Introduction

Causal powers have been posited to ground and explain activity in nature. And yet, powers are subject to scrutiny and criticism today as they were in the seventeenth century and for more or less the same reasons. The detailed and substantive Introduction sketches the key conceptions of, and arguments for and against, powers from Aristotle up to the present. In the first part (Sections 1.1–1.5), there is an account of the history of the powers debate, starting with the Aristotelian conception and moving through medieval accounts to the revolt against powers by the novatores of the seventeenth century. Various criticisms of powers, notably by Descartes, the occasionalists, Boyle and Newton, as well as endorsements, notably by Leibniz, are presented. Then there is an account of Hume’s systematic critique of the epistemology and ontology of powers, of the transition from a power-based to a law-based conception of nature (notably in the work of Mill) and finally a recounting of the various attempts to eliminate or reduce powers and dispositions in the twentieth century. Sections 1.6–1.9 describe the key reasons for the comeback of powers in the last quarter of the twentieth century, notably the issues concerning the nature of properties and the ontic status and necessity of the laws of nature. Sections 2.1–2.12 offer a detailed summary of the twelve contributions to the volume. Finally, the chapter concludes with questions for moving forward.

The Ontological Status of Causal Powers

Benjamin Hill seeks to initiate deeper contemporary discussion of the ontological challenges that drove early modern philosophers (namely, several early Cartesians, Berkeley, and Hume) to accept the negative thesis of occasionalism, that no physical object can truly be an efficient cause. He argues that we should be looking past Hume and his empiricist’s approach to secondary causation to bring the core metaphysical, issues he believes are still lingering, into sharper focus. Hill walks us backwards from Hume’s empirical critiques of powers in the Enquiry and Treatise to Locke’s presentation of the ‘popular’ view that experience lead us to postulate powers as a response to occasionalism. This, he suggests, reveals that the early modern debate about causal powers tracked not the divide between scholastics and mechanical philosophers but the divide between realists and occasionalists and revolved around a confusion between them regarding what was the underlying question of the debate. For the occasionalists, it was not really about whether or not causal powers did exist, but about explaining how they could exist. This leads Hill to explore the metaphysical worries animating seventeenth-century occasionalists.

Induction and Natural Kinds Revisited

Howard Sankey reconsiders a special issue closely connected with causal powers—the problem of induction. He addresses a deep version of problem of circularity originally raised by Psillos, and argues that the circularity can be avoided. The key is recognizing certain epistemically externalist results of the Megaric consequences of the commitment to dispositional essentialism. Circularity can be avoided, Sankey argues, because it is the way the world is, rather than the inductive inference itself, that grounds the reliability of the inductive inference in his previous account. What are doing the work for Sankey here are the Megaric consequences of his adoption of Ellis’s dispositional essentialism. The uniformity in question is one that stretches across possible worlds: nature is uniform in the precise sense that there are natural kinds whose members all possess a shared set of essential properties. The significance of this commitment lies in how the possible and the temporal intersect through restrictions placed on the accessibility relation between the actual and the possible. Ipso facto, when considering questions about the future behaviours of objects, which is how Sankey understands the problem of induction to be, the uniformity of nature can ground the reliability of beliefs about those future behaviours precisely because the domain of possibility is restricted to those worlds accessible to the actual world, which is fixed by the commitments of dispositional essentialism.

The Case against Powers

Walter Ott argues that there is a central, underlying metaphysical worry that animated early modern thinking against causal powers that is still unresolved. He emphasizes that there is a spectre of power holism embedded in the Aristotelian’s commitment to intrinsicality. Ott sees this as the worry expressed in Descartes’s ‘little souls’ argument against powers, which he sees as an important precursor to Neil Williams’s recent ‘problem of fit’ objection to neo-Aristotelian causal powers. And it is this ‘problem of fit’ that highlights the connection between intrinsicality and powers holism. According to Ott, however, the lesson to take from the ‘problem of fit’ is not that Humeanism is the proper position. He suggests instead that we return to a mitigate powers ontology like that found in Boyle and Locke. The idea here is that powers are really relational properties that cannot be reduced to any one of their relata; they simply ‘are internal relations among the intrinsic properties of things’.

Qualities, Powers, and Bare Powers in Locke

Lisa Downing focuses on the important issue of the metaphysics of Locke’s primary–secondary qualities distinction. In recent years this has returned as a topic of scholarly contention. Downing is concerned by the anti-realist trends in recent work on the metaphysics of Locke primary–secondary qualities distinction, and she is keen to defend the claims that Locke was ‘putting forward a kind of realism about secondary qualities’ and that his realism does not readily appear to be a reductive form of realism. Downing begins with the traditional claim that Locke’s distinction was driven by his understanding of matter theory within the new science, like many others in the seventeenth century. From this perspective, she criticizes recent work on the nature and priority of primary qualities, which fail to root the primary in a metaphysical base or connect them to the metaphysical base in the wrong way. Next, she turns toward explaining her own understanding of the subordinate status of the secondary qualities, which brings Downing to Locke’s claim that secondary qualities are ‘mere powers’ and what this meant metaphysically to him.

