (particularly a story about the ontology of mathematics) and what they call methodological or mathematical structuralism — a term which “is meant to capture a distinctive way of doing mathematics”. “Roughly,” we are told, “it consists of doing mathematics by ‘studying abstract structures’”, something a mathematician can do without explicitly considering metaphysical questions about the nature of stuctures. And then “One main goal of the present collection of essays is to clarify the origins, and with it the nature, of methodological/mathematical structuralism up to the rise of category theory”.

So what, in a little more detail, do Reck and Schiemer count as being involved in methodological structuralism? (1) The use of concepts like ‘group,’ ‘field,’ ‘3-dimensional Euclidean space’, which (2) are characterized axiomatically and “typically specify global or ‘structural’ properties”, and where (3) we importantly study systems falling under these concepts by relating them to each other by iso/homomorphisms, and (4) by considering ‘invariants’, and where (5) “there is the novel practice of ‘identifying’ isomorphic systems”.

[T]his can all be seen as culminating in the view that what really matters in mathematics is the ‘structure’ captured axiomatically, on the one hand, and preserved under relevant morphisms, on the other hand.

But having said this, Reck and Schiemer allow that not all of features (1) to (5) are required for what they call methodological structuralism: they say that we are dealing with “family resemblances” between cases.

Plainly, themes (1) to (4) did indeed emerge in nineteenth century mathematics. I rather discount (5), though, as the supposed novel practice of identifying isomorphic systems seems typically to amount to no more than ignoring specific differences between isomorphic Xs for certain purposes when doing the theory of Xs. For example, while focusing on the pure group-theoretic properties of some structures, we do ignore the non-group-theoretic differences between isomorphic groups. But of course, when we start to apply the group theory, e.g. in Galois theory, such differences become salient again; it is crucial, for example, that that certain groups are permutations of roots of equations, a property not shared with isomorphic groups.

There are various stripes of mathematical problem. To stick to nineteenth-century ones, at one end of the spectrum, there are very specific problems. For instance, we might be interested in finding a closed form solution for some integral: we tackle our problem using a rather specific bag of tricks we’ve developed (like substituting variables, integrating by parts, etc.). Again, we might be interested in finding an approximate solution to a specific application of the Navier-Stokes equation, and there’s another bag of tricks to develop and use. At the other end of the spectrum, there are considerably more abstract problems — is there a global solution in radicals for quintic equations? is the parallel postulate independent of the other Euclidean axioms? if a and k are co-prime, must there be an infinite number of primes in the arithmetic progression a, a + k, a + 2k, a + 3k, …? Not surprisingly, such abstract general questions often need abstract general ideas to answer them — and these ideas, being sufficiently abstract, will typically have other applications too, so may indeed well not need to be tied to a particular subject matter (i.e. they will be, in a sense, structural ideas).

So, at least those mathematicians interested in sufficiently general questions (far from all of all mathematicians, then) will naturally find themselves deploying abstract concepts as arm-wavingly gestured at by Reck and Schiemer’s (1) to (4). And the successes in doing this were impressive. But were (1) to (4) pursued with enough unity and clarity of purpose to constitute a methodological “ism”? We shall see.

The second part of Reck and Schiemer’s editorial introduction gives thumbnail sketches of the remaining fifteen essays in the book. So I won’t delay over that here, but will move on to the first contribution, Paola Cantù’s ‘Grassmann’s Concept Structuralism’.

I have to report, however, that I really got very little from this. Not knowing Grassmann’s work, I found Cantù’s sketched exposition of some his ideas quite opaque. So I imagine this essay will only be of interest to those who already know something of her topic; it certainly didn’t engage this reader. So I’ll skip over it.

The Pre-history of Mathematical Structuralism

There is a new collection of essays edited by Erich H. Reck and Georg Schiemer being published by OUP, officially next month. Here’s the book description: “Since the 1960s, there has been a vigorous and ongoing debate about structuralism in English-speaking philosophy of mathematics. But structuralist ideas and methods go back further in time; that is, there is a rich prehistory to this debate, also in the German- and French-speaking literature. In the present collection of essays, this prehistory is explored in a twofold way: by reconsidering various mathematicians in the 19th and early 20th centuries (Grassmann, Dedekind, Pasch, Klein, Hilbert, Noether, Bourbaki, and Mac Lane) who contributed to structuralism in a methodological sense; and by re-examining a range of philosophical reflections on such contributions during the same period (also by Peirce, Poincaré, Russell, Cassirer, Bernays, Carnap, and Quine), which led to suggestions about logical, epistemological, and metaphysical aspects that remain relevant today. Overall, the collection makes evident that structuralism has deep roots in the history of modern mathematics, that mathematical and philosophical views about it have often been closely intertwined, and that the range of philosophical options available in this context is significantly richer than a mere focus on current debates may make one believe.”

