Camillo Fiore

Often, two logical systems differ in what seems a merely superficial way; for example, in the symbols they choose for a given operation. When this happens, we say that the systems are notational variants of each other and that, as a consequence, they are presentations of the same logical theory. In recent years, increasing attention has been paid to certain logical systems that are radically substructural, in the sense that they abandon the reflexivity and/or transitivity of consequence. The contribution of this paper is twofold. First, I argue that such systems pose serious challenges to our extant criteria for deciding when two systems are notational variants. Second, I give some steps towards a new, improved criterion. Intuitively, I suggest that two logical systems are notational variants just in case, once we translate them properly, they give rise to the same non-logical theories—where, crucially, a non-logical theory can contain not only statements, but also inferences

The emergence of the strict-tolerant logic ST (advocated by Cobreros, Egré, Ripley and van Rooij) and of the hierarchy of increasingly classical logics based upon it (advanced by Barrio, Pailos and Szmuc) sparked many new questions, developments, and debates. One of the central points of discussion is how to identify a logic, or in other words, when two logics can be said to be alternative presentations of one another. Recently, Brian Porter tentatively suggested a new criterion of identity, premised on the intuitive idea that two logics cannot be the same if they admit different applications. In this article, I argue that, although Porter’s criterion is on the right track, it is still unsatisfactory by the author’s own lights. Then, I propose a different criterion which better captures the mentioned intuitive idea.

Validity is usually taken to be a bivalent property: every inference is either valid or invalid, and never both. We argue for the controversial thesis that, if one endorses a many-valued semantics for the object language, then one likely has good reasons to also endorse a manyvalued notion of validity. We present several logical systems (based on Belnap’s algebra 4) whose notion of validity is non-bivalent: there are inferences that are both valid and invalid, and/or inferences that are neither valid nor invalid. We show that, under natural assumptions, the validity of metainferences is also non-bivalent in many of these systems. Lastly, we claim that our framework has fruitful applications to philosophical issues, such as the link between logical consequence and the conditional, and the link between logic and metalogic.

In philosophical logic and proof theory, we often find multiple-conclusion systems that induce a conjunctive reading of premises and a disjunctive reading of conclusions. In mathematical logic, in contrast, we often find multiple-conclusion systems that induce a conjunctive reading of both premises and conclusions. This paper studies some technical and philosophical aspects of this latter approach to multiple-conclusion consequence. The takeaway is that, while the importance of disjunctive multiple conclusions is beyond doubt, conjunctive multiple conclusions also have philosophical interest. First, because there is some evidence that there are arguments with conjunctive multiple conclusions in natural language. Second, because conjunctive multiple conclusions are compatible with the reflexivity and transitivity of logical consequence, and this allows them to cohere better with some of our best accounts of what logical consequence is.

Recently, it has been proposed to understand a logic as containing not only a validity canon for inferences but also a validity canon for metainferences of any finite level. Then, it has been shown that it is possible to construct infinite hierarchies of ‘increasingly classical’ logics—that is, logics that are classical at the level of inferences and of increasingly higher metainferences—all of which admit a transparent truth predicate. In this paper, we extend this line of investigation by taking a somehow different route. We explore logics that are different from classical logic at the level of inferences, but recover some important aspects of classical logic at every metainferential level. We dub such systems meta-classical non-classical logics. We argue that the systems presented deserve to be regarded as logics in their own right and, moreover, are potentially useful for the non-classical logician.

Connexive logics are based on two ideas: that no statement entails or is entailed by its own negation (this is Aristotle’s thesis) and that no statement entails both something and the negation of this very thing (this is Boethius' thesis). Usually, connexive logics are contra-classical. In this note, I introduce a reading of the connexive theses that makes them compatible with classical logic. According to this reading, the theses in question do not talk about validity alone; rather, they talk in part about (a property related to) the soundness of arguments.

A metainference is usually understood as a pair consisting of a collection of inferences, called premises, and a single inference, called conclusion. In the last few years, much attention has been paid to the study of metainferences—and, in particular, to the question of what are the valid metainferences of a given logic. So far, however, this study has been done in quite a poor language. Our usual sequent calculi have no way to represent, e.g. negations, disjunctions or conjunctions of inferences. In this paper we tackle this expressive issue. We assume some background sentential language as given and define what we call an inferential language, that is, a language whose atomic formulas are inferences. We provide a model-theoretic characterization of validity for this language—relative to some given characterization of validity for the background sentential language—and provide a proof-theoretic analysis of validity. We argue that our novel language has fruitful philosophical applications. Lastly, we generalize some of our definitions and results to arbitrary metainferential levels.

