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.

From 1912, C. I. Lewis attempted to construct a logic of entailment. In doing so, he created his modal logics, S1–S5, of which his chosen logic of entailment was S2. Although his logics avoid the so-called paradoxes of material implication, they still fall prey to the problem of explosion (that every proposition follows from any contradiction) and the problem of implosion (that every tautology follows from every proposition). These problems, and the inadequate treatment of nested entailments, make Lewis’s logics of limited use as logics of entailment. The chapter also discusses the systems devised by Lewis’s students Everett Nelson and William Parry. Nelson’s connexive logic avoids many of the problems with Lewis’s system but is found to have severe difficulties of its own, and Parry’s analytic implication, although it introduces an interesting version of the notion of meaning containment, does not adequately avoid the problems with Lewis’s logics.

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”.

One difficulty with Lewis’s logics favourite systems, S1–S3, is that they have no intuitive semantics or proof theory. Another approach to constructing a logic of entailment is to begin with a semantic intuition and then adopt the logic characterized by the semantics. This is the approach of Carnap’s Meaning and Necessity. On Carnap’s view, entailment is just strict implication in the sense of the logic S5. The chapter examines Carnap’s semantics and its successor developed by Nino Cocchiarella, and finds that whereas they may give a good representation of the notion of logical truth, they do not provide an adequate analysis of entailment. Once again, the problem of implosion and nested entailments are problematic. This chapter also looks at attempts to solve these difficulties using worlds at which the logical truths differ, and raises philosophical worries about them.

This chapter discusses the application of the logic to actual theories. In particular, it discusses the use of this relevant logic to make inferences about classical theories. It also examines the internal structure of theories and the nature of the conditionals in those theories. At the end of the chapter, some suggestions are made about generalizing the semantical theory of the book to treat the application of background scientific theories to other theories and some consequences for scientific confirmation.

A logic of entailment is one in which it represents its own derivability relation as a connective. This allows it to express nested claims about what is derivable from what. It can say, for example, that if B is derivable from A and if C is derivable from B, then C is derivable from A. Mathematicians and logicians think this way when making proof plans. This chapter sets out the problem of constructing proof plans and looks at the other uses to which logics of entailment have been put. One key use, especially in the context of this book, is as a theory of the closure of scientific theories.