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This Element addresses the viability of categoricity arguments in philosophy by focusing with some care on the specific conclusions that a sampling of prominent figures have attempted to draw – the same theorem might successfully support one such conclusion while failing to support another. It begins with Dedekind, Zermelo, and Kreisel, casting doubt on received readings of the latter two and highlighting the success of all three in achieving what are argued to be their actual goals. These earlier uses of categoricity arguments are then compared and contrasted with more recent work of Parsons and the co-authors Button and Walsh. Highlighting the roles of first- and second-order theorems, of external and internal theorems, the Element concludes that categoricity arguments have been more effective in historical cases that reflect philosophically on internal mathematical matters than in recent questions of pre-theoretic metaphysics.
This paper reconstructs Steel’s multiverse project in his ‘Gödel’s program’ (Steel, 2014), first by comparing it to those of Hamkins (2012) and Woodin (2011), then by detailed analysis what’s presented in Steel’s brief text. In particular, we reconstruct his notion of a ‘natural’ theory, describe his multiverse axioms and his translation function, and assess the resulting status of the Continuum Hypothesis. In the end, we reconceptualize the defect that Steel thinks
might suffer from and isolate what it would take to remove it while working within his framework. As our goal is to present as coherent and compelling a philosophical and mathematical story as we can, we allow ourselves to augment Steel’s story in places (e.g., in the treatment of Amalgamation) and to depart from it in others (e.g., the removal of ‘meaning’ from the account). The relevant mathematics is laid out in the appendices.
Logic might chart the rules of the world itself; the rules of rational human thought; or both. Husserl had a very broad concept of logic that embraces our usual modern idea of logic as well as something he called pure logic, which we can loosely characterise as something like the fundamental forms of experience. For Husserl, the fundamental forms of pure logic are an in-eliminable part of experience: i.e. experience encompasses direct apprehension of these inferential relationships. The apprehended structures are abstract and platonic; discovered, rather than constructed. Theory, empirical observation, and experience are in this sense fallible: they may or may not get it right and reveal the actual independent structure of logic. Both logic and mathematics as they are characterised by Husserl, should encounter the realist problem of independence, neither are the sort of thing we can simply take as part of human cognition.
This talk surveys a range of positions on the fundamental metaphysical and epistemological questions about elementary logic, for example, as a starting point: what is the subject matter of logic—what makes its truths true? how do we come to know the truths of logic? A taxonomy is approached by beginning from well-known schools of thought in the philosophy of mathematics—Logicism, Intuitionism, Formalism, Realism—and sketching roughly corresponding views in the philosophy of logic. Kant, Mill, Frege, Wittgenstein, Carnap, Ayer, Quine, and Putnam are among the philosophers considered along the way.
The Annual European Meeting of the Association for Symbolic Logic, also known as the Logic Colloquium, is among the most prestigious annual meetings in the field. The current volume, Logic Colloquium 2007, with contributions from plenary speakers and selected special session speakers, contains both expository and research papers by some of the best logicians in the world. This volume covers many areas of contemporary logic: model theory, proof theory, set theory, and computer science, as well as philosophical logic, including tutorials on cardinal arithmetic, on Pillay's conjecture, and on automatic structures. This volume will be invaluable for experts as well as those interested in an overview of central contemporary themes in mathematical logic.
The Logic Colloquium 2007, the European Summer Meeting of the Association for Symbolic Logic, was held in Wrocław, Poland, from 14 to 19 July 2007. It was colocated with the following events: The thirty-fourth International Colloquium on Automata, Languages and Programming (ICALP), the twenty-second Annual IEEE Symposium on Logic in Computer Science (LICS) and the ninth ACM-SIGPLAN International Symposium on Principles and Practice of Declarative Programming (PPDP). There was an agreement with LICS on running joint sessions for one day.
More than 200 participants from all over the world took part in the Logic Colloquium. The programme consisted of 3 tutorials, 11 invited plenary talks, 6 joint talks with LICS (2 long, 4 short) and 21 talks in 5 special sessions on set theory, proof complexity and nonclassical logics, philosophical and applied logic at the JPL, logic and analysis and model theory. In addition to these invited talks, there were 63 contributed talks.
The programme committee consisted of Alessandro Andretta (Turin), Françoise Delon (Paris 7), Ulrich Kohlenbach (Darmstadt), Steffen Lempp (Madison, Chair), Penelope Maddy (UC Irvine), Jerzy Marcinkowski (Wrocław), Ludomir Newelski (Warsaw), Andrew Pitts (Cambridge), Pavel Pudlák (Prague), Sławomir Solecki (Urbana-Champaign), Frank Stephan (Singapore) and Göran Sundholm (Leiden). The local organizing committee consisted of Tobias Kaiser, Piotry Kowalski, Jan Kraszewski, Amador Martin-Pizarro, Serge Randriambololona and Roman Wencel.
The Logic Colloquium 2007 wants to acknowledge its sponsors for their generous support of the event: the Association for Symbolic Logic, the Polish Academy of Sciences and the University of Wrocław.
This paper traces the evolution of thinking on how mathematics relates to the world—from the ancients, through the beginnings of mathematized science in Galileo and Newton, to the rise of pure mathematics in the nineteenth century. The goal is to better understand the role of mathematics in contemporary science.
Despite some discomfort with this grandly philosophical topic, I do in fact hope to address a venerable pair of philosophical chestnuts: mathematical truth and existence. My plan is to set out three possible stands on these issues, for an exercise in compare and contrast. A word of warning, though, to philosophical purists (and perhaps of comfort to more mathematical readers): I will explore these philosophical positions with an eye to their interconnections with some concrete issues of set theoretic method.
