Artificial Intelligence, in press. Abstract: For some time we have been developing, and have had significant practical success with, a time-sensitive, contradiction-tolerant logical reasoning engine called the active logic machine (ALMA). The current paper details a semantics for a general version of the underlying logical formalism, active logic. Central to active logic are special rules controlling the inheritance of beliefs in general (and of beliefs about the current time in particular), very tight controls on what can be derived from direct (...) contradictions (P &￢P), and mechanisms allowing an agent to represent and reason about its own beliefs and past reasoning. Furthermore, inspired by the notion that until an agent notices that a set of beliefs is contradictory, that set seems consistent (and the agent therefore reasons with it as if it were consistent), we introduce an “apperception function” that represents an agent’s limited awareness of its own beliefs, and serves to modify inconsistent belief sets so as to yield consistent sets. Using these ideas, we introduce a new definition of logical consequence in the context of active logic, as well as a new definition of soundness such that, when reasoning with consistent premises, all classically sound rules remain sound in our new sense. However, not everything that is classically sound remains sound in our sense, for by classical definitions, all rules with contradictory premises are vacuously sound, whereas in active logic not everything follows from a contradiction. (shrink)

Highlights of this volume from the 2004 Annual European Meeting of the Association for Symbolic Logic (ASL) include a tutorial survey of the recent highpoints of universal algebra, written by a leading expert; explorations of foundational questions; a quartet of model theory papers giving an excellent reflection of current work in model theory, from the most abstract aspect "abstract elementary classes" to issues around p-adic integration.

This book constitutes the refereed proceedings of the 4th International Conference on Frontiers of Combining Systems, FroCoS 2002, held in Santa Margherita Ligure, Italy, in April 2002.The 14 revised full papers presented together with 3 invited papers were carefully reviewed and selected from 35 submissions. Among the topics covered are combination of logics, combination of constraint solving techniques, combination of decision procedures, combination problems in verification, modular problems of theorem proving, and the integration of decision procedures and other solving processes (...) into constraint programming and deduction systems. (shrink)

(or not oveA-complete.) . Let * be a unary operator defined on the set F of formulas of the language £ (ie, if A is a formula of £, then *A is also a ...

International Studies in the Philosophy of Science, Volume 26, Issue 3, Page 345-348, September 2012.

L. E. HAHN and P. A. SCHILPP (eds.), The philosophy of W. V. Quine. La Salle, Illinois: Open Court, 1986. xvi + 705 pp. $35.95 cloth/$16.50 (paper).

L. E. HAHN and P. A. SCHILPP (eds.), The philosophy of W. V. Quine. La Salle, Illinois: Open Court, 1986. xvi + 705 pp. $35.95 cloth/$16.50 (paper).

An experiment is reported examining dual-process models of belief bias in syllogistic reasoning using a problem complexity manipulation and an inspection-time method to monitor processing latencies for premises and conclusions. Endorsement rates indicated increased belief bias on complex problems, a finding that runs counter to the “belief-first” selective scrutiny model, but which is consistent with other theories, including “reasoning-first” and “parallel-process” models. Inspection-time data revealed a number of effects that, again, arbitrated against the selective scrutiny model. The most striking inspection-time (...) result was an interaction between logic and belief on premise-processing times, whereby belief - logic conflict problems promoted increased latencies relative to non-conflict problems. This finding challenges belief-first and reasoning-first models, but is directly predicted by parallel-process models, which assume that the outputs of simultaneous heuristic and analytic processing streams lead to an awareness of belief - logic conflicts than then require time-consuming resolution. (shrink)

A comprehensive model for describing various forms of developments in science is defined in precise, set-theoretic terms, and in the spirit of the structuralist approach in the philosophy of science. The model emends previous accounts in centering on single systems in a homogenous way, eliminating notions which essentially refer to sets of systems. This is achieved by eliminating the distinction between theoretical and non-theoretical terms as a primitive, and by introducing the notion of intended links. The force of the model (...) is demonstrated by formally incorporating many of the important, precise meta-theoretic concepts occurring in the literature. (shrink)

The contributions in the first part Re-modelling logic of this volume take account of formal logic in the theory of rational argumentation.