I’m going to be really interested to read probably most of the essays in the book. So I plan to start blogging about them here. (I certainly don’t promise to have anything especially illuminating or insightful to say: but writing blog-posts makes me read a bit more carefully and helps me fix ideas.) And now the good news: you don’t have to fork out £64 in order to follow along! For this is being published as an open access title. The collection is already free to read at Oxford Scholarship Online and is offered as a free PDF download from OUP. Which is excellent!

Hellman and Shapiro on Mathematical Structuralism

CUP are publishing a series of short books (about 100 pages) under the title Cambridge Elements in the Philosophy of Mathematics. The blurb says that the series “provides an extensive overview of the philosophy of mathematics in its many and varied forms. Distinguished authors will provide an up-to-date summary of the results of current research in their fields and give their own take on what they believe are the most significant debates influencing research, drawing original conclusions.” Which sounds ambitious. So far, though, the series is only stuttering into life. Just two Elements have been published, Mathematical Stucturalism (2018) by Geoffrey Hellman and Stewart Shapiro, and A Concise History of Mathematics for Philosophers (2019) by John Stillwell. And no further books are yet announced on the web-page for the series.

The hyper-active Stillwell has already written a well-known and accessible Mathematics and Its History (3rd edn, 2010) as well as a number of other non-specialist books (alongside his singificantly better hard-core maths texts). It seems a bit of a failure of imagination for the series editors to ask him to write another history; and to me, the result looks pretty unexciting.

Again, getting Hellman and Shapiro to write on structuralism is hardly adventurous! But I have now read their book. It’s not clear, though, who the intended readership really is. The series blurb — “up-to-date summary”, “current research”, “original conclusions” — might suggest a book aimed at e.g. graduate students. But little of the book approaches that sort of level. (One odd feature: we aren’t told who wrote what, even though some of the passages are in the first person and are characteristic of just one of the authors.)

There is a short Introduction giving initial characterizations of some forms of mathematical structuralism, and setting out some questions that we’d want any structuralism to address. Chapter 2 gives Historical Background. This is over twice the length of the next longest chapter and it is nicely done, with a good selection of quotations;   it will provide very helpful reading for undergraduates. Chapter 3 is then on Set-Theoretic Structuralism, the view that “structures are isomorphism types (or representatives thereof) within the set-theoretic hierarchy”.

Of course, this view won’t in itself give us structuralism for mathematics across the board: the status of set theory itself is left up for grabs. Indeed, on the obvious story “the foundational theory [set theory] is an exception to the theme of structuralism. But, the argument continues, every other branch of mathematics is to be understood in eliminative structuralist terms.” The authors don’t do much, however, to explain why set theory should get this foundational role, or illuminate how this reductionist story squares with the familiar fact that most working mathematicians can get by in cheerful ignorance of set theory, etc.  A student could be better pointed to e.g. some of Maddy’s work for a more nuanced account of the role of set theory in mathematics.

Chapter 4 is on Category Theory as a Framework for Mathematical Structuralism. But who is this short chapter addressed to? The philosophy of maths student (at any level) should have some initial grasp on what set theory is about. But most won’t have much clue about what Category Theory might be, and these brisk arm-waving pages are unlikely to help at all. On the other hand, the few who are in the game will be familiar with the usual suggestions from Awodey and others which are gestured to here: for them, there will be no news, certainly no “original conclusions”.

Chapter 5 and 6 discusses Structures as Sui Generis Structuralism (Shapiro-style) and The Modal-Structural Perspective (Hellman-style). Both authors have been presenting their respective lines for well over twenty years, and so we are not going to expect any exciting new insights, criticisms or developments — and in under a dozen printed pages for each chapter, we don’t get them.

The final Chapter 7 is on Modal Set-Theoretic Structuralism, in particular as developed by Øystein Linnebo. This topic is at least relatively novel and is interesting; but since the authors are nowhere near as good at explaining Linnebo’s approach as that particularly lucid author is, the sufficiently equipped student reader would do a lot better to go to the original paper “The Potential Hierarchy of Sets,” Review of Symbolic Logic (2013).

