Introduction
Benjamin Hill, Henrik Lagerlund, and Stathis Psillos
The present volume brings together twelve scholars from different philosophical disciplines (metaphysics, philosophy of science, and history of philosophy) with the aim to discuss the ontic status of causal powers, blending together historical and conceptual perspectives. Most of the authors in this volume had the privilege of discussing early drafts of our papers among ourselves and with students during a Summer Institute organized by the Rotman Institute of Philosophy—Engaging Science, of the University of Western Ontario and partly funded by an SSHRC grant. The event lasted for two weeks in July 2014, and took place in London, Ontario, and on the shores of the Lake Huron.
The motivating idea behind this detailed introduction and the twelve specially written papers is that understanding the resurgence of powers in current metaphysics and philosophy of science, as well as their role in the current scientific image of the world, is best achieved by mapping the trajectory (both conceptually and historically) of the movement away from powers and the dominant Aristotelian conception of nature in the seventeenth century and towards powers at the end of the twentieth century. This mapping will cast light on the key arguments against powers in the seventeenth century, will unravel their philosophical and scientific presuppositions, and will examine their relevance (or lack thereof) for the current debate about powers. At the same time, this mapping will locate the current trend to revitalize powers within its proper historical and conceptual context and attempt a comparison of the current conceptions of powers with the ones offered by Aristotle and the medieval Aristotelians and criticized by the novatores. Along the way, the attempted mapping will offer reappraisals of dominant views about, or dominant interpretations of, various important philosophers, such as Descartes, Hobbes, Malebranche, Locke, and Hume.
When all is said and done, it might appear that the elimination or reconceptualization of powers that took place in the seventeenth century has been a long(ish) interval in an otherwise continuous appeal to sui generis causal
Benjamin Hill, Henrik Lagerlund, and Stathis Psillos, Introduction In: Reconsidering Causal Powers: Historical and Conceptual Perspectives. Edited by: Benjamin Hill, Henrik Lagerlund, and Stathis Psillos, Oxford University Press (2021). © Benjamin Hill, Henrik Lagerlund, and Stathis Psillos. DOI: 10.1093/oso/9780198869528.003.0001
powers 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.
In the rest of this long introduction and before a summary of the individual papers, there is an attempt (not wholly impartial) to sketch the key conceptions of, and arguments for and against, powers from Aristotle up to the present. This narrative is, of essence, selective and incomplete, but it hopefully offers the basic contours of the movement away from powers and back to them, and the backdrop against which the essays are set. The papers offer more nuanced accounts as well as alternative interpretations of parts of this narrative.
1. The History of the Powers Debate
1.1. Aristotelian Powers
Once upon a time powers ruled the world. In his Sophist, Plato had the Eleatic Stranger tie real existence with the power to produce change or to undertake change. Here is the characteristic quotation: ‘everything which possesses any power of any kind, either to produce a change in anything or to be affected even in the least degree by the slightest cause, though it be only on one occasion, has real existence’ (Plato 1997, 247d–e, 269; translation amended). To be, then, is to be powerful. Later, Aristotle (A 1046a10–11, 2:1652) developed the first systematic theory of power. A power is ‘a principle of change in something else or in itself qua something else’. Principles are causes; hence powers are causes. Powers are posited for explanatory reasons—they are meant to explain activity in nature: change and motion. Action requires agency. For X to act on Y, X must have the (active) power to bring a change to Y, and Y must have the (passive) power to be changed (in the appropriate way) by X. Powers have modal force: they ground facts about necessity and possibility. Powers necessitate their effects: when a (natural) power acts (at some time and in the required way) and if there is ‘contact’ with the relative passive power, the effect necessarily (that is, inevitably) follows. Here is Aristotle’s (324b8, 1:530) example: ‘and that that which can be hot must be made hot, provided the heating agent is there, i.e. comes near.’
