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I start by reconsidering two familiar arguments against modal realism. The argument from epistemology relates to the issue whether we can infer the existence of concrete objects by a priori means. The argument from pragmatics purports to refute the analogy between the indispensability of possible worlds and the indispensability of unobserved entities in physical science and of numbers in mathematics. Then I present two novel objections. One focusses on the obscurity of the notion of isolation required by modal realism. The other stresses the arbitrary nature of the rules governing the behaviour of Lewisean universes. All four objections attack the reductive analysis of modality that is supposed to be the chief merit of modal realism.

David Lewis' modal counterpart theory falls prey to the famous Saul Kripke's objection, and this is mostly due to his 'static' ontology (divergence) of possible worlds. This paper examines a genuinely realist but different, branching ontology of possible worlds and a new definition of the counterpart relation, which attempts to provide us with a better account of de re modality, and to meet satisfactorily Kripke's claim, while being also ontologically more 'parsimonious'.

Some argue that Lewisian realism fails as a reduction of modality because in order to meet some criterion of success the account needs to invoke primitive modality. I defend Lewisian realism against this charge; in the process, I hope to shed some light on the conditions of success for a reduction. In §1 I detail the resources the Lewisian modal realist needs. In §2 I argue against Lycan and Shalkowski’s charge that Lewis needs a modal notion of ‘world’ to ensure that worlds correspond to possibilities. In §3 I respond to Divers and Melia’s objection that Lewis needs to invoke primitive modality to give a complete account of what worlds there are. In §4 I ask what it is for a notion to ‘involve’ modality. I conclude that the question is either in bad standing or at best offers little traction on the debate, and propose a different way of assessing when materials are appropriately included in a reductive base.

Kit Fine (1994. “Essence and Modality”, Philosophical Perspectives 8: 1-16) argues that the standard modal account of essence as de re modality is ‘fundamentally misguided’ (p. 3). We agree with his critique and suggest an alternative counterfactual analysis of essence. As a corollary, our counterfactual account lends support to non-vacuism the thesis that counterpossibles (i.e., counterfactual conditionals with impossible antecedents) are not always vacuously true.

Metaphysical modalities are definable from counterfactual conditionals, and the epistemology of the former is a special case of the epistemology of the latter. In particular, the role of conceivability and inconceivability in assessing claims of possibility and impossibility can be explained as a special case of the pervasive role of the imagination in assessing counterfactual conditionals, an account of which is sketched. Thus scepticism about metaphysical modality entails a more far-reaching scepticism about counterfactuals. The account is used to question the significance of the distinction between a priori and a posteriori knowledge.

This book is a survey, fortified by original material, of metaphysical theories of modality set in terms of possible worlds. Those theories include what Divers calls “genuine realism”, or “GR” — this is David Lewis’s “genuine modal realism” — and what Divers calls “actualist realism”, or “AR” — this seems to be the same as what Lewis called “ersatz modal realism”, which has also become widely know as “ersatzism”. Two important kinds of theory are not included: those that treat modality by means not involving possible worlds at all (for example, modalism, various kind of non-cognitivism, and Quinean eliminativism about modality); and those that, while treating modality by talking about possible worlds, go on to deny that there are any possible worlds in one way or another (for example, Rosen’s fictionalism, and Divers’ own agnosticism about possible worlds). These get a short discussion towards the end of the introductory part of the book (sections 2.4–2.6), and Divers is planning to write a companion volume covering them.

In recent work Timothy Williamson argues that the epistemology of metaphysical modality is a special case of the epistemology of counterfactuals. I argue that Williamson has not provided an adequate argument for this controversial claim, and that it is not obvious how what he says should be supplemented in order to derive such an argument. But I suggest that an important moral of his discussion survives this point. The moral is that experience could play an epistemic role which is more epistemically significant than a mere 'enabling' role but not equivalent to an evidential role.

According to Lewis, causal claims must be analysed in terms of counterfactual conditionals, and these in turn are understood in terms of relations of comparative similarity among single concrete possible worlds. Lewis also claims that there is no trans-world causation because there is no way to make sense of trans-world counterfactuals without automatically making them come out to be false. In this paper I argue against this claim. I show how to make sense of trans-world counterfactuals in a non-trivial way that can make them come out to be true, by appealing to relations of comparative similarity among concrete possible worlds (i.e., assuming modal realism). I argue that either merely making such sense of a relevant counterfactual is not enough to have causation, or that Lewis’ modal realism must be given up.

The paper compares the suitability of two different epistemologies of counterfactuals—(EC) and (W)—to elucidate modal knowledge. I argue that, while both of them explain the data on our knowledge of counterfactuals, only (W)—Williamson’s epistemology—is compatible with all counterpossibles being true. This is something on which Williamson’s counterfactual-based account of modal knowledge relies. A first problem is, therefore, that, in the absence of further, disambiguating data, Williamson’s choice of (W) is objectionably biased. A second, deeper problem is that (W) cannot satisfactorily elucidate modal knowledge. Third, from a naturalistic perspective, the nature of this second problem favours (EC) against (W).

What is our epistemic access to metaphysical modality? Timothy Williamson suggests that the epistemology of counterfactuals will provide the answer. This paper challenges Williamson's account and argues that certain elements of the epistemology of counterfactuals that he discusses, namely so called background knowledge and constitutive facts, are already saturated with modal content which his account fails to explain. Williamson's account will first be outlined and the role of background knowledge and constitutive facts analysed. Their key role is to restrict our imagination to rule out irrelevant counterfactual suppositions. However, background knowledge turns out to be problematic in cases where we are dealing with metaphysically possible counterfactual suppositions that violate the actual laws of physics. As we will see, unless Williamson assumes that background knowledge corresponds with the actual, true laws of physics and that these laws are metaphysically necessary, it will be difficult to address this problem. Furthermore, Williamson's account fails to accommodate the distinction between conceivable yet metaphysically impossible scenarios, and conceivable and metaphysically possible scenarios. This is because background knowledge and constitutive facts are based strictly on our knowledge of the actual world. Williamson does attempt to address this concern with regard to metaphysical necessities – as they hold across all possible worlds – but we will see that even in this case the explanation is questionable. These problems, it will be suggested, cannot be addressed in a counterfactual account of the epistemology of modality. The paper finishes with an analysis of Williamson's possible rejoinders and some discussion about the prospects of an alternative account of modal epistemology.