Let us call an integer part of an ordered field any subring such that every element of the field lies at distance less than 1 from a unique element of the ring. We show that every real closed field has an integer part.

Shepherdson [14] showed that for a discrete ordered ring I, I is a model of IOpen iff I is an integer part of a real closed ordered field. In this paper, we consider integer parts satisfying PA. We show that if a real closed ordered field R has an integer part I that is a nonstandard model of PA (or even IΣ₄), then R must be recursively saturated. In particular, the real closure of (...) I, RC (I), is recursively saturated. We also show that if R is a countable recursively saturated real closed ordered field, then there is an integer part I such that R = RC(I) and I is a nonstandard model of PA. (shrink)

It is common to assimilate Marx’s and Spinoza’s conceptions of democracy. In this chapter, I assess the relation between Marx’s early idea of “true democracy” and Spinozist democracy, both the historical influence and the theoretical affinity. Drawing on Marx’s student notebooks on Spinoza’s Theological-Political Treatise, I show there was a historical influence. However, at the theoretical level, I argue that a sharp distinction must be drawn. Philosophically, Spinoza’s commitment to understanding politics through real concrete powers does not support with (...) Marx’s anti-institutional conception of true democracy. And as a matter of social theory, the gap between civil society and the state which so troubles Marx is a development of modernity that has not entered Spinoza's premodern field of view. Marx’s true democracy was also influenced by his study of Rousseau, and theoretically, it is just as close if not closer to Rousseau as to Spinoza. (shrink)

We give an intuitionistic axiomatisation of real closed fields which has the constructive reals as a model. The main result is that this axiomatisation together with just the decidability of the order relation gives the classical theory of real closed fields. To establish this we rely on the quantifier elimination theorem for real closed fields due to Tarski, and a conservation theorem of classical logic over intuitionistic logic for geometric theories.

The following conjecture is due to Shelah–Hasson: Any infinite strongly NIP field is either real closed, algebraically closed, or admits a non‐trivial definable henselian valuation, in the language of rings. We specialise this conjecture to ordered fields in the language of ordered rings, which leads towards a systematic study of the class of strongly NIP almost real closed fields. As a result, we obtain a complete characterisation of this class.

We show that for any real number, the class of real numbers less random than it, in the sense of rK-reducibility, forms a countable real closed subfield of the real ordered field. This generalizes the well-known fact that the computable reals form a real closed field. With the same technique we show that the class of differences of computably enumerable reals (d.c.e. reals) and the class of computably approximable reals (c.a. reals) (...) form real closed fields. The d.c.e. result was also proved nearly simultaneously and independently by Ng (Keng Meng Ng. Master's Thesis. National University of Singapore, in preparation). Lastly, we show that the class of d.c.e. reals is properly contained in the class or reals less random than Ω (the halting probability), which in turn is properly contained in the class of c.a. reals, and that neither the first nor last class is a randomness class (as captured by rK-reducibility). (shrink)

Jan Krajíček posed the following problem: Is there is a generalization result in the theory of real closed fields of the form: If A is provable in length k for all n ϵ ω , then A is provable? It is argued that the answer to this question depends on the particular formulation of the “theory of real closed fields.” Four distinct formulations are investigated with respect to their generalization behavior. It is shown that there is (...) a positive answer to Krajíček's question for 1. the axiom system RCF of Artin-Schreier with Gentzen's LK as underlying logical calculus, 2. RCF with the variant LK B of LK allowing introduction of several quantifiers of the same type in one step, 3. LK B and the first-order schemata corresponding to Dedekind cuts and the supremum principle. A negative answer is given for 4. any system containing the schema of extensionality. (shrink)

We introduce a new construction of real closed fields by using an elementary extension of an ordered field with an integer part satisfying. This method can be extend to a finite extension of an ordered field with an integer part satisfying. In general, a field obtained from our construction is either real closed or algebraically closed, so an analogy of Ostrowski's dichotomy holds. Moreover we investigate recursive saturation of an o‐minimal extension of (...) a real closed field by finitely many function symbols. (shrink)