At the same time, what is possible for X to do is exactly what it has the power to do. Power (qua attribute of substances) belongs to the category of relative (pros ti). This is because ‘their very essence includes in its nature a reference to something else’ (1021a27–8, 2:1612). Positing (both active and
passive) powers implies that they ‘point to’ (are directed towards) something outside themselves, viz. something that will suffer change when they act (having the suitable passive power) or something that will cause change in them (having the suitable active power).
Aristotelian powers are real, irreducible, inherent in substances, causal, and relative. Moreover, powers are possessed by their objects even when they are not acting. In fact, that Χ has the power to Φ and yet X is not Φing captures what it is for X to have the power to Φ. Hence, what it is for X to possess the power to Φ is not the same as X actually Φing (acting). X’s having the power to Φ grounds and explains X’s Φ–ing.
This theoretical account of powers was retained, more or less intact, throughout the Middle Ages and became the major explanatory framework for all kinds of accounts of the phenomena. Powers were taken to be natural tendencies (inclinations) that objects possess in virtue of their (Godgiven) nature. Hence, objects possess specific natural tendencies and lack others. These natural inclinations act of necessity (so long as the power is unimpaired) and act uniformly. Importantly, powers are directed to their acts (Aquinas also highlights this, see ST 1.77.3, 1:385–6.). Hence, powers ‘point’ to something outside themselves. Qua relatives, they have a being toward something else. They have an ‘esse ad.’ This towardness of powers reinforces the teleological element in nature that characterized Aristotelianism. Take gravity, viz. the downward movement of (heavy) bodies. To this, Aquinas (1a.36.1, 1:748) assigned a ‘twofold cause’. The final cause of a heavy body’s downward movement is the lower place: the body is seeking the lower place. The efficient cause (‘principle of movement’) is a natural inclination resulting from gravity.
The locus of powers of an object was its substantial form. Forms are active: the active principle of a thing is a form. As Aquinas put it, ‘Heat is a disposition to the form of fire’ (1a.74.4, 1:921). The form was the very nature of simple things or a constituent of the nature of a thing, which was composed of matter and form. Given the diversity of forms, there is no general and unified explanation of natural phenomena: each particular causal explanation is distinct from any other in that it appeals to an individual sui generis form.
The Middle Ages brought with it an important and ineliminable agent: God. He fixes the natural order by giving all creatures their natures, that is, their causal powers. This creates a network of relations of necessitation among distinct existences and hence a network of natural necessities. These could be violated by God (for example, God could make fire not burn the three boys in the fiery furnace), but if nature is left to run its own course, natural necessity
rules. As John Buridan (quoted in King 2001, 16) put it, ‘There is another necessity which is called “natural”, which is not necessity simpliciter, but which would be necessity with all supernatural cases put to one side.’ For Aquinas, natural necessity has an ineliminable teleological element. It is an impression from God—God directs things to their ends. God fixes the nature of things and they tend towards their ends. Natural necessity is evidence for divine providence.
There are two ways in which something may happen outside the natural order. The first is when there is action against the natural powers or tendencies of things—as when an agent throws a heavy thing upwards against its natural inclination to move downwards. This action is against nature. The second is when there is action against the natural inclinations of a thing but is caused by the agent on whom these natural inclinations depend—as when in an ebb of tide there is movement of the water against its natural inclination to go downwards. Aquinas (ST 1.105.6, 1:519–20), who uses this example, notes that this kind of action, dependent as it is on the influence of a heavenly body, is not against nature. God then acts on nature in the second way. Hence, if God does something outside the natural order (hence against the natural inclinations of things), this is not against nature.