According to a prevalent view among philosophers formal logic is the philosopher’s main tool to assess the validity of arguments, i.e. the philosopher’s ars iudicandi. By drawing on a famous dispute between Russell and Strawson over the validity of a certain kind of argument – of arguments whose premises feature definite descriptions – this paper casts doubt on the accuracy of the ars iudicandi conception. Rather than settling the question whether the contentious arguments are valid or not, Russell and Strawson, (...) upon discussing the proper logical analysis of definite descriptions, merely contrast converse informal validity assessments rendered explicit by nonequivalent logical for-malizations. (shrink)

ABSTRACT: This paper traces the evidence in Galen's Introduction to Logic (Institutio Logica) for a hypothetical syllogistic which predates Stoic propositional logic. It emerges that Galen is one of our main witnesses for such a theory, whose authors are most likely Theophrastus and Eudemus. A reconstruction of this theory is offered which - among other things - allows to solve some apparent textual difficulties in the Institutio Logica.

There is a long-standing debate whether propositions, sentences, statements or utterances provide an answer to the question of what objects logical formulas stand for. Based on the traditional understanding of logic as a science of valid arguments, this question is firstly framed more exactly, making explicit that it calls not only for identifying some class of objects, but also for explaining their relationship to ordinary language utterances. It is then argued that there are strong arguments against the proposals commonly put (...) forward in the debate. The core of the problem is that an informative account of the objects formulas stand for presupposes a theory of formalization; that is, a theory that explains what formulas may adequately substitute for an inference in proofs of validity. Although such theories are still subject to research, some consequences can be drawn from an analysis of the reasons why the common accounts featuring sentences, propositions or utterances fail. Theories of formalization cannot refer to utterances qua expressions of propositions; instead they may refer to sentences and rely on additional information about linguistic structure and pragmatic context. (shrink)

There is an exponential speed-up in the number of lines of the quantified propositional sequent calculus over Substitution Frege Systems, if one considers proofs as trees. Whether this is true also for the number of symbols, is still an open problem.

This publication refers to the proceedings of the Seventh Latin American on Mathematical Logic held in Campinas, SP, Brazil, from July 29 to August 2, 1985. The event, dedicated to the memory of Ayda I. Arruda, was sponsored as an official Meeting of the Association for Symbolic Logic. Walter Carnielli. -/- The Journal of Symbolic Logic Vol. 51, No. 4 (Dec., 1986), pp. 1093-1103.

This is a paper on borderline cases and the law of Excluded Middle. In it I try to make use of some long forgotten, but perhaps valuable, work on the topic – a bit of Hegel for instance.

This book is intended to be used as a textbook by students of mathematics, and also within limitations as a reference work.

We present a semantic proof of Löb's theorem for theories T containing ZF. Without using the diagonalization lemma, we construct a sentence AUT T, which says intuitively that the predicate autological with respect to T (i.e. applying to itself in every model of T) is itself autological with respect to T. In effect, the sentence AUT T states I follow semantically from T. Then we show that this sentence indeed follows from T and therefore is true.

Peacocke proposes a criterion for logical constancy in terms of a priori knowability conditions. An a priori knowability condition, Peacocke claims, meets a condition of adequacy for any criterion of logical constancy: expressions satisfying the criterion are topic-neutral. I’ll raise the objection that certain a posteriori knowability conditions would satisfy this adequacy condition. For the requirement of topic-neutrality is ambiguous between two conceptions. Under one conception, a truth is topic-neutral if it is characterized by its indifference to all worldly facts (...) or its abstraction from all semantic content whatsoever. According to another conception of topic-neutrality, to claim that a truth is topic-neutral is not to characterize it by its abstraction from all content whatsoever but rather to characterize it by its abstraction from the specific identities of things. A posteriori knowability conditions could yield expressions which are topic-neutral in this second sense, and so a priori knowability conditions are unnecessary to yield expressions which are topic-neutral in some sense or other. (shrink)