Several researchers have recently established that for every Turing degree \, the real closed field of all \-computable real numbers has spectrum \. We investigate the spectra of real closed fields further, focusing first on subfields of the field \ of computable real numbers, then on archimedean real closed fields more generally, and finally on non-archimedean real closed fields. For each noncomputable, computably enumerable set C, we produce a (...) real closed C-computable subfield of \ with no computable copy. Then we build an archimedean real closed field with no computable copy but with a computable enumeration of the Dedekind cuts it realizes, and a computably presentable nonarchimedean real closed field whose residue field has no computable presentation. (shrink)

We study the model theory of fields k carrying a henselian valuation with real closed residue field. We give a criteria for elementary equivalence and elementary inclusion of such fields involving the value group of a not necessarily definable valuation. This allows us to translate theories of such fields to theories of ordered abelian groups, and we study the properties of this translation. We also characterize the first-order definable convex subgroups of a given ordered abelian group and (...) prove that the definable real valuation rings of k are in correspondence with the definable convex subgroups of the value group of a certain real valuation of k. (shrink)

By RCA 0 , we denote a subsystem of second order arithmetic based on Δ0 1 comprehension and Δ0 1 induction. We show within this system that the real number system R satisfies all the theorems (possibly with non-standard length) of the theory of real closed fields under an appropriate truth definition. This enables us to develop linear algebra and polynomial ring theory over real and complex numbers, so that we particularly obtain Hilbert’s Nullstellensatz in RCA (...) 0. (shrink)

We give a valuation theoretic characterization for a real closed field to be recursively saturated. This builds on work in [9], where the authors gave such a characterization forκ-saturation, for a cardinal$\kappa \ge \aleph _0 $. Our result extends the characterization of Harnik and Ressayre [7] for a divisible ordered abelian group to be recursively saturated.

In this paper, we prove that a pseudo-exponential field has continuum many nonisomorphic countable real closed exponential subfields, each with an order-preserving exponential map which is surjective onto the nonnegative elements. Indeed, this is true of any algebraically closed exponential field satisfying Schanuel’s conjecture.

For the classA of uncountable Archimedian real closed fields we show that the statement “TheL <ω-theory ofA is complete” is independent of ZFC. In particular we have the following results:Assuming the Continuum-Hypothesis (CH) is incomplete. Conversely it is possible to build a model of set theory in which is complete and decidable. The latter can also be deduced from the Proper Forcing Axiom (PFA). In this case turns out to be equivalent to the elementary theory of the (...) class='Hi'>real numbers ℝ (by a quantifier-elimination procedure).Formally: is incomplete. is complete and decidable. (shrink)

The paper shows elimination of imaginaries for real closed valued fields to suitable sorts. We also show that this result is in some sense optimal. The paper includes a quantifier elimination theorem for real closed valued fields in a language with sorts for the field, value group and residue field.

We define a notion of residue field domination for valued fields which generalizes stable domination in algebraically closed valued fields. We prove that a real closed valued field is dominated by the sorts internal to the residue field, over the value group, both in the pure field and in the geometric sorts. These results characterize forking and þ-forking in real closed valued fields (and also algebraically closed valued fields). We lay (...) some groundwork for extending these results to a power-bounded T-convex theory. (shrink)

We present some results concerning elimination of quantifiers and elementary equivalence for Boolean products of real closed valuation rings and fields. We also study rings of continuous functions and rings of definable functions over real closed valuation rings under this point of view.

We present a canonical form for definable subsets of algebraically closed valued fields by means of decompositions into sets of a simple form, and do the same for definable subsets of real closed valued fields. Both cases involve discs, forming "Swiss cheeses" in the algebraically closed case, and cuts in the real closed case. As a step in the development, we give a proof for the fact that in "most" valued fields F, if f(x),g(x) (...) ∈ F[ x] and v is the valuation map, then the set {x : v(f(x)) ≤ v(g(x))} is a Boolean combination of discs; in fact, it is a finite union of Swiss cheeses. The development also depends on the introduction of "valued trees", which we define formally. (shrink)

Abstract.In this paper we show that the theory of fields together with an integrally closed subring, the theory of formally real fields with a real holomorphy ring and the theory of formally \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $p$\end{document}-adic fields with a \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $p$\end{document}-adic holomorphy ring have no model companions in the language of fields augmented by a unary predicate for the corresponding ring.