1.2. The Revolt against Powers
This, in broad outline, is the power ontology against which the novatores of the seventeenth century revolted—a revolt that had partly started already in the fourteenth century. With the mechanical account of nature and its two catholic principles—matter and motion—in place, the critique of the medievalAristotelian account of powers acquires centre stage. The novatores put forward a different conception or explanation of natural phenomena in terms of the microscopic (and hence unobservable) parts of matter and their interactions (collisions) subject to universal laws. Matter is divested of its sui generis causal powers. Descartes, in a now famous passage of his posthumously published Le monde, states boldly:
Someone else may if he wishes imagine the ‘form’ of fire, the ‘quality’ of heat, and the ‘action’ of burning to be very different things in the wood. For my own part, I am afraid of going astray if I suppose there to be in the wood anything more than what I see must necessarily be there, so I am satisfied to confine myself to conceiving the motion of its parts. For you may posit ‘fire’
and ‘heat’ in the wood, and make it burn as much as you please: but if you do not suppose in addition that some of its parts move about and detach themselves from their neighbours, I cannot imagine it undergoing any alteration or change. (AT 11:7; CSM 1:83)
Fire burns wood not because it has some sui generis burning power based on its substantial form but because its minute parts move rapidly and constantly and collide with the minute parts of wood. Motion is the principle of change and not substantial forms, which are taken to be ‘occult qualities’. Indeed, Descartes wrote to Regius in January 1642, ‘If they say that some action proceeds from a substantial form, it is as if they said that it proceeds from something they do not understand; which explains nothing’ (AT 3:506; CSMK 208–9).
But the most puzzling feature of powers was their towardness. Descartes put forward the wellknown ‘little souls’ argument. Only minds can have content which is about something else. Hence to attribute powers to things in virtue of which they behave is like attributing little minds to them. In a letter to Mersenne dated 26 April 1643, he noted: ‘I do not suppose there are in nature any real qualities, which are attached to substances, like so many little souls to their bodies, and which are separable from them by divine power’ (AT 3:648; CSMK 216). Gravity is a case in point. His medieval predecessors, Descartes thought, took gravity to be a real quality ‘of which all we know is that it has the power to move the body that possesses it towards the centre of the earth’ (AT 3:667; CSMK 219), thereby wrongly attributing to it soullike attributes.
If powers can no longer play the role of necessitating principles, what plays this role? The laws of nature. Descartes, in the Principles of Philosophy, introduces the idea that matter and its motion are governed by universal and fewinnumber laws of nature, which follow directly from God’s immutability and constitute the ‘the secondary and particular causes of the diverse movements which we notice in individual bodies’ (AT 8a:62; CSM 1:240–1). The idea that laws can be causes of motion is so alien to the current mind that it does not seem to make sense. And yet it does (proving that Descartes’s conception of laws of nature is so different from ours). The (rough) idea is this: if bits of matter are inert and powerless something must causally determine their behaviour when they move on their own and when they collide with others. Descartes’s radical claim is that God does this (qua the primary and universal cause of all motions) via laws. Laws then are God’s tools (instruments) for the redistribution of quantities of motion to bits and pieces of matter, while, due to God’s immutability, the total amount of quantity of motion in the universe, set by God during the Creation, is conserved.
In the Cartesian occasionalism, God—via simple and comprehensive laws, which He chooses—is the sole cause of all motion. Father Malebranche argued that Aristotle’s idea of nature, that is the idea that things have natures in virtue of which they behave—of necessity—the way they do is a ‘pure chimera’. We have no conception of power of secondary things; no object can move itself, let alone another thing. Occasionalism emerges as the view that bodies lack motor force and God acts on nature via general laws. Not all novatores wanted to eliminate powers. But most took it that power has a place in the new mechanical theory only if it is suitably connected with the primary qualities of matter and their motion. In his famous lock and key case, Robert Boyle takes it that both the key and the lock, qua pieces of metal with certain sizes and shapes, acquire a new capacity, when by means of motion they are ‘applied to one another after a certain manner’ (B 23): the key unlocks the lock. But this capacity is new not in the sense that any new real or physical entity was added to the key or the lock. It’s new in the sense that it does not supervene on either the key or the lock, but it supervenes on both and their relations. Boyle is adamant that, though his scholastic predecessors would look at this capacity as ‘a peculiar faculty and power in the key that it was fitted to open and shut the lock’ (23), nothing of the kind is the case. Nothing real (a new real quality) was added to each object taken separately. Nothing ‘distinct from the figure it had before those keys were made’ (23). But, of course, ‘nothing new’ does not imply nothing. Due to being part of a relational structure, the body—qua matter—acquires a modification (and not a res) which enables it ‘to produce various effects’, which it would not have been able to have had it not been part of this relational structure. Given these new effects, we ‘make bodies to be endowed with qualities’, ‘upon whose account’ the effects follow. But these qualities are not ‘any real or distinct entities, or differing from the matter itself furnished with such a determinate bigness, shape, or other mechanical modifications’ (24).