Let me begin with a brief look at what to count as ‘philosophy’. To some extent, this is a matter of usage, and mathematicians sometimes classify as ‘philosophical’ any considerations other than outright proofs. So, for example, discussions of the propriety of particular mathematical methods would fall under this heading: should we prefer analytic or synthetic approaches in geometry? Should elliptic functions be treated in terms of explicit representations (as in Weierstrass) or geometrically (as in Riemann)? Should we allow impredicative definitions? Should we restrict ourselves to a logic without bivalence or the law of the excluded middle? Also included in this category would be the trains of thought that shaped our central concepts: should a function always be defined by a formula? Should a group be required to have an inverse for every element? Should ideal divisors be defined contextually or explicitly, treated computationally or abstractly? In addition, there are more general questions concerning mathematical values, aims and goals: Should we strive for powerful theories or low-risk theories? How much stress should be placed on the fact or promise of physical applications? How important are interconnections between the various branches of mathematics? These philosophical questions of method naturally include several peculiar to set theory: should set theorists focus their efforts on drawing consequences for areas of interest to mathematicians outside mathematical logic? Should exploration of the standard axioms of ZFC be preferred to the exploration and exploitation of new axioms? How should axioms for set theory be chosen? What would a solution to the Continuum Problem look like?
Does mathematics need new axioms? was the second of three plenary panel discussions held at the ASL annual meeting, ASL 2000, in Urbana-Champaign, in June, 2000. Each panelist in turn presented brief opening remarks, followed by a second round for responding to what the others had said; the session concluded with a lively discussion from the floor. The four articles collected here represent reworked and expanded versions of the first two parts of those proceedings, presented in the same order as the speakers appeared at the original panel discussion: Solomon Feferman (pp. 401–413), Penelope Maddy (pp. 413–422), John Steel (pp. 422–433), and Harvey Friedman (pp. 434–446). The work of each author is printed separately, with separate references, but the portions consisting of comments on and replies to others are clearly marked.
The nature of classes – particularly proper classes, collections “too large” to be sets – is a perennial problem in the philosophy and foundations of set theory. Logicians worry that the unrestricted quantifiers of set theory must range over the collection of all sets, a collection that cannot itself be a set, and hence, a collection that is ill-understood; philosophers puzzle over the existence of properties (such as x ∉ x) that seem to have no extensions; set theorists ponder heuristic arguments that involve performing operations on the entire universe, V, of sets as if it were a set. Existing theories of sets and classes seem unsatisfactory because their ‘proper classes’ are either indistinguishable from extra layers of sets or mysterious entities in some perpetual, atemporal process of becoming. In the spirit of Cantor's bold introduction of the completed infinite, we might hope for a theory of sets and classes that both distinguishes plausibly between the two and treats classes as bona fide entities. In the end, this may be too much to ask, but it seems at least the right place to begin.
Several of Charles Parsons' papers have addressed the difficult problem of sets and classes in insightful and influential ways. One central thrust of his treatment has been to emphasize the strong analogy between paradoxes of truth, such as the liar, and paradoxes of classes, such as Russell's paradox.
My aim in this paper is to propose what seems to me a distinctive approach to set theoretic methodology. By ‘methodology’ I mean the study of the actual methods used by practitioners, the study of how these methods might be justified or reformed or extended. So, for example, when the intuitionist's philosophical analysis recommends a wholesale revision of the methods of proof used in classical mathematics, this is a piece of reformist methodology. In contrast with the intuitionist, I will focus more narrowly on the methods of contemporary set theory, and, more importantly, I will certainly recommend no sweeping reforms. Rather, I begin from the assumption that the methodologist's job is to account for set theory as it is practiced, not as some philosophy would have it be. This credo lies at the very heart of the so-called ‘naturalism’ to be described here.
A philosopher looking at set theoretic practice from the outside, so to speak, might notice any number of interesting methodological questions, beginning with the intuitionist's ‘why use classical logic?’, but this sort of question is not a live issue for most practicing set theorists. One central question on which the philosopher's and the practitioner's interests converge is this: what is the status of independent statements like the continuum hypothesis (CH)? A number of large questions arise in its wake: what criteria should guide the search for new axioms? For that matter, what reasons support our adoption of the old axioms?
Does V = L? Is the Axiom of Constructibility true? Most people with an opinion would answer no. But on what grounds? Despite the near unanimity with which V = L is declared false, the literature reveals no clear consensus on what counts as evidence against the hypothesis and no detailed analysis of why the facts of the sort cited constitute evidence one way or another. Unable to produce a well-developed argument one way or the other, some observers despair, retreating to unattractive fall-back positions, e.g., that the decision on whether or not V = L is a matter of personal aesthetics. I would prefer to avoid such conclusions, if possible. If we are to believe that L is not V, as so many would urge, then there ought to be good reasons for this belief, reasons that can be stated clearly and subjected to rational evaluation. Though no complete argument has been presented, the literature does contain a number of varied argument fragments, and it is worth asking whether some of these might be developed into a persuasive case.
One particularly simple approach would be to note that the existence of a measurable cardinal (MC) implies that V ≠ L,1 and to argue that there is a measurable cardinal. The drawback to this approach is that its implying V ≠ L cannot then be counted as evidence in favor of MC, as it often is. Indeed, there seems to have been considerable sentiment against V = L even before the proof of its negation from MC,2 and this sentiment must either be accounted for as reasonable or explained away as an aberration of some kind.