When logicians work with multiple-conclusion systems, they use a metalinguistic comma ‘,’ to aggregate premises and/or conclusions. In this note, I present an analogy between this comma and Prior’s infamous connective tonk. The analogy reveals that these expressions have much in common. I argue that, indeed, the comma can be seen as a structural incarnation of tonk. The upshot is that, whatever story one has to tell about tonk, there are good reasons to tell a similar story about the comma in typical multiple-conclusion systems, and vice versa.

An old and well-known objection to non-classical logics is that they are too weak; in particular, they cannot prove a number of important mathematical results. A promising strategy to deal with this objection consists in proving so-called recapture results. Roughly, these results show that classical logic can be used in mathematics and other unproblematic contexts. However, the strategy faces some potential problems. First, typical recapture results are formulated in a purely logical language, and do not generalize nicely to languages containing the kind of vocabulary that usually motivates non-classical theories—for example, a language containing a naïve truth predicate. Second, proofs of recapture results typically employ classical principles that are not valid in the targeted non-classical system; hence, non-classical theorists do not seem entitled to those results. In this paper, we analyze these problems and provide solutions on behalf of non-classical theorists. To address the first problem, we provide a novel kind of recapture result, which generalizes nicely to a truth-theoretic language. As for the second problem, we argue that it relies on an ambiguity and that, once the ambiguity is removed, there are no reasons to think that non-classical logicians are not entitled to their recapture results.

Logical pluralism is a general idea that there is more than one correct logic. Carnielli and Rodrigues [2019a] defend an epistemic interpretation of the paraconsistent logic N4, according to which an argument is valid in this logic just in case it necessarily preserves evidence. The authors appeal to this epistemic interpretation to briefly motivate a kind of logical pluralism: “different accounts of logical consequence may preserve different properties of propositions”. The aim of this paper is to study the prospect of a logical pluralism based on different interpretations of logical systems. First, we give our analysis of what it means to interpret a logic – and make some hopefully useful distinctions along the way. Second, we present what we call an interpretational logical pluralism: there is more than one correct logic and a logic is correct only if it has some adequate interpretation. We consider four variants of this idea, bring up some possible objections, and try to find plausible solutions on behalf of the pluralist. We will argue that interpretations of logical systems provide a promising – albeit not unproblematic – route to logical pluralism.

Saul Kripke proposed a skeptical challenge that Romina Padró defended and popularized by the name of the Adoption Problem. The challenge is that, given a certain definition of adoption, there are some logical principles that cannot be adopted—paradigmatic cases being Universal Instantiation and Modus Ponens. Kripke has used the Adoption Problem to argue that there is an important sense in which logic is not revisable. In this essay, I defend two independent claims. First, that the Adoption Problem does not entail that logic is never revisable in the sense that Kripke addresses. Second, that, to assess whether an agent can revise their logic in the sense that Kripke addresses, it is best to consider a different definition of adoption, according to which Universal Instantiation and Modus Ponens are sometimes adoptable.

En este capítulo ofrecemos una introducción sistemática e histórica a la lógica, disciplina que contribuyó en gran medida a la producción del conocimiento en general y a la formación del pensamiento científico en particular. La primera sección contiene la introducción sistemática: primero, presentamos las distintas disciplinas que forman parte de la lógica en el sentido amplio del término; luego, identificamos a la lógica en sentido canónico o estricto como el estudio la validez; por último, explicamos en qué sentido la validez es una propiedad formal. La segunda sección contiene la introducción histórica; nos detenemos, en particular, a estudiar dos hitos: el nacimiento de la lógica como disciplina con Aristóteles, y la intervención de Gottlob Frege, a quien se debe, en gran parte, el surgimiento de la lógica tal como la conocemos. Finalmente, damos un breve comentario del estado actual de la disciplina.

The infinitary function of the truth predicate consists in its ability to express infinite conjunctions and disjunctions. A transparency principle for truth states the equivalence between a sentence and its truth predication; it requires an introduction principle—which allows the inference from “snow is white” to “the sentence ‘snow is white’ is true”—and an elimination principle—which allows the inference from “the sentence ‘snow is white’ is true” to “snow is white”. It is commonly assumed that a theory of truth needs to satisfy a transparency principle to fulfil the infinitary function. Picollo and Schindler (Erkenntnis 83:899–928, 2017) argue against this idea. They prove that, given certain assumptions, an elimination principle is sufficient for the purpose. Then, they pose a challenge: to show why we should incorporate introduction principles to our theory of truth. In this essay I take on the challenge. I show that, given the authors’ assumptions, an introduction principle is also sufficient to perform the infinitary function.