Much of the last fifty years of scholarship on Aristotle’s syllogistic suggests a conceptual framework under which the syllogistic is a logic, a system of inferential reasoning, only if it is not a theory or formal ontology, a system concerned with general features of the world. In this paper, I will argue that this a misleading interpretative framework. The syllogistic is something sui generis: by our lights, it is neither clearly a logic, nor clearly a theory, but rather exhibits certain (...) characteristic marks of logics and certain characteristic marks of theories. In what follows, I will present a debate between a theoretical and a logical interpretation of the syllogistic. The debate centers on the interpretation of syllogisms as either implications or inferences. But the significance of this question has been taken to concern the nature and subject-matter of the syllogistic, and how it ought to be represented by modern techniques. For one might think that, if syllogisms are implications, propositions with conditional form, then the syllogistic, in so far as it is a systematic taxonomy of syllogisms, is a theory or a body of knowledge concerned with general features of the world. Furthermore, if the syllogistic is a theory, then it ought to be represented by an axiomatic system, a system deriving propositional theorems from axioms. On the other hand, if syllogisms are inferences, then the syllogistic is a logic, a system of inferential reasoning. And furthermore, it ought to be represented as a natural deduction system, a system deriving valid arguments by means of intuitively valid inferences. I will argue that one can disentangle these questions—are syllogisms inferences or implications, is the syllogistic a logic or a theory, is the syllogistic a body of worldly knowledge or a system of inferential reasoning, and ought we to represent the syllogistic as a natural deduction system or an axiomatic system—and that we must if we are to have a historically accurate understanding of Aristotle. (shrink)

It is demonstrated how Kripke models for intuitionistic predicate logic can be applied in order to prove classical theorems. As examples proofs of the independence of the axiom of constructibility, of the omitting types theorem and of Shelah's ultrapower theorem are sketched.

Marian David defends the correspondence theory of truth against the disquotational theory of truth, its current major rival. The correspondence theory asserts that truth is a philosophically rich and profound notion in need of serious explanation. Disquotationalists offer a radically deflationary account inspired by Tarski and propagated by Quine and others. They reject the correspondence theory, insist truth is anemic, and advance an "anti-theory" of truth that is essentially a collection of platitudes: "Snow is white" is true if and only (...) if snow is white; "Grass is green" is true if and only if grass is green. According to disquotationalists the only profound insight about truth is that it lacks profundity. David contrasts the correspondence theory with disquotationalism and then develops the latter position in rich detail--more than has been available in previous literature--to show its faults. He demonstrates that disquotationalism is not a tenable theory of truth, as it has too many absurd consequences. (shrink)

First published as part of the renowned Routledge Encyclopedia of Philosophy , this complete glossary of logical terms and notation is the definitive handbook for students of the subject. Logic from A to Z features over 500 short definitional entries on terms used in the most highly technical areas of philosophy--philosophical logic and the philosophy of mathematics. Readers will find key terms such as Argument; Turing Machine; Isomorphism; Function; Algorithm; Variable ; and much more, plus a complete table of logical (...) symbols. Easy-to-use and extensively cross-referenced, this new work is a boon for undergraduates and postgraduate students. (shrink)

We investigate judgment aggregation by assuming that some formulas of the agenda are singled out as premisses, and the Independence condition (formula-wise aggregation) holds for them, though perhaps not for others. Whether premiss-based aggregation thus de…ned is non-degenerate depends on how premisses are logically connected, both among themselves and with other formulas. We identify necessary and su¢ cient conditions for dictatorship or oligarchy on the premisses, and investigate when these results extend to the whole agenda. Our theorems recover or strengthen (...) several existing ones and are formulated for in…nite populations, an innovation of this paper. JEL identi…cation numbers: D70, D71. (shrink)

Austin rejects the contention that every proposition has a contradictory. This paper finds problems with the case Austin makes for rejecting the contention in question.

The great contribution Marcus has made to several of intensely discussed topics in philosophy might not have been noticed fully without this collection of some of her most important articles that makes it evident that her achievement is not limited to inventing the famous Barcan formula.

There are certain logical abilities that any rational creature must have. I call this thesis the Universality of Logic (UL). Something like UL is presupposed in Quinean and Davidsonian uses of the Principle of Charity. Their arguments for the Principle of Charity might be thought of as top−down arguments, establishing UL on the basis of very general considerations about meaning and belief. In this paper, I intend to argue for UL constructively, from the bottom up, as it were, by showing (...) just why and how rationality demands certain specific logical abilities. I shall begin by looking at two sources of opposition to UL, in order to locate it better in the current philosophical landscape and underline the interest and importance of the thesis. Next, I shall deal with important methodological issues surrounding my arguments for UL. This will involve addressing such questions as: what is a rational creature?; and, what are logical abilities? As a result, we will be led to a certain weakening of UL. Then, I shall argue for the weakened version of UL constructively, by demonstrating the universality of certain particular logical abilities. Finally, I shall examine some of the consequences of UL for other areas of philosophy. (shrink)