Causal Powers and Structures

Brian Ellis provides a detailed and systemic overview of his version of dispositional essentialism. Ellis is famous for having developed and defended a mixed ontology for scientific realism. This is a robustly Aristotelian ontology that involves a mix of categorical and essentially dispositional properties inhering as universal in individualized entities. In this contribution, Ellis briefly defends this sort of ontology by arguing that it, or something very much like it, is necessary to provide an account of the system of reality discovered through modern science. It is, moreover, entirely adequate for the job of accounting for the ontology of modern science. He then turns to consider three objections to his ontology. The first is what other contributors will call the ‘directedness problem’, which is the idea that powers are directed at their manifestation in a way analogous to the directedness of intentionality. The second is what other contributors call the ‘intrinsicness problem’, which is the idea that causal powers are intrinsic to or inhering in their subject. The third is what other contributors call the ‘necessity problem’, which is the idea that there is something important and distinctive about metaphysical necessity vis-à-vis logical necessity.

Hume on Causation and Causal Powers

Peter Millican addresses the issue of how to best interpret Hume’s iconic passages on causation and causal powers and aims to cut through the various interpretations by fixing twelve ‘key points’ and arguing that a reductivist reading makes best sense of them. With these twelve points regarding Hume’s theory fixed, Millican turns toward adjudicating between reductivist, subjectivist, and projectivist interpretations. First, Millican attacks subjectivist interpretations on the grounds that they emphasize melodramatic passages in tension with Hume’s more considered claims, especially the first definition of necessity. Millican backs up the critical comments about subjectivism with a plausibly Humean account of what his ‘impression of power or necessary connexion’ might be. Then he turns to projectivist interpretations. Here, he argues that projectivist readings can be accommodated by the reductivist reading he is defending. After that, he turns to the ‘New Hume’, who allegedly accepted ‘thick’ causal powers, which push beyond the two definitions of cause. However, Millican emphasizes that Hume did accept causal powers in some thinner sense, powers that reduce to causal structures in the world that allow the discovery of laws and enable predictive success.

The Return of Causal Powers?

Andreas Hüttemann disagrees with Hill and Ott regarding the relevance of the early modern critiques of causal powers for contemporary practitioners. He argues that the contemporary acceptance of powers and dispositions is insulated against the early modern criticism because the emergence of powers nowadays is not a ‘revival of’ or ‘return to’ the Aristotelian or scholastic version of causal powers. Hüttemann traverses two lines of argumentation in his defence of the contemporary metaphysics of powers. First, he maintains that the early modern critics utilized a version of causation that, because it was rooted in the doctrine of substantial forms, was quite strong and restrictive and that, consequently, their criticisms don’t apply to contemporary notions of powers, which utilize a counterfactual conception of causation. Then, he turns in a different direction to defend Nancy Cartwright and Jeremy Hardie’s use of the Extrapolation Argument in favour of the postulation of dispositions and powers.

Agency, Force, and Inertia in Descartes and Hobbes

Deborah Brown looks at how Hobbes and Descartes used the language of ‘tendency’ in their natural philosophies. She contrasts the problems that arise for Hobbes as he tries to reduce tendencies away with a Descartes, for whom ‘tendency talk is not a mere façon de parler’ but rather is ‘real and causally explanatory’, and who strives to incorporate inherent tendencies into his broader mechanistic ontological framework. The resulting interpretation of Descartes sees him as much closer in his conception of natural laws to Nancy Cartwright than to David Lewis. One of the real benefits of this interpretation, claims Brown, is that it ‘might just help to demystify Descartes’s references to active forces’. Having thus establishing that Descartes was a realist about tendencies who nevertheless remained committed to a non-teleological, mechanistic account of nature, Brown contrasts this Cartesian picture with Hobbes’s reductive mechanics that eliminated forces and tendencies by equating them with actual motions, including even the ‘force of a body at rest’.

Aristotelian Powers, Mechanism, and Final Causes in the Late Middle Ages

Henrik Lagerlund explores the topic of final causality in the High and later Middle Ages. He argues that the seventeenth-century mechanists weren’t the only ones critiquing and rejecting final causality. There were earlier figures who developed a form of mechanical materialism that eschewed final causes, most notably William of Ockham and John Buridan. Lagerlund begins with the way that Ockham and Buridan in the fourteenth century understood the mereology of the body. Bodily substances were composed of essential parts and integral parts. Essential parts were its metaphysical constituents, its matter and substantial form. Integral parts were its various extended bits. This distinction generated a metaphysical divide between material objects with extended substantial forms and simple, immaterial substances like God, angels, and the human soul. And this divide raises a number of philosophical puzzles for the entities on either side of it. Of special concern to Lagerlund is the numeric identity and unity of material substances across time. Lagerlund shows how Buridan in particular struggled to make sense of the identity and unity of material substances through time. In the end, Buridan could only say that material substances are successively identical through time; they are not totally or partially identical.