Let ℛ be an o-minimal expansion of a real closed field R. We continue here the investigation we began in [11] of differentiability with respect to the algebraically closed field [Formula: see text]. We develop the basic theory of such K-differentiability for definable functions of several variables, proving theorems on removable singularities as well as analogues of the Weierstrass preparation and division theorems for definable functions. We consider also definably meromorphic functions and prove that every (...) definable function which is meromorphic on Kn is necessarily a rational function. We finally discuss definable analogues of complex analytic manifolds, with possible connections to the model theoretic work on compact complex manifolds, and present two examples of "nonstandard manifolds" in our setting. (shrink)

We approach the subject of o-minimality from the point of view of tame systems, following the work of Charbonnel and Wilkie. This gives some general sufficient conditions for a system to be model complete and o-minimal. We are then able to obtain the following generalisation of a recent result of Gabrielov : A polynomially bounded o-minimal expansion of the real ordered field by a collection of restricted C∝ functions, which is closed under partial differentiation, is model complete.

In the first half of this paper, we study the way that sets of real numbers closed under Turing equivalence sit inside the real line from the perspective of algebra, measure and orders. Afterwards, we combine the results from our study of these sets as orders with a classical construction from Avraham to obtain a restriction about how non trivial automorphism of the Turing degrees (if they exist) interact with 1-generic degrees.

The Expanse does not provide an easy answer to the vexing question on making a decision when competing, but considering conflicts of values on the show can help us reason about tough choices in real life. Sometimes, scientific progress conflicts with the prudential value of self‐preservation. This chapter explains three ways of understanding value conflicts: as situations in which every option is forbidden, situations in which every option is permissible, or situations in which some options are obligatory and some (...) options are forbidden. The idea that all these different kinds of value can be traded with a common currency is tempting. But contra Keats, beauty is not truth; and it certainly is not goodness, as Tolstoy says. Epistemic, moral, prudential, and aesthetic values are very different. (shrink)

D’Aquino et al. (J Symb Log 75(1):1–11, 2010) have recently shown that every real-closed field with an integer part satisfying the arithmetic theory IΣ4 is recursively saturated, and that this theorem fails if IΣ4 is replaced by IΔ0. We prove that the theorem holds if IΣ4 is replaced by weak subtheories of Buss’ bounded arithmetic: PV or \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\Sigma^b_1-IND^{|x|_k}}$$\end{document}. It also holds for IΔ0 (and even its subtheory IE2) under (...) a rather mild assumption on cofinality. On the other hand, it fails for the extension of IOpen by an axiom expressing the Bézout property, even under the same assumption on cofinality. (shrink)

The collaboration of Language and Computing nv (L&C) and the Institute for Formal Ontology and Medical Information Science (IFOMIS) is guided by the hypothesis that quality constraints on ontologies for software ap-plication purposes closely parallel the constraints salient to the design of sound philosophical theories. The extent of this parallel has been poorly appreciated in the informatics community, and it turns out that importing the benefits of phi-losophical insight and methodology into application domains yields a variety of improvements. L&C’s LinKBase® (...) is one of the world’s largest medical domain ontologies. Its current primary use pertains to natural language processing ap-plications, but it also supports intelligent navigation through a range of struc-tured medical and bioinformatics information resources, such as SNOMED-CT, Swiss-Prot, and the Gene Ontology (GO). In this report we discuss how and why philosophical methods improve both the internal coherence of LinKBase®, and its capacity to serve as a translation hub, improving the interoperability of the ontologies through which it navigates. (shrink)

This paper explores the ethical insights provided by Anne Truitt's minimalist sculptures, as viewed through the phenomenological lenses of Hannah Arendt's investigations into the co-constitution of reality and Maurice Merleau-Ponty's investigations into perception. Artworks in their material presence can lay out new ways of relating and perceiving. Truitt's works accomplish this task by revealing the interactive motion of our embodied relations and how material objects can actually help to ground our reality and hence human potentiality. Merleau-Ponty shows how our prereflective (...) bodies allow incompossible perceptions to coexist. Yet this same capacity of bodies to gather multiple perceptions together also lends itself to the illusion that we see from only one perspective. If an ethical perspective becomes reified into one position, it then becomes detached from reality, and the ethical potential is actually lost. At the same time, phenomenologically understood, the real world does not exist in terms of static matter, but is instead a web of contextual relations and meanings. An ethics that does not take embodied relations into account—that allows for only one perspective—ultimately loses its capacity for flexibility, and for being part of a common and shared reality. (shrink)

