Possible Worlds

Rod Girle

• Publisher: Routledge
• Year: 2014
• eBook ISBN:
  9781317489405
• Topic: Philosophy
• Subtopic: Philosophy History & Theory

Ever since Saul Kripke and others developed a semantic interpretation for modal logic, 'possible worlds' has been a much debated issue in contemporary metaphysics. To propose the idea of a possible world that differs in some way from our actual world - for example a world where the grass is red or where no people exist - can help us to analyse and understand a wide range of philosophical concepts, such as counterfactuals, properties, modality, and of course, the notions of possibility and necessity. This book examines the ways in which possible worlds have been used as a framework for considering problems in logic and argument analysis. The book begins with a non-technical introduction to the basic ideas of modal logic in terms of Kripke's possible worlds and then moves on to a discussion of 'possible for' and 'possible that'. The central chapters examine questions of meaning, epistemic possibility, temporal logic, metaphysics, and impossibility. Girle also investigates how the idea of a possible world can be put to use in different areas of philosophy, the problems it may raise, and the benefits that can be gained.

Chapter 1
Introduction

1.1 What might have been

Possible worlds - the very phrase can set the speculative imagination alight. Leibniz suggested that this world was the best of all possible worlds. The suggestion has enraged some, bewildered many, satisfied some and set others to pondering. What is this idea of possible worlds?

Many works of narrative fiction, such as novels, films and even television programmes, describe possible worlds. Such worlds usually have some sort of internal consistency, or some sort of internal logic, even when they are quite unrealistic. Although realism is not always important, it can be. This is particularly so with the genre of historical novels and films. Works such as Pride and Prejudice and No Barrier are highly realistic and depend on a setting that is historically accurate. By contrast, some novels, and the films derived from them, such as The Lord of the Rings and Harry Potter and the Philosopher's Stone are works of sheer fantasy. Their setting is quite unlike the real world in crucial ways. They are valued just because they are not realistic. But there is an internal logic to the possible worlds described in these works.

in the television series Sliders there is explicit use of the idea of possible worlds. The series is built on the idea of possible worlds parallel to ours, worlds to which the heroes can "slide". The heroes have their adventures in these possible worlds, in a different one each episode. In one of the possible worlds the French conquered North America, in one the British still rule all the North American "colonies", in one there are Cro-Magnon humanoids instead of Homo sapiens, and in another almost everything is the same except that the Golden Gate Bridge in San Francisco is painted blue.

There are limitations in Sliders on the variations from possible world to possible world. The environment remains the same and allows the heroes to live on the alternative planet Earths. The alternative planet Earths remain in the same solar system. The sun and moon remain the same. The geography of the planet is much the same. Some things are different, some things are the same from world to world.

The differences are the ways in which things might have been or might have turned out. The samenesses include the laws of nature and a wide range of natural phenomena. It might even be said that, in Sliders, the laws of nature remain the same from possible world to possible world of necessity. In fact, it would be quite difficult to understand what is going on if the laws of nature were different.

It has also been argued that the worlds of Sliders are not, in fact, distinct possible worlds. They are worlds between which there can be traffic, worlds that can be connected by the slide hole through which the heroes and villains can slide. They might at first look distinct, but they must all be part of one huge multi-universe in several dimensions. The worlds to which one can slide are some fifth or sixth dimensional distance away from the real world. There is a machine that allows people to link to one and to travel to it.

It has been argued that one of the key features of possible worlds is that they are utterly and completely distinct from each other from a causal and spatiotemporal point of view across all spatiotemporal dimensions. Lewis writes that things in the same possible world are "worldmates". And further that "whenever two possible individuals are spatiotemporally related, they are worldmates. If there is any distance between them - be it great or small, spatial or temporal - they are parts of one single world".¹ And that "there isn't any trans-world causation... if there is no trans-world causation, there is no trans-world travel. You can't get into a 'logical-space' ship and visit another possible world."²

Whatever the finer, and even crucially important, philosophical points here, the worlds of Sliders are at least very like possible worlds. I will return to some of these points, especially in Chapter 9.