Strawson offers three accounts of singular predication: a grammatical, a category and a mediating account. I argue that the grammatical and mediating accounts are refuted by a host of counter-examples and that the latter is worse than useless. In later works Strawson defends only the category account. This account entails that singular terms cannot be predicates; it excludes non-denoting singular terms from being logical subjects, except by means of an ad hoc analogy; it depends upon a notion of identification that (...) is too vague; and it is unnecessarily complicated, relying on analogies where a more uniform explanation should be possible. But I show how the account can be corrected to avoid all these difficulties and to provide an accurate account of singular predication. (shrink)

It is often taken for granted by writers who propose--and, for that matter, by writers who oppose--'justifications' of inductions, that deduction either does not need, or can readily be provided with, justification. The purpose of this paper is to argue that, contrary to this common opinion, problems analogous to those which, notoriously, arise in the attempt to justify induction, also arise in the attempt to justify deduction.

Benjamin Schnieder has argued that several traditional definitions of truth-functionality fail to capture a central intuition informal characterizations of the notion often capture. The intuition is that the truth-value of a sentence that employs a truth-functional operator depends upon the truth-values of the sentences upon which the operator operates. Schnieder proposes an alternative definition of truth-functionality that is designed to accommodate this intuition. We argue that one traditional definition of ‘truth-functionality’ is immune from the counterexamples that Schnieder proposes and is (...) preferable to Schnieder’s alternative. (shrink)

A recent article by Jeff Kochan contains a discussion of modus ponens that among other thing alleges that the paradox of the heap is a counterexample to it. In this note I show that it is the conditional major premise of a modus ponens inference, rather than the rule itself, that is impugned. This premise is the contrapositive of the inductive step in the principle of mathematical induction, confirming the widely accepted view that it is the vagueness of natural language (...) predicates, not modus ponens , that is challenged by Sorites. (shrink)

In Tractatus, Wittgenstein held that there are null sentences – prominently including logical truths and the truths of mathematics. He says that such sentences are without sense (sinnlos), that they say nothing; he also denies that they are nonsensical (unsinning). Surely it is what a sentence says which is true or false. So if a sentence says nothing, how can it be true or false? The paper discusses the issue.

A case against Prior’s theory of propositions goes thus: (1) everyday propositional generalizations are not substitutional; (2) Priorean quantifications are not objectual; (3) quantifications are substitutional if not objectual; (4) thus, Priorean quantifications are substitutional; (5) thus that Priorean quantifications are not ontologically committed to propositions provides no basis for a similar claim about our everyday propositional generalizations. Prior agrees with (1) and (2). He rejects (3), but fails to support that rejection with an account of quantification on which there (...) could be quantifications that are neither substitutional nor objectual. The paper draws from the work of Lorenzen an alternative conception of quantification in terms of which that needed account can be given. (shrink)

Colin Howson argues that (1) my sociologistic reliabilism sheds no light on the objectivity of epistemic content, and that (2) sorites does not threaten the reliability of modus ponens . I reply that argument (1) misrepresents my position, and that argument (2) is beside the point.

I shall make two main claims. My first main claim is that Frege started out with a view of logic that is closer to Kant’s than is generally recognized, but that he gradually came to reject this Kantian view, or at least totally to transform it. My second main claim concerns Frege’s reasons for distancing himself from the Kantian conception of logic. It is natural to speculate that this change in Frege’s view of logic may have been spurred by a (...) desire to establish the logicality of the axiom system he needed for his logicist reduction, including the infamous Basic Law V. I admit this may have been one of Frege’s motives. But I shall argue that Frege also had a deeper and more interesting reason to reject his early Kantian view of logic, having to do with his increasingly vehement anti-psychologism. (shrink)

Following an idea from Gödel and Carnap we show how we can speak with absolute generality even if we cannot quantify with absolute generality.