John Locke took it that objects have powers (active and passive). He (Essay II.xxi.1, 233) says: ‘Fire has a power to melt Gold, i.e. to destroy the consistency of its insensible parts, and consequently its hardness, and make it fluid; and Gold has a power to be melted.’ But these powers are not like scholastic faculties. Why is meat digested in stomachs? To say, like the scholastic philosophers did, that the stomach has a digestive faculty is trivial and nonexplanatory. It amounts to saying, ‘Digestion is performed by something that is able to digest’ (II.xxi.20, 244). They are powers suitable for mechanical philosophy: they depend on the microconstitution of objects. Like Boyle, Locke takes it that ‘Power includes in it some kind of relation’ (II.xxi.3, 234).
This is analysed in Draft B of the Essay, where Locke talks about the purgative power of rhubarb. This is grounded in the microstructure of rhubarb. It is relative nonetheless because whether or not this microstructure has a purging power depends on whether there are forms of life with the right kind of digestive system. For Locke (1990, 262) ‘the purgeing power in Rhubarb is relative for rhubarb would still be the same were there noe animal in the world capable of being purged.’ The ascription of power then depends on the relational structure in which rhubarb is embedded.
Significantly, Leibniz took it that extension is not enough for matter and that some notion of inherent power should be added. Based on a general metaphysical principle that ‘there is neither more nor less power in an effect than there is in its cause’ (Leibniz 1989, 125), he argued that a ‘formal principle’ must be added to ‘material mass’ as an irreducibly dynamical notion, which accounts for the power there is in matter. This notion of ‘power’ in bodies, Leibniz explains, is twofold. It is passive force when it is thought of as ‘matter or mass’ and active force when it is thought of as constituting ‘entelechy or form.’ Matter is dynamical precisely because it is characterized by impenetrability and resistance to motion (inertia). But there is active power too. Leibniz takes this to be distinct from the scholastic potentia. He takes the scholastic notion of power to be passive in the sense that it amounts to ‘receptivity to action’. Whereas Leibniz’s power is active already in the sense that it involves an actual conatus or tendency toward action such that ‘unless something else impedes it, action results’ (252). The notion of active power plays a role both in metaphysics and in physics, with the former role being the primary and the latter being the derivative. The primary sense of active power is metaphysical, since it is what makes a substance what it is—its form. This form is dynamical but at the same time mental; it is ‘either a soul or a form analogous to a soul’ (Leibniz 1989, 162). But if the primary sense of active power is what makes a substance what it is, its relevance to physics is nowhere to be seen unless it somehow gets connected with natural forces. Hence Leibniz talks about a derivative power, known as ‘impetus, conatus, or a striving [tendentia] . . . toward some determinate motion’ (Leibniz 1989, 253).
Opposing occasionalism, Leibniz argued that powers are ‘in bodies’ and they cause (or causally contribute) to the motions of bodies. Though for Leibniz the motion of matter is lawgoverned, where the laws are instituted by God on the basis of maximizing simplicity and perfection (comprehensiveness), bodies have innate forces by virtue of which they act according to laws. Perhaps surprisingly, in his unpublished text De gravitatione, Isaac Newton took a rather hard line on powers. Criticizing Descartes’s account of matter as
extension, Newton (2004, 21) argued that, though it is not customary to ‘define substance as an entity that can act upon things, yet everyone tacitly understands this of substances’. He then went on to define a body as something with three kinds of power: impenetrability, mobility, and capacity to affect our senses. To be sure, these are conditions imposed by God on determined quantities of extension. But they are powers nonetheless. Newton doesn’t say much about their ontic status. But he does say two important things. First, they are not like the scholastic substantial form, and second, they are subject to laws. In fact, the key power of bodies to move amounts to the transference of impenetrability from one part of space to another ‘according to certain laws’ (28). Besides, ‘we do not move our bodies by a proper and independent power but by laws imposed on us by God’ (30).