Which all sounds rather carping. But overall I found this an extremely disappointing book.

Structuralism in the Philosophy of Mathematics

Just to note the arrival at the Stanford Encyclopedia of a new entry ‘Structuralism in the Philosophy of Mathematics’ by Erich Reck and Georg Schiemer.

Two features of this entry are potentially particularly useful. First, there is an attempt to provide a helpful taxonomy of the many varieties of structuralism about mathematics. Second, there’s a discussion of the relation between category theory and structuralism.

There’s quite often a problem of level and speed of coverage with SEP entries. I’d say that might apply here — the piece necessarily goes quickly over a lot of ground, and I’d guess that many a last-year undergraduate/first-year graduate student wanting a way into the topic will find that this entry goes too fast. For just one example, I don’t imagine someone getting much of an idea of Charles Parsons’s position from the brief discussion here. My guess is that an entry perhaps 30% longer, going more slowly over the same material, could have been 100% more useful to many student readers.

However, if you do already know a little about structuralism in the philosophy of mathematics, and want a well organized guide to help you get to know a lot more (and to spur you on, perhaps, to finding out something about category theory!), with very helpful pointers to the literature, then this will be a quite excellent place to start.

Thin Objects

Øystein Linnebo’s book Thin Objects: An Abstractionist Account is out from OUP. If you’ve been following his contributions to debates on neo-Fregean philosophy of mathematics and related issues over some fifteen years, you won’t be surprised by the general line; but you will be pleased to have the strands of thought brought together in a shortish and (at least relative to the topic) accessible book. If you are new to Linnebo’s brand of neo-neo-neo-Fregeanism, this is your chance to catch up!

I confess that, having read Frege at an impressionable age, I still rather want something broadly Fregean to be right. I want there to be mathematical objects OK, but for them to be “thin”, to use Linnebo’s word — which he cashes out (perhaps not entirely happily, I’d say) as “not making a substantial demand on the world”. Linnebo gets his thin objects, Frege-style — his Platonism on the cheap — by conjuring the objects into being by abstraction principles. But unlike Frege and the Hale/Wright neo-Fregeans, Linnebo insists that the defensible and harmless principles need to be predicative. And it is well known that predicative abstraction principles by themselves are too weak to be useful for the foundations of much mathematics. Linnebo’s distinctive response is to allow indefinite iteration of abstraction principles (what he calls “dynamic” abstraction). But allowing completed infinite iterations would seem to entangle us in some pretty robust commitments again; so Linnebo wants to regiment his ideas about dynamic abstraction by going modal.

Does this work? Is getting thinness at the cost of going modal a good trade? I’ll let you know (given world enough and time).

“Multiversism and Concepts of Set” revisited

A month ago I posted here a link to an interesting paper here by Neil Barton. There’s now a discussion exchange, which it would be a pity to leave buried unread in comments on an old posting: so here it is.

From Rowsety Moid. It’s an interesting paper, but to me it seems there are many questionable steps in its arguments, and I would like to know what people who know more than I do about set-theoretic multiverses would say.

The “algebraic” interpretation strikes me as incoherent, or else slight of hand. When he explains it on page 10, he seems to be saying it is not involved with existence and reference, but he then talks of “a group G”, of “elements” of G, and of “constructing new groups from old”, which all involve existence and reference. His way out of this seem to be to say the algebraic view is not “concerned with” such things (which is largely a matter of attitude, focus and interest), and that we can understand operations on groups “not as making any claims about existence and reference” (as if that were the only way issues of existence or reference could come in).

Or, page 11: “We do not make any claims as to what exists within the Multiverse, rather it is seen as an intuitive picture to facilitate algebraic reasoning concerning sets.” Even the sets don’t exist? “Given a structure” The structure doesn’t exist?

Also, it’s not clear whether he is addressing Hamkins’s actual view or a maximally “radical” alternative. For instance, on page 8 “we are interested in Multiversism in its most radical form” is given as a reason for assuming that every level of metalanguage is indeterminate.