‘Quantified pure existentials’ are sentences (e.g., ‘Some things do not exist’) which meet these conditions: (i) the verb EXIST is contained in, and is, apart from quantificational BE, the only full (as against auxiliary) verb in the sentence; (ii) no (other) logical predicate features in the sentence; (iii) no name or other sub-sentential referring expression features in the sentence; (iv) the sentence contains a quantifier that is not an occurrence of EXIST. Colin McGinn and Rod Girle have alleged that standard (...) first-order logic cannot adequately deal with some such existentials. The article defends the view that it can. (shrink)

It is almost universally acknowledged that first-order logic (FOL), with its clean, well-understood syntax and semantics, allows for the clear expression of philosophical arguments and ideas. Indeed, an argument or philosophical theory rendered in FOL is perhaps the cleanest example there is of “representing philosophy”. A number of prominent syntactic and semantic properties of FOL reflect metaphysical presuppositions that stem from its Fregean origins, particularly the idea of an inviolable divide between concept and object. These presuppositions, taken at face value, (...) reflect a significant metaphysical viewpoint, one that can in fact hinder or prejudice the representation of philosophical ideas and arguments. Philosophers have of course noticed this and have, accordingly, sought to alter or extend traditional FOL in novel ways to reflect a more flexible and egalitarian metaphysical standpoint. The purpose of this paper, however, is to document and discuss how similar “adaptations” to FOL—culminating in a standardized framework known as Common Logic —have evolved out of the more practical and applied encounter of FOL with the problem of representing, sharing, and reasoning upon information on the World Wide Web. (shrink)

Bertrand Russell’s 1906 article ‘The Theory of Implication’ contains an algebraic weak completeness proof for classical propositional logic. Russell did not present it as such. We give an exposition of the proof and investigate Russell’s view of what he was about, whether he could have appreciated the proof for what it is, and why there is no parallel of the proof in Principia Mathematica.

Judgment aggregation theory, or rather, as we conceive of it here, logical aggregation theory generalizes social choice theory by having the aggregation rule bear on judgments of all kinds instead of merely preference judgments. It derives from Kornhauser and Sager’s doctrinal paradox and List and Pettit’s discursive dilemma, two problems that we distinguish emphatically here. The current theory has developed from the discursive dilemma, rather than the doctrinal paradox, and the final objective of the paper is to give the latter (...) its own theoretical development along the line of recent work by Dietrich and Mongin. However, the paper also aims at reviewing logical aggregation theory as such, and it covers impossibility theorems by Dietrich, Dietrich and List, Dokow and Holzman, List and Pettit, Mongin, Nehring and Puppe, Pauly and van Hees, providing a uniform logical framework in which they can be compared with each other. The review goes through three historical stages: the initial paradox and dilemma, the scattered early results on the independence axiom, and the so-called canonical theorem, a collective achievement that provided the theory with its specific method of analysis. The paper goes some way towards philosophical logic, first by briefly connecting the aggregative framework of judgment with the modern philosophy of judgment, and second by thoroughly discussing and axiomatizing the "general logic" built in this framework. (shrink)

This proposal for Philosophy of Science Society Japan and its members presents recommendations toward improvement of logic education, outline of logic curriculum to be shared among community, and requisite components of logical skills and knowledge for philosophers of each field. It also provides information on the past workshops on logic education by PSSJ as well as a summary of ASL guideline and ASL inquiry on logic education (1995).

Raul Orayen's book _Lógica, significado y ontología_ is a deep study into a range of issues in the philosophy of logic, taking Quine as the main interlocutor. It goes into subjects such as Truth-bearerss, Logical Truth, Validity, Propositions, Quine's Extensionalism, Relevant Logic and disjunctive syllogism, and Castañeda's ontology of Guises.

In this paper we give some formal examples of ideas developed by Penco in two papers on the tension inside Frege's notion of sense (see Penco 2003). The paper attempts to compose the tension between semantic and cognitive aspects of sense, through the idea of sense as proof or procedure – not as an alternative to the idea of sense as truth condition, but as complementary to it (as it happens sometimes in the old tradition of procedural semantics).

Gödel began his 1951 Gibbs Lecture by stating: “Research in the foundations of mathematics during the past few decades has produced some results which seem to me of interest, not only in themselves, but also with regard to their implications for the traditional philosophical problems about the nature of mathematics.” (Gödel 1951) Gödel is referring here especially to his own incompleteness theorems (Gödel 1931). Gödel’s first incompleteness theorem (as improved by Rosser (1936)) says that for any consistent formalized system F, (...) which contains elementary arithmetic, there exists a sentence GF of the language of the system which is true but unprovable in that system. Gödel’s second incompleteness theorem states that no consistent formal system can prove its own consistency. (shrink)