The naive theory of properties states that for every condition there is a property instantiated by exactly the things which satisfy that condition. The naive theory of properties is inconsistent in classical logic, but there are many ways to obtain consistent naive theories of properties in nonclassical logics. The naive theory of classes adds to the naive theory of properties an extensionality rule or axiom, which states roughly that if two classes have exactly the same members, they are identical. In (...) this paper we examine the prospects for obtaining a satisfactory naive theory of classes. We start from a result by Ross Brady, which demonstrates the consistency of something resembling a naive theory of classes. We generalize Brady’s result somewhat and extend it to a recent system developed by Andrew Bacon. All of the theories we prove consistent contain an extensionality rule or axiom. But we argue that given the background logics, the relevant extensionality principles are too weak. For example, in some of these theories, there are universal classes which are not declared coextensive. We elucidate some very modest demands on extensionality, designed to rule out this kind of pathology. But we close by proving that even these modest demands cannot be jointly satisfied. In light of this new impossibility result, the prospects for a naive theory of classes are bleak. (shrink)

Students often have difficulty connecting theoretical and text-based scholarship to the real world. When teaching in Asia, this disconnection is exacerbated by the European/American focus of many canonical texts, whereas students' own experiences are primarily Asian. However, in my discipline of political philosophy, this problem receives little recognition nor is it comprehensively addressed. In this paper, I propose that the problem must be taken seriously, and I share my own experiences with a novel pedagogical strategy which might offer a (...) possible path forward. Recent scholarship has championed an active learning approach, where students engage in their own research, and deliver outward-facing products that have a meaning and purpose beyond the confines of the student-professor relationship. In this spirit, I have put into practice a strategy of course design, where active learning is used to overcome students' disconnection with the course content. In particular, as a major component of course assessment, students are required to write an 'opinion piece', which is then showcased on a public website. The opinion piece must address a real-world issue which the student himself or herself selects and deems important; furthermore, it must build on the theoretical tools of the course and be written in a style which makes it accessible to a wider audience. I discuss the implementation of this strategy in two political philosophy courses, including strategies to avoid 'dumbing down' and ‘diluting’ the process of critical thinking. While no formal analysis of impact of the strategy on learning outcomes has been conducted, an anonymous pedagogical survey has yielded an overwhelmingly positive response for students' self-reported perceptions of the curricular innovations. (shrink)

"Combining aspects of his acclaimed street work with an innovative approach to portraiture, Chicago-based photographer Jed Fielding has concentrated closely on these children's features and gestures, probing the enigmatic boundaries between surface and interior. Design, composition, and the play of light and shadow are central elements in these photographs, but the images are much more than formal experiments; they confront disability in a way that affirms life. Fielding's sightless subjects project a vitality that seems to extend beyond the limits of (...) self-consciousness. In collaborative, joyful participation with the children, he has made pictures that reveal essential gestures of absorption and the basic expressions of our creatureliness. Fielding's work achieves what only great art, and particularly great portraiture can: it launches and then complicates a process of identification across the barriers that separate us from each other. (shrink)

It is really impossible to say anything worth saying about Plato in general within the limits of a single article. Indeed, the more one studies Plato the more impossible does it become—if the concept of degrees of impossibility may be used in a philosophical journal. The reasons for this are manifold. The first lies in the supreme greatness of Plato as a thinker. Hardly anyone who has made a serious effort to study Plato has escaped receiving the impression of him (...) as probably the greatest thinker of all ages. The difficulty is intensified by the particular form in which his greatness is conveyed to us. One may make a distinction, a relative distinction at least, between different philosophers in this respect. With some, their message is conveyed to us with comparative rapidity, even perhaps on the first careful reading, and subsequent study adds little except in the way of detail. Among these it would perhaps not be unfair to place Hume. Others, on first reading, will appear obscure and difficult, sometimes even repellent. It is only after a much more careful study that their real greatness emerges, and they may go on increasing in stature at every subsequent reading. Such, I believe, to be the experience of the majority of readers with Kant. Plato is almost unique in that he makes his impression in both ways. Almost everyone is captivated by him at first reading, but it is only to those who take the trouble of reading him again and again with the intellectual effort that he himself would have demanded. (shrink)