Even when the aim doesn’t seem to be the maximalisation of radicalism, there are a number of questionable interpretations or restatements. On page 9: “One way to understand Hamkins’ suggestion is to hold that we refer to several universes at once via description”. By page 11, the “one way to understand” has dropped out. Hamkins saying “in this article I shall simply identify a set concept with the model of set theory to which it gives rise”, quoted on page 7, becomes “it was noted that the Multiversist thought that every model of set theory constituted a set concept” on page 13. The idea that the concept gives rise to the model has been lost, and the idea that every model constitutes a concept has been added. Between pages 7 and 8, Hamkins’s “Often the clearest way to refer to a set concept is to describe the universe of sets in which it is instantiated” becomes “each model is correlated with a set concept, and we refer (to?) this concept through a description.” Such examples can be multiplied.

On page 14, it turns out to be important for Neil Barton’s argument that every model (or, perhaps, every “cloud” of models) corresponds to a concept and so to a description. I don’t see how that could work. Aren’t there more models than descriptions? (Hamkins seemed to have it that there was a model for every concept, not that there was a concept for every model.)

Another important step in Barton’s argument is the idea that we (or at least “Hamkinsians”) can use only first-order descriptions and so “lack the conceptual resources to pin down a single universe precisely”. From that, via the “One way to understand Hamkins” mentioned above, we reach the idea that we end up referring to “clouds”. For some purposes, that restriction seems correct. But when we’re trying to ground reference and so avoid a vicious infinite regress? That’s not so clear. Natural language (English, for example) isn’t restricted to FOL, for a start.

In the background, there seems to be an ideological element to the argument. It’s difficult to pin it down, but I think it may become visible on page 15 when arguing that “the Hamkinsian can give no particular reason to focus on one stopping point rather than another” and then saying “the response that we simply stop somewhere (without being able to give any reason for a particular stopping point) seems, like Go ̈del, to ascribe unexplained powers to the human mind.”

The power to stop somewhere? Is that supposed to be mysterious? I supposed that, in a sense, it is unexplained; but only because pretty much everything about the mind is currently unexplained, if you push hard enough.

From Neil Barton First, let me say a big “Thank You!” to Peter for publicising my paper, and to Rowsety Moid for some excellent comments. Indeed, your remarks were very timely, as they highlighted a mistake that I corrected in the proofs (shameless self-promotion: the paper will come out in this volume.

You said, RM: “The “algebraic” interpretation strikes me as incoherent, or else slight of hand. When he explains it on page 10, he seems to be saying it is not involved with existence and reference, but he then talks of “a group G”, of “elements” of G, and of “constructing new groups from old”, which all involve existence and reference. His way out of this seem to be to say the algebraic view is not “concerned with” such things (which is largely a matter of attitude, focus and interest), and that we can understand operations on groups “not as making any claims about existence and reference” (as if that were the only way issues of existence or reference could come in).”

Sure: we can have the algebraic interpretation relate to issues of existence and reference. The point is just that one requires additional assumptions first. For example, if we assume that the relevant objects exist, then there is a class of structures which instantiate the relevant algebraic properties about which the algebraist talks. Indeed, then various claims she makes might be a good way of proving the existence of certain objects, and better than trying to construct these things `absolutely’ (I think non-standard models are possibility a vivid case here). But those additional assumptions are needed before her view gets off the ground.

To bring this out, imagine that nominalism about sets were true. The ontological interpretation would then be null and void, there simply is no multiverse. It seems that the algebraic interpretation might still live on, despite the fact that the relevant algebraic properties are uninstantiated. For, we could still say that IF we were given some objects satisfying the relevant properties, we would be able to do such and such operations (and similarly with other algebraic theories like group theory).

[N.B. It’s an interesting question how the algebraic interpretation relates to if-then-ism. It’s not clear that these are wholly the same because of the algebraists acceptance of indeterminacy in metalogical notions.]

RM: “Even the sets don’t exist? “Given a structure” The structure doesn’t exist?”

We say things like this all the time though. “If space-time is discrete, then such and such will hold, and it is theoretically possible to construct such and such kind of object.” That seems like a perfectly valid claim to make, even if space-time is not discrete. Similarly with sets. Give me a structure, and I will be able to do these sorts of operations. I make no claim on whether the structure exists.

[N.B. As someone who is generally of a realist persuasion, I tend to think that this sort of response depends on the existence of the structures anyway. But this is just a refusal to engage with the position, not a dialectically convincing response.]

RM: “Also, it’s not clear whether he is addressing Hamkins’ actual view or a maximally “radical” alternative. For instance, on page 8 “we are interested in Multiversism in its most radical form” is given as a reason for assuming that every level of metalanguage is indeterminate.”

