In this chapter, the semantical framework of Chapter 7 is fine-tuned in order to provide a semantics for Anderson and Belnap’s logic E of relevant entailment. It is shown how a form of logical necessity can be represented in the semantics, and this is used to motivate the postulates that are needed to characterize that logic. A labelled deduction system (introduced in Chapter 7) is also used to show how to derive the axioms of E easily.

The semantical framework for the positive view of this book is one in which entailment is understood primarily in terms of theory closure. This chapter outlines both the history of the notion, beginning with Alfred Tarski’s theory of closure operators, and the relationship between closure operators and the entailment connective. At the end of the chapter, it is shown how closure operators can be used to model a simple logic, Graham Priest’s logic N4.

This chapter generalizes the ideas given in the previous chapter. It sets out the notion of a model based on a set of theories. One of these theories is the logic itself. It is, so to speak, the correct theory of theories. It correctly states the principles under which all the theories (including itself) are closed. But each theory has associated with it a closure operator. Some of these operators get the principles of theory closure quite wrong in the sense that they do not apply correctly to every theory in the model. The interaction between these closure operators can be altered in various ways, giving rise to different logical systems. The resulting formal semantics can be represented in the manner of Kit Fine’s “Models for Entailment”.

In its most general form, logical pluralism is the view that there is more than one correct logic. I call this generic claim "the plurality thesis". Different versions of logical pluralism emerge with different implementations of that thesis and, most notably, of its key components logic and correctness. On some readings of the plurality thesis, logical pluralism is completely uncontroversial, on others it may turn out to be a rather exciting position. In this opening chapter, I identify an interesting, revisionist reading of the plurality thesis that is inconsistent with both logical monism and logical nihilism. Logical pluralism, so understood, claims that there are at least two correct theories of logical consequence. The chapter sketches historical developments of the view and gives an outline of the arguments defended in the book.

The previous chapter highlighted the difficulties of combining logical pluralism with a semantic account of rivalry between correct logics. This chapter discusses the weaker conception of applicational rivalry and its relation to the idea that logical consequence has a certain kind of normative force. I argue that all variants of logical pluralism that meet the following three conditions are susceptible to what has been called the collapse problem for logical pluralism: (i) that there are at least two correct logical systems characterized in terms of different consequence relations, (ii) that there is applicational rivalry among the correct logics, and (iii) that logical consequence is normative. I argue that if a position satisfies all these conditions, then that position is unstable in the sense that it collapses into competing positions. In a final step, I show how the collapse problem persists even without an explicitly logical normativity constraint, leaving only conditions (i) and (ii). The problem can therefore be viewed as a result of two core assumptions: plurality and a very weak sense of rivalry that is endorsed by virtually all logical pluralists.

The chapter is concerned with a commitment of the logical pluralist: if there are at least two correct logics, then these logics will either involve different logical vocabulary or they will assign different meanings to the shared vocabulary. A central question is how this plurality in meaning can be implemented within a pluralist framework. Pluralists typically endorse claims to the effect that (i) the connectives of the logics have different meanings or (ii) that the notion of validity employed by one logic may be relevantly different from the one employed by the other. A further important question is whether the plurality of meanings is confined to the theoretical level only or whether a corresponding plurality is postulated regarding the extra-systematic counterparts of the logical vocabulary of correct logics. I argue that both connectives pluralism and consequence pluralism are implausible when construed as pluralistic theories. Meta-contextualism–the view that the question of whether the meaning of the logical terminology is the same or different in different contexts is itself subject to semantic variability–is shown to have exceedingly radical implications.

This final chapter summarizes the main arguments given in the book. The central aim has been to defend logical monism–the view that there is only one correct answer to the question of whether or not a given argument is valid–against the challenges raised by the logical pluralist. The first task was to get clear on what, exactly, those challenges amount to. It turned out that pluralism, understood as the thesis that there is more than one correct logic, is not necessarily a controversial view. In some readings, it is obviously true. Crucially, logical monism, properly understood, needs no defense against those readings. But there are other versions of logical pluralism that do conflict with logical monism. Those are the readings I call revisionist. The account offered in this book allows for the obviously true readings of logical pluralism while resisting the revisionist approaches pursued by some pluralists. The basic tenets of this account are (i) that there is exactly one notion of extra-systematic logical consequence and (ii) that there is exactly one logical theory that provides the best account of this notion.