This idea of lawconstituted powers Newton did not develop further in Principia except in a crucial respect when it comes to gravity. In his famous ‘General Scholium’ from Principia, Newton declared that he feigns no hypotheses for the cause of gravity. He added rather boldly that hypotheses have no role to play in experimental philosophy. And he stressed:
In this experimental philosophy, propositions are deduced from the phenomena and are made general by induction. The impenetrability, mobility, and impetus of bodies, and the laws of motion and the law of gravity have been found by this method. And it is enough that gravity really exists and acts according to the laws that we have set forth and is sufficient to explain all the motions of the heavenly bodies and of our sea. (1999, 589)
What’s important for our purposes is Newton’s claim that gravity exists and acts according to laws. The inverse square law constitutes gravity, at least when it comes to its causalexplanatory role. That there may be an unknown causal basis for this power is irrelevant. In an unsent letter written circa May 1712 to the editor of the Memoirs of Literature, Newton stressed that it is not necessary for the introduction of a power—such as gravity—to specify anything other than the law it obeys; no extra requirements should be imposed, and in particular no requirement for a mechanical grounding. He wrote:
And therefore if any man should say that bodies attract one another by a power whose cause is unknown to us, or by a power seated in the frame of nature by the will of God, or by a power seated in a substance in which bodies move and float without resistance and which has therefore no vis inertiae but acts by other laws than those that are mechanical: I know not why he
should be said to introduce miracles and occult qualities and fictions into the world. (2004, 116)
In 1713, in the preface to the second edition of Principia, Roger Cotes noted emphatically that gravity is a ‘primary affection of bodies’ and that, like other primary affections such as extension, mobility, and impenetrability, is one of the primary qualities of all bodies universally (Newton 1999, 391). Indeed, Newton employed the famous third rule of philosophy, which is a rule of induction, in order to argue that ‘all bodies gravitate toward one another’. And yet he added that he is not affirming that ‘gravity is essential to bodies’ (796).
In a 1693 letter to Richard Bentley, Newton (2004, 102) had stressed that it is ‘inconceivable that inanimate brute matter should, without the mediation of something else which is not material, operate upon and affect other matter without mutual contact, as it must be, if gravitation in the sense of Epicurus, be essential and inherent in it’. It is an absurdity, he later added in print, that ‘gravity should be innate, inherent, and essential to matter, so that one body may act upon another at a distance through a vacuum, without the mediation of anything else, by and through which their action and force may be conveyed from one to another’ (1999, 796). The only inherent force is ‘the force of inertia’. Newton took it that, if gravity were essential to bodies, then there would be action at a distance. If action at a distance is to be avoided, gravity should not be taken to be essential to bodies, qua material substances.
But this does not imply that gravity is not a universal quality of bodies. Gravity depends on the mass of bodies, which is an essential and universal quality of bodies. And Newton has shown how exactly gravity depends on mass in the law of universal attraction. In the ‘General Scholium’ Newton sums up his findings about gravity thus:
Thus far I have explained the phenomena of the heavens and of our sea by the force of gravity, but I have not yet assigned a cause to gravity. Indeed, this force arises from some cause that penetrates as far as the centers of the sun and planets without any diminution of its power to act, and that acts not in proportion to the quantity of the surfaces of the particles on which it acts (as mechanical causes are wont to do) but in proportion to the quantity of solid matter, and whose action is extended everywhere to immense distances, always decreasing as the squares of the distances. (1999, 943)
If the action is extended everywhere to immense distances and it obeys a law, gravity is hardly a nonuniversal force. In fact, gravity exists insofar as it acts
according to laws. This is the lawconstitutive approach to powers introduced by Newton.