Although Church and Turing presented their path-breaking undecidability results immediately after their explication of effective decidability in 1936, it has been generally felt that these results do not have any direct bearing on ordinary mathematics but only contribute to logic, metamathematics and the theory of computability. Therefore it was such a celebrated achievement when Yuri Matiyasevich in 1970 demonstrated that the problem of the solvability of Diophantine equations is undecidable. His work was building essentially on the earlier work by Julia (...) Robinson, Martin Davis and Hilary Putnam (1961), who had showed that the problem of solvability of exponential Diophantine equations is undecidable. One should note, however, that although it was only Matiyasevich’s result which finally solved Hilbert’s tenth problem, already the earlier result had provided a perfectly natural problem of ordinary number theory which is undecidable. (shrink)

Algorithmic information theory, or the theory of Kolmogorov complexity, has become an extraordinarily popular theory, and this is no doubt due, in some part, to the fame of Chaitin’s incompleteness results arising from this field. Actually, there are two rather different results by Chaitin: the earlier one concerns the finite limit of the provability of complexity (see Chaitin, 1974a, 1974b, 1975a); and the later is related to random reals and the halting probability (see Chaitin, 1986, 1987a, 1987b, 1988, 1989.

The aim of this paper is to comprehensively question the validity of the standard way of interpreting Chaitin''s famous incompleteness theorem, which says that for every formalized theory of arithmetic there is a finite constant c such that the theory in question cannot prove any particular number to have Kolmogorov complexity larger than c. The received interpretation of theorem claims that the limiting constant is determined by the complexity of the theory itself, which is assumed to be good measure of (...) the strength of the theory.I exhibit certain strong counterexamples and establish conclusively that the received view is false. Moreover, I show that the limiting constants provided by the theorem do not in any way reflect the power of formalized theories, but that the values of these constants are actually determined by the chosen coding of Turing machines, and are thus quite accidental. (shrink)

Este artigo tem como objetivo geral apresentar alguns aspectos básicos da filosofia da matemática de Charles Sanders Peirce, com o intuito de suscitar discussão posterior. Especificamente, são ressaltados: o lugar da matemática na classificação das ciências do autor; a diferença entre matemática e filosofia como cenoscopia; a relação entre as categorias da fenomenologia e matemática; o conceito de experiência e sua formalização possível; a distinção geral entre lógica, como parte da investigação filosófica, e matemática.

Provided here is an account, both syntactic and semantic, of first-order and monadic second-order quantification theory for domains that may be non-atomic. Although the rules of inference largely parallel those of classical logic, there are important differences in connection with the identification of argument places and the significance of the identity relation.

Comment on S. Neale's, "Facts and Free Logic".

Although the truth value (falsity) of "Henry knows that (dogs live in trees and beavers chew wood)" remains unchanged no matter what sentence is substituted in it for "beavers chew wood", we want not to regard the second as a truth functional component (tfc) of the first. Many definitions of "tfc" (e.g., Quine's) fail to insure satisfaction of the following principle: if p is a component of r which is in turn a component of q, then p is a tfc (...) of q if and only if 1) p is also a tfc of r, and 2) r is also a tfc of q. (shrink)

There are well-known quasi-formal arguments that identity is a "strict" relation in at least the following three senses: (1) There is a single identity relation and a single distinctness relation; (2) There are no contingent cases of identity or distinctness; and (3) There are no vague or indeterminate cases of identity or distinctness. However, the situation is less clear cut than it at first may appear. There is a natural formal theory of identity that is very close to the standard (...) classical theory but which does not validate the formal analogues of (1)-(3). The core idea is simple: We weaken the Principle of the Indiscernibility of Identicals from a conditional to an entailment and we adopt a weakly classical logic. This paper investigates this weakly classical theory of identity (and related theories) and discusses its philosophical rami cations. It argues that we can accept a reasonable theory of identity without committing ourselves to the uniqueness, necessity, or determinacy of identity. (shrink)

A traditional pastime of philosophers is the analysis of rhetoric and the repudiation of sophistry. Nevertheless, some of what philosophers call sophistry might rather be a subtle repudiation of the traditional principles of rationality. In this paper I start by granting the Sophist his repudiation and outline some of the obstacles to settling the dispute between Sophists and Rationalists. I then suggest that we should distinguish pragmatic Sophism from nihilistic Sophism. In the hope of driving a wedge between these two (...) I illustrate the way in which the two are confounded when Sophism is advanced by use of a particular rhetorical manoeuvre and then give an argument against nihilistic Sophism which even a nihilistic Sophist should be persuaded by. (shrink)