Chapter 1 SYNOPSIS: MODALITY AND EXPLANATORY REASONING I aim to shed light on a broad family of modal notions whose most widely discussed representative is the concept of metaphysical necessity. This choice of topic requires little justification or explanation. Since the work of Lewis, Kripke and others ushered in the modal turn in analytic philosophy, the concept of necessity has been at the forefront of the communal philosophical mind. Today, modality is one of the most active areas of research in metaphysics, and modal notions are central to philosophical theorizing across the board—from the foundations of logic to moral theory. Given this trend, it seems to be more important than ever to gain a clear philosophical understanding of modality, not least in order to determine whether modal concepts can bear the weight that so much of recent philosophical practice has placed on them. There are different ways of elucidating a concept. For example, one can shed some light on it by clarifying its formal and logical properties, and its inferential and analytic connections to other notions. A lot of excellent work like this has been done on modal concepts. More ambitiously, one can try to give a full-blown analysis of modal notions. This task has been attempted less frequently and, in my opinion, less successfully. David Lewis’ account is the most developed and famous, but it rests on problematic ontological assumptions and it runs counter to actualist intuitions that are widely shared and, in my opinion, correct. Consequently, I think that we should look for a different analysis of necessity. We do not start this project with a guarantee of success. It may turn out that no informative analysis of modal concepts is possible, and that we should be primitivists or eliminativists about modality. But at this stage it is too early to throw in the towel. It is an important aim of this book is to advertise a new analysis of modal concepts. I will combine this objective with two other goals. Firstly, I think that a comprehensive theory of the modal domain should not stop at analyzing modal notions. It should also explain why we go in for modal thinking at all. What purpose does this practice serve? Why have creatures with our concerns and interests developed the capacity for modal thought? A comprehensive account should offer both a theory of modality and a theory of the practice of modalizing. It should, as it were, combine an internal with an external viewpoint on this practice. Before we start to philosophize about modality, we have a set of (often implicit) assumptions about modality. Philosophers provide this pre-philosophical system of beliefs with a foundation, and refine, extend and correct it from within. They act as participants of our practice of modalizing, i.e. their standpoint is internal to this practice. But philosophers should also make the practice of modalizing itself an object of study. They should, so to speak, take a standpoint external to the practice, in order to describe the practice, and explain what its function is, i.e. why it exists. It is a familiar fact that these two interests can pull in opposite directions. If we concentrate exclusively on giving a metaphysical account of modality, we might end up with a theory that makes it hard to explain why we are interested in modality. It is a common charge, justified or not, against Lewis’ account that it fails in this way.1 Even if there were other possible worlds in Lewis’ sense, so the objection goes, we would have no reason to be interested in them, and Lewis’ account therefore makes it a mystery why we should bother to have modal thoughts at all. At the other end of the spectrum there are theories that do well at explaining the purpose of our modal notions, but at the expense of giving implausible metaphysical accounts of the nature of modality. One example is the theory that for a proposition to be necessary is for it to owe its truth to a convention, perhaps the convention to regard it as true come what may. If some propositions are indeed conventionally exempted from empirical testing, then a concept that singles them out is undeniably useful for our epistemic practices. It is therefore easy for the conventionalist to explain the point of our modal notions. But this advantage is purchased at the price of an implausible theory of what necessity is. 1 For discussions of the point, see Kripke 1980, p. 45, Hazen 1979, pp. 320ff., Blackburn 1984, pp. 214f., Hale 1986, p. 77, McFetridge 1990, sct. 2, and Rosen 1990, pp. 349ff. 2 The task before us, then, is to develop a theory that permits us to achieve both goals, a credible metaphysical account of modality and a plausible account of the practice of modalizing. The best way of doing this is to integrate the two goals in a single enterprise: an account of the nature of modality can be guided by a hypothesis about the ordinary- life practice in which modal concepts originated, while assumptions about the nature of modality can in turn suggest ways of developing one’s ideas about the practice of modalizing. That is what I will try to do in this book. Secondly, it will be one of my aims to illuminate the relationship of modal notions to another family of concepts that are of central interest to metaphysics: the class of notions that contains the concept of causation and related explanatory notions. These concepts have had a career in philosophy that is similar to that of modal notions. The notion of causation was once subjected to a rigorous empiricist critique that convinced many philosophers that we can make sense of causal talk, if at all, only if we construe it as derived talk about regular succession, nomic entailment, or something of that kind. Consequently, for a long time causal notions were made to do very little work in philosophical theorizing. That changed around the 1960’s, when causal theories began to proliferate in many areas in the philosophy of mind, epistemology, metaphysics, and related fields. Suddenly, causation was used in theories about mental states, perception, knowledge, explanation, reference, persistence, the direction of time, and many other topics. It would not be exaggerated to talk of a ‘causal turn.’ This historical trend also has a second, more recent chapter: the rise to prominence of another group of concepts that have an important affinity to that of causation, such as metaphysical explanation and grounding, and related notions like essence, real definition and fundamentality. A cursory glance at the literatures on modality and on causation and explanation leaves little doubt that there are important connections between the two areas, but the nature of these ties and their philosophical implications are controversial. While many philosophers have tried to exploit the connections to give modal accounts of causation and explanation (often counterfactual temrs), I will argue for the opposite order of explanation. Many philosophers have taken Kripke’s work to demonstrate a deep and important distinction between epistemic necessity or a prioricity on the one hand and metaphysical 3 necessity on the other. I essentially agree with this assessment, but I would describe the fundamental divide as separating, not just two notions of necessity, but two broad families of modal notions, one epistemic and metaphysical or ontic. The ontic modalities include, not merely what is commonly known as metaphysical necessity, but an entire spectrum of other notions of necessity as well. It is the ontic modal notions that form the subject matter of this book. I will argue that, just as the notions of a prioricity and epistemic necessity are constitutively tied to the epistemic domain, the notions of ontic necessity are constitutively connected to the realm of causation and explanation. As we will see, the dependence of the modal on the causal and explanatory takes several different forms. Causal and explanatory concepts figure in the analyses of the various forms of ontic necessity. The ontic modal status of individual facts derives to a large extent from their position in the network of causal and explanatory relationships. And our practice of ontic modal thinking has its origin at least in part in causal and explanatory reasoning: notions like metaphysical necessity developed as useful auxiliaries to such reasoning. What follows is a brief synopsis of the main elements of my position. My exposition in this summary will be sketchy and occasionally a little dogmatic. Detailed arguments will be given in later chapters. 1.1 Ontic modality To say that a proposition is necessary, in either an epistemic or an ontic sense, is to say that, in some sense, its truth is very secure, firm or unshakable. How this thought is to be spelled out in detail depends on the form of modality that is under consideration. To say that a proposition is epistemically necessary is to say that its truth is, in some sense, very epistemically secure. To say that a proposition is metaphysically necessary, by contrast, is to say that its truth is very firm or secure in a metaphysical, non-epistemic sense. We can approach the task of analyzing ontic necessity by trying to develop a non-metaphorical way of understanding this idea of metaphysical security or unshakability. I will argue in Chapter 2 that it is the same notion that we are operating with when we raise the question, concerning some fact, how easily it could have failed to obtain: the less easily P could have been otherwise, the more secure its truth. To say that P is necessary, i.e.