The use of possible worlds to account for what goes on in fiction has been challenged. It might look like the way to go, but it does not really give a satisfactory account. For example, in the case of a child's pretending that a tree stump is a bear, "the real world circumstance that Johnny has put a rope around the stump makes it fictional that a bear has been lassoed".³ It is not that there is a possible world in which Johnny lassoed a bear; Johnny has lassoed a "pretend bear" in the real world. So we need to bear in mind that possible worlds might not give a good account of exactly what we think they will.

Quite apart from fiction like Sliders, we find no difficulty in considering what might have been, and in thinking about what might be. Each might be and each might have been is a possible world. Many everyday activities depend on being able to consider the many things that might be: several future possible worlds. When someone plans a dinner, looks at clothes, organizes a meeting or decides what courses to take at university, they are considering a range of possible worlds. They ask themselves about the possibilities, the possible worlds, and then set about trying to make one of them become actual.

A person can look back on things that have happened, and regret that other possibilities were not realized. They think about what might have been had they acted differently. They can be pleased that they avoided some of things that might have happened, and satisfied with the way things actually turned out. They can even be astonished and elated at the unexpected outcomes, outcomes that had not even been considered as a reasonable possibility.

There is another side to this coinage of mights and possibles: the musts and necessities. A person will consider these also when they are looking back or looking forwards. There will be some things that must be done when they organize a dinner, buy clothes, call a meeting or enrol at university. They can look back and see that, however much they would have wished otherwise, there are some things that just had to happen no matter what. Some things happen of necessity.

1.2 Possible worlds technology

What are these possibilities and necessities? The basic tenet of possible worlds logic and of possible worlds metaphysics is that possibilities are to be seen as possible worlds. At the heart of the technical and the philosophical use of possible worlds is the simple idea that something is possible if it is so in at least one possible world and something is necessary if it is so in all possible worlds. We can express these ideas in terms of the truth of possibility and necessity statements or propositions. In terms of truth:

• possibly P is true if P is true in at least one possible world
• necessarily P is true if P is true in every possible world.

For example, if proposition P is a trivial logical truth, a tautology, a logical necessity such as if it's raining then it's raining, then P will be true in every possible world, and so necessarily P is true. If P is a contingently true proposition, such as Wellington is the capital of New Zealand, then P is in fact true in at least one possible world, the actual world, and so possibly P is true.

The actual world, the one in which we live and move and have our being, is one of many possible worlds. Other possible worlds are unactualized possible worlds. The technical use of possible worlds, based on the simple ideas just set out, has had an enormous impact on the study of the logical systems known as modal logics. Modal logics can be understood without reference to possible worlds, but possible worlds provide one of the easiest ways of understanding modal logics.

It turns out that possible worlds are seen as useful for helping people to think about a wide range of things. Reality and actuality, possibility and necessity, action and process, knowledge and consciousness, obligation and permission and identity and essence, have all been considered from the perspective of possible worlds.

The first clear detailed technical and formal use of possible worlds is in the work of Saul Kripke.⁴ His account appeared in the middle of the twentieth century. Kripke's account of possible worlds spilled over from formal logic into philosophy. It has provided philosophers with a powerful explanatory apparatus for discussing questions about possibility, necessity, knowledge, time, reality and many other concepts. There has been much argument, discussion and debate about the idea of possible worlds, and the explanations that use possible worlds. Our main task in this book is to look at some of those ideas and explanations and the many faceted debate that they have generated.

It will turn out that possible worlds, and the detailed technical apparatus that has been generated by using them (the possible worlds "technology", as I will call it), are not an unmixed blessing. Not everyone sees possible worlds as useful or helpful. This volume will urge that possible worlds be considered more critically than has been the recent tendency.

1.3 From arguments to logic and back again

In this chapter I consider some of the argument analysis background to th...

Table of contents

• Cover
• Title
• Copyright
• Contents
• Preface
• 1 Introduction
• 2 Possible worlds
• 3 Possible worlds and quantifiers
• 4 Possible worlds, individuals and identity
• 5 Possibility talk
• 6 The possible worlds of knowledge
• 7 The possible worlds of belief
• 8 Time and many possible worlds
• 9 Real possibility
• 10 Impossible possible worlds
• 11 Unfinished story
• Notes
• Bibliography
• Index