A Study Of David Lewis' Thought On Modal Philosophy

Modern modal logic was founded by C. I. Lewis at the beginning of the 20th century. Ever since then, and especially since his thinking on the five model systems SI to S5, how to establish the semantics of modal logic and its legitimacy has always been an important issue for modern logicians. In the 1950's and 1960's, Kripke established the semantics of possible worlds. Afterwards, the status and value of modal logic gradually became recognized by scholars. More and more modal logicians joined in the research of the concept of possible worlds and brought forward some competing theories. The modal realism of David Lewis was one of them. However, among these competing theories, modal realism received much critique because of its non-consistence with intuition. But it still showed it's powerful vitality. This thesis tries to show that the criticisms on David Lewis'modal realism are due to the misunderstanding of his thought on modal philosophy to a great extent, and it is partly because of the character of Lewis'expression with the theory. By clarfying the original and by comparision, we can find that David Lewis' thought on modal philosophy is systematic and coherent. It just shows that modal realism is superior to other theories of modal philosophy, which makes it play an important role in sovling the questions.The modal philosophy of David Lewis is mainly represented by modal realism, which consists of three main components:possible-world theory, counterpart theory, and reductionism. In addition, Lewis's modal philosophy includes the analysis of counterfactuals, which is the application of modal realism. Thus, the main body of this paper is divided into the following five chapters:The first chapter describes the historical development of modal logic and its research background, points out the value of studying modal philosophy, introduces briefly Lewis' modal philosophy and provides an explanation of the methods of this innovative research. The second chapter focuses on Lewis' possible-world theory. In the theory of possible worlds, Lewis claims that possible worlds are real and irreducible. These worlds are just like the actual world.In view of the species of reality, possible worlds and the actual world have no difference. According to Lewis, possible worlds have four characteristics:isolation, concreteness, plentitude and actuality. It is precisely because of these characteristics that modal realism has been called extreme modal realism and been much criticized. Based on the analysis of Lewis' use of these concepts, this thesis shows that Lewis did not use them in the normal sense, especially the concepts of "reality", "actuality" and "concreteness". Reality is an existence, relative to the existence of laws or fact. It can be divided into three levels:the actual existence, actual possible reality in relation to actual laws, and logical possible realty in relation to logical laws. The distinction between these levels cannot deny the commonality in respect to the quality of reality, that is, the real world and possible worlds have no difference in respect to the quality of reality. "Actuality" can have two kinds of understanding. One is reality in the sense of logical possibility, and the other is reality from the point of view of actuality, i.e. the expression of a real possibility in relation to actual laws. "Concreteness" does not necessarily have the actual world as its premise. Non-actuality can also be concrete. Understood by those who live in the actual world, modal realism represents an actual possibility. Thus, the reality of possible worlds is undeniable.The third chapter studies the counterpart theory of Lewis. On trans-world issues, Lewis believes that there is no possibility for this, neither for trans-world identity. He proposes the counterpart relation to replace trans-world identity. Counterpart relationships are not identity relationships, neither are they one to one relationships. Rather, they are relationships of similarity, in which external similarity plays an important role. Lewis supports the theory of counterpart relations for two reasons. The first is associated with the characteristics of possible worlds. Possible worlds are real and isolated. Therefore, individuals in possible worlds are world-bound individuals and they are concrete. The recognition of trans-world identity would certainly lead to theoretical conflicts. The second reason is the insistence on Leibniz's principle of the indiscernibility of identicals. Individuals in different worlds must have different properties. If we claim trans-world identity, it will certainly violate the principle of the indiscernibility of identicals. On this basis, Lewis builds the counterpart logical system, and discusses the advantages of counterpart theory from the view of its explanatory power.This paper argues that the two sides of the dispute both misuse the principle, as this principle cannot be applied to trans-world individuals. That counterpart relation replaces trans-world identity is necessary for the consistency of the possible-world theory. Facing the criticism of counterpart relations, this paper argues, from the point of view of epistemology and the judgment of modal propositions that the theory of counterpart relationships reflects an understanding of modality. In view of the standard of similarity of the counterpart relation theory, especially the role of external similarity in determining the counterpart relationship, the counterpart relation theory reflects a similarity of situation.The fourth chapter researches Lewis'reductionism. With the back-ground of nominalism, Lewis advocates that concepts such as proposition and property can be expressed by the reduction to the possible worlds, individuals, and sets. When he interprets these concepts by method of reduction, Lewis proposes a standard based on the role they play, rather than their identity. This paper argues that from the point of view of truth, especially in modal discourse, it is reasonable to explain concepts, such as proposition and property by reducing them to a set of individuals. But if the two sides of reduction are identified, this method may be unacceptable.The fifth chapter analyses Lewis's counterfactual theory. Counterfactual theory is a result of Lewis'use of possible world semantics and modal realism in the analysis of counterfactual conditionals. Firstly, Lewis thinks that counterfactual implication is a strict implication. Different from others, its strict implication changes according to different contexts. After analyzing, this paper thinks that the variability of counterfactual implication, as a variable and strict implication, is reflected in changes in the worlds within its reach. The reason why changes in the worlds within its reach happen, lies in the fact that the accessibility relation of counterfactual implication is not only associated with laws, but also with prerequisites and backgrounds. It is changes of prerequisites and backgrounds which lead to changes in the worlds within reach and the strictness of counterfactual implication. The judgment of counterfactual conditionals is the judgment of the truth or falsity of propositions belonging to similar possible worlds or situations. The study of similar possible worlds and situations is an application of modal realism and possible world semantics. The success of Lewis' counterfactual analysis also reflects the justifiability of possible world semantics and modal realism in the analysis of practical problems.By studying modal realism and counterfactual theory, this paper claims that David Lewis' modal realism is not extreme and it is not as absurd as some scholars' criticisms claim it to be. As an explanation theory of possible worlds, the true value of modal realism lies in the understanding of modal problems. Moreover, in view of the equality of possibility and of the judgment of modal propositions, modal realism is more convincing than moderate realism.