This chapter explores a number of ways to understand the key notions of the plurality thesis. First, I disambiguate three readings of the term logic: (i) purely formal systems, (ii) interpreted logical theories, and (iii) the subject matter of logical theories. I argue that this distinction is relatively lightweight and should be acceptable on all prominent views about the nature of logical consequence. Building on those readings of logic, I then explore different conceptions of what it means for a logic to be correct. In particular, I present a generic view of correctness of logical theories which is broad enough not to exclude pluralists who claim that the plurality thesis should better be put in terms of the legitimacy or the usefulness of a logic. I propose different ways to strengthen the generic view by means of a weak or a strong version of the correspondence view or the logic-as-modeling view. Finally, I introduce different implementations of the plurality thesis resulting from the different readings of logic and of correctness and identify the interesting version of the thesis which will be the subject of the rest of the book.

Logical pluralism is sometimes motivated by the claim that it affords a more charitable interpretation of important debates in philosophical logic than monism does. This chapter argues that this claim is false. Pluralists are unable to account for important parts of logical and mathematical practice since they are in no position to account for potential rivalry between different logics. This is a problem for the charity-based project because most non-classical logicians formulate their theories of logical consequence as rivals to classical logic. I introduce three approaches to rivalry that focus on semantics, metasemantics, and applications, respectively. I argue in this chapter that neither the semantic nor the metasemantic approach offer a sense of rivalry that is plausible from a pluralist perspective, leaving the applicational approach for the following chapter. I further argue that this supports the conclusion that, at least as far as issues concerning the meanings of logical vocabulary are concerned, the correct logics are ultimately compatible. If we are interested in potential rivalry between the logics, we will have to look elsewhere.

In previous chapters, I construed logical pluralism as the view that there are multiple correct theories of extra-systematic logical consequence. Against this background, it may be tempting to think that logical pluralists are committed to the postulation of a plurality of extra-systematic logical consequence relations. In this chapter I argue that further options are available. I first show that, depending on the underlying notion of correctness, logical pluralism is compatible with any account of the cardinality of extra-systematic logical consequence. I then identify readings of the plurality thesis that give rise to the revisionist reading of logical pluralism that is the target of this book. The most obvious one is genuine plurality—the view that there is more than one extra-systematic consequence relation. A less obvious one acknowledges monism about extra-systematic consequence but argues that there cannot be a single precise theory that captures this relation. I propose a monist approach to logic in both the theory sense and the subject of investigation sense that rejects revisionist logical pluralism.

This chapter explores three dimensions on which logical plurality may arise. The first is concerned with the application of logic. Traditionally, logic was taken to be universally applicable in the sense that a deductively valid argument can be applied in any discourse or inquiry whatsoever. Some pluralists oppose that view by arguing that there are arguments which, though deductively valid, cannot be applied across the board. Deductive validity, on that view, is domain-dependent. The second dimension concerns semantics. Typically, if logics differ in their logical vocabulary, then they will draw the line between valid and invalid arguments in different ways. Even if the logical vocabulary of two logics is superficially the same, the sets of arguments the logics classify as valid may differ due to differences in the meaning of the logical vocabulary. The third dimension concerns the nature of validity. The most substantial kind of pluralism amounts to claiming that there is more than one extra-systematic relation that qualifies as a relation of logical consequence. The chapter outlines both the pluralist and the monist positions on those dimensions and identifies some core commitments.

This chapter presents a dialogical rationale based on the Prover–Skeptic model for the three main features of deduction identified in Chapter 1: necessary truth-preservation, perspicuity, and belief-bracketing. Moreover, it addresses four important ongoing debates in the philosophy of logic: the normativity of logic, logical pluralism, logical paradoxes, and logical consequence. It is shown that the Prover–Skeptic model provides a promising vantage point to address the questions raised in these debates.