Ederson Melo

Semantic paradoxes, like the Liar Paradox, are one of the best-known motivations for the dialetheists' claim that there are true contradictions. Liar-like arguments arise in natural language and dialetheists argue that the Liar sentence is true and false, i.e., it bears a glut as its truth-value. However, in a recent paper, JC Beall [4] argued that, by parallel reasoning, one should also be led from the resources of natural language to triviality by the use of validity paradoxes (Curry-style paradoxes involving a pre-theoretical notion of validity). Priest [11] answers Beall's challenge based on a typical move against Curry paradoxes: we avoid triviality by avoiding structural contraction, which is one of the steps in the derivation of triviality. We shall argue that this strategy poses some further difficulties for the dialetheist: i) it is hard for a dialetheist to motivate restriction of structural contraction; ii) structural contraction, if properly motivated, on the other hand, may lead one to avoid the Liar paradox too, and thus, the main motivation for the existence of gluts seems threatened; iii) the asymmetry in the treatment of paradoxes (take the Liar at face value, avoid the Curry) raises difficulties for the desideratum that pre-theoretical concepts should play a prominent role in motivating dialetheism. © 2019 Elsevier B.V., All rights reserved.

Dialetheism is the thesis that some contradictions are true. In this paper, we explore claims to such a thesis to be found in some early philosophical writings of Newton da Costa, most notably in his book Ensaio Sobre os Fundamentos da Lógica (hereafter mentioned as the Essay). We present and explore da Costa’s claims that, in a certain sense, abstract notions such as ‘set’ and ‘truth’ may clearly involve true contradictions. Additionaly, we reconstruct da Costa’s argument to the effect that some contradictions may also be true of concrete reality. In a nutshell, da Costa suggests that some paradoxes, the aporias, are so deeply problematic that they may be pointing to reality as the source of the contradiction; i.e. aporias may ground a form of metaphysical dialetheism. This goes further than most dialetheistic claims found in the current literature. We compare these claims to a standard approach to dialetheism as Graham Priest’s, and conclude that da Costa offers a version of dialetheism too in the Essay. This opens the door for interesting explorations of some valuable yet often overlooked contributions of da Costa’s thought to the contemporary debate on dialetheism.

With the rise of paraconsistent logics, contradictions need no longer be seen as a threat to rationality: one may embrace them, without going trivial. This, however, brings a methodological challenge when one is facing a paradox. Broadly speaking, paradoxes are derivations leading from the plausible to the implausible; when the conclusion is a contradiction, the implausible is an explicit contradiction. Now, with paraconsistent logics, such implausibility need no longer be seen as logically trivializing. As a result, the paradox may be embraced as plausible reasoning, with the contradiction accommodated in our overall system. This conflicts with more traditional approaches, requiring that contradictions be eliminated by revision of the derivation. Given that now two paths are available, do we have any reason to prefer one option over the other? In this paper, we argue that the accommodation camp faces difficulties when a form of methodological naturalism is embraced.

Este artigo discute dois tratamentos dados por Charles Sanders Peirce ao paradoxo do Mentiroso, estabelecendo conexões com o debate atual acerca do tema. Nas Harvard Lectures de 1865, Peirce considera que a proposição do mentiroso é tanto verdadeira quanto falsa, o que, de acordo com sua visão sobre a lógica, torna a proposição sem sentido. Esse tratamento antecipa parcialmente a posição dialeteísta contemporânea, que considera que o Mentiroso é uma evidência de que algumas proposições são verdadeiras e falsas. Em textos posteriores, Peirce revisa seu tratamento do problema, considerando que a contradição do Mentiroso é falsa, alinhando-se à tradição da lógica de sua época. A discussão deste artigo evidencia que concepções lógicas específicas pressupostas na investigação de Peirce são as responsáveis pelas interpretações que ele fornece para a contradição envolvida no paradoxo. A partir dessa discussão, este artigo argumenta que o tratamento dado ao Mentiroso depende das concepções lógicas de fundo. Assim, as limitações encontradas por Peirce em suas abordagens ao problema são introduzidas pela lógica tradicional, que, diferentemente de algumas lógicas contemporâneas, não oferece aparato compatível com contradições verdadeiras.

Systems of paraconsistent logics violate the law of explosion: from contradictory premises not every formula follows. One of the philosophical options for interpreting the contradictions allowed as premises in these cases was put forward recently by Carnielli and Rodrigues, with their epistemic approach to paraconsistent logics. In a nutshell, the plan consists in interpreting the contradictions in epistemic terms, as indicating the presence of non-conclusive evidence for both a proposition and its negation. Truth, in this approach, is consistent and is dealt with by classical logic. In this paper we discuss the fate of the Liar paradox in this picture. While this is a paradox about truth, it cannot be accommodated by the classical part of the approach, due to trivialization problems. On the other hand, the paraconsistent part does not seem fit as well, due to the fact that its intended reading is in terms of non-conclusive evidence, not truth. We discuss the difficulties involved in each case and argue that none of the options seems to accommodate the paradox in a satisfactory manner.

Liar-like paradoxes are typically arguments that, by using very intuitive resources of natural language, end up in contradiction. Consistent solutions to those paradoxes usually have difficulties either because they restrict the expressive power of the language, or else because they fall prey to extended versions of the paradox. Dialetheists, like Graham Priest, propose that we should take the Liar at face value and accept the contradictory conclusion as true. A logical treatment of such contradictions is also put forward, with the Logic of Paradox, which should account for the manifestations of the Liar. In this paper we shall argue that such a formal approach, as advanced by Priest, is unsatisfactory. In order to make contradictions acceptable, Priest has to distinguish between two kinds of contradictions, internal and external, corresponding, respectively, to the conclusions of the simple and of the extended Liar. Given that, we argue that while the natural interpretation of LP was intended to account for true and false sentences, dealing with internal contradictions, it lacks the resources to tame external contradictions. Also, the negation sign of LP is unable to represent internal contradictions adequately, precisely because of its allowance of sentences that may be true and false. As a result, the formal account suffers from severe limitations, which make it unable to represent the contradiction obtained in the conclusion of each of the paradoxes.

Dialetheism is the view that some true sentences have a true negation as well. Defending dialetheism, Graham Priest argues that the correct account of negation should allow for true contradictions and \) without entailing triviality. A negation doing precisely that is said to have ‘surplus content’. Now, to defend that the correct account of negation does have surplus content, Priest advances arguments to hold that classical Boolean negation does not even make sense without begging the question against the dialetheist. We shall argue that Priest’s arguments may be turned upon themselves, and that he may also be accused of begging the question against the classical logician. We then advance an argument to the effect that Priest’s account of negation falls short of satisfying his own desiderata on a correct account of a negation: a theory of negation that attempts to represent contradictions cannot coherently allow surplus content, and vice-versa, a negation allowing for surplus content bans contradiction.