You raise a good point here, that I think is a general feature of the debate: the exact positions on the table are unclear. The Hamkins paper, though both highly interesting and ingenious, is notoriously slippery when it comes to being fully precise about the commitments of his view. So: am I addressing Hamkins or a radical alternative? I don’t know: you’d have to ask Joel how well the view put forward coheres with his (who, it has to be mentioned, was very helpful in discussing the paper and I’ve found very approachable). However, the views I present in the paper are ones that can be extracted from some of the things that he says, and he’s often keen to embrace the radical consequences of his view. He’s very clear that he thinks that indeterminacy infects the metalanguage, and there is no definite concept of natural number.

RM: “there are a number of questionable interpretations or restatements.”

I think there are going to have to be with the literature as it stands. Nowhere is Hamkins explicit about the kinds of epistemology he envisages, or the full metaphysical character of his view. My paper is intended to be just as much filling out possible ways of taking Hamkins’ view as a criticism of some of the things he says. I would welcome it if there are alternative interpretations out there, or I have got something wrong in exegesis—that way we can be more precise about what views are available and tenable. But I need to see these additional interpretations before I can weigh them up against the ones I have put forward.

RM: “The idea that the concept gives rise to the model has been lost, and the idea that every model constitutes a concept has been added. Between pages 7 and 8, Hamkins’s “Often the clearest way to refer to a set concept is to describe the universe of sets in which it is instantiated” becomes “each model is correlated with a set concept, and we refer (to?) this concept through a description.” Such examples can be multiplied.

On page 14, it turns out to be important for Neil Barton’s argument that every model (or, perhaps, every “cloud” of models) corresponds to a concept and so to a description. I don’t see how that could work… (Hamkins seemed to have it that there was a model for every concept, not that there was a concept for every model.)”

This was sloppy on my part, and I made some alterations in the proofs as a result. You are right, we should say that we refer to the concept through the model. However, this is done through *description*: we refer to the concept by *describing* the model. However, since we can only use first-order descriptions, we can’t pin down a single model, so our reference must be indeterminate, but this requires fixing some other model (in which some concept is instantiated; I take it that every model instantiates a concept) and so on.

I think there’s a lot more to be said here (in fact it doesn’t seem impossible to me that we get a loop of concepts or models), but the challenge at least represents an invitation for the Hamkinsian Multiversist to be clear on their commitments, and explain why there’s no such problem. As it stands, the Ontological Interpretation is not developed in sufficient detail to explain how these problems are avoided.

RM: “Aren’t there more models than descriptions?”

That depends on where you live for the Hamkinsian. Since every universe’s multiverse is countable from the perspective of some other universe, the universes of a multiverse are bijective with the descriptions from a suitable perspective (though not through the natural mapping of a universe with a description of it, and not within any particular multiverse). In any case, to press the challenge I only require infinitely many concepts being used, so countable is enough. There’s also the question of whether or not Hamkins is allowing parameters, which would in turn make the issue a whole lot more complicated (given the emphasis on ultrapowers, I’m guessing he is allowing parameters for the ultrafilters), as then we could have proper-class-many descriptions. Again, this would be another area I would like clarification from the Hamkinsian.

RM: “But when we’re trying to ground reference and so avoid a vicious infinite regress? That’s not so clear. Natural language (English, for example) isn’t restricted to FOL, for a start.”

I’m in full agreement here! In fact I find the restriction to FOL excessive. Again though, this is a case where the dialectic with Hamkinsian is important. If he/she wants to admit non-FOL, s/he has to explain what is acceptable and what isn’t. Why is it okay for him/her to use non-FOL resources in giving an account of reference, yet an account of the semantic referents of the natural numbers or set theory in terms of properties/plurals/the ancestral relation/sets is not allowed? I want to know what the rules of the game are by which the Hamkinsian has a stronger position compared to the Universist.

RM:“but only because pretty much everything about the mind is currently unexplained, if you push hard enough.”

Sure! But here we’re seeing if the Hamkinsian has a decent response to Benacerraf’s challenge through description. So, given that this is the background for the paper, we can demand an explanation.

I think in general the paper shouldn’t be viewed as an all out attack on the Hamkinsian, but rather a request for him/her to clarify a number of points.

Thank you ever so much for the comments! I found them very helpful. Best Wishes, Neil

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