1.3. Hume’s Critique of Powers
By the time Hume’s Treatise appeared, sui generis causal powers were far from abandoned. This made Hume offer a systematic critique of the ontic and epistemic status of powers. Echoing Malebranche, Hume put forward a disagreement argument: there is a diversity of philosophical views about how powers play their role and ‘none of them has any solidity or evidence’. The whole panoply of substantial forms, accidents, and faculties is perfectly unintelligible and inexplicable (T 158). Then, in line with his overall empiricist approach which implies that legitimately held ideas require prior impressions, he advanced a verificationist argument: there is no impression of efficacy or power; ‘we never have an impression that contains any power or efficacy’ (160). Hume argued that ‘’tis impossible we can have any idea of power or efficacy, unless some instances can be produc’d, wherein this power is perceiv’d to exert itself.’ Hence, there cannot be unmanifested powers, that is, powers which exist, even though there are no impressions of their manifestations. In fact, the very ascription of a power to a thing requires that this power has been exercised. Hence, ‘The distinction, which we often make betwixt power and the exercise of it, is equally without foundation’ (171).
To all this he added what might be called the ‘unnecessary duplication argument’: power or efficacy is not to be confused with an unknown quality of the objects—it is taken to be signify something that grounds the action and causation of bodies. But the constant conjunction of ordinary qualities of the bodies is enough to account for action and causation as well as for the idea of efficiency (171).
Finally, Hume advanced the key argument that powers cannot be necessitymakers. Powers were supposed to ground the necessary connection between cause and effect. But what kind of necessity is this? Hume takes it that if there were such a necessity, it would be such that the effect would be entailed by the cause—and the entailment would amount to a demonstration of the effect on the basis of the cause—hence the impossibility of the cause without the effect. But it is conceivable that the cause is unaccompanied by the effect and conversely. Hence, there is no metaphysical necessity in causation.
Could it be some weaker kind of necessity—natural necessity—which characterizes the powerbased connection among distinct existences? In line
with the standard distinction between the absolute and the ordained power of God, there are philosophers who attribute efficacy to secondary causes (natural bodies) and hence a ‘derivative, but a real power and energy to matter’ (T 161). It could then be argued that, though it is metaphysically possible that though A has the power to produce B, A exists without B, it is naturally necessary. Hume’s farreaching point was precisely that this appeal to natural necessity can be questioned.
For Hume this kind of issue takes the following form: can there be knowledge of Principle of Uniformity of Nature (PUN) based on experience, and in particular on probable reasoning? His famous answer is that this kind of move would be questionbegging. Probable reasoning begins with experience and memory (89–90) and should provide a connection between something given in experience and something not given in experience. Hence, probable reasoning is causal inference: the only ‘just inference from one object to another’. But then here is the problem: probable inference of the sort required, being causal inference, is founded on the ‘presumption of a resemblance betwixt those objects, of which we have had experience, and those, of which we have had none’. Hence, this presumption (PUN) cannot come from probable reasoning. The very same principle cannot be the ground for an inference and the product of this inference. Or, as Hume put it, ‘The same principle cannot be both the cause and effect of another; and this is, perhaps, the only proposition concerning that relation, which is either intuitively or demonstratively certain’ (90).
Could it be that powers are posited to ground causal inference? This might well be a legitimate metaphysical inflation of the world by means of powers. Here is the relevant argument, as summed up by Hume: ‘The past production implies a power: The power implies a new production: And the new production is what we infer from the power and the past production’ (90). Hence, the argument for powers might be the following:
1. A has been constantly conjoined with B.
2. A has the power to produce B.
3. A necessarily produces B.
4. Hence All As are B.
It’s no surprise that Hume argues that this argument begs the question, since it relies on a powerbased PUN. The move from 2 to 3 and then to 4 requires that ‘the same power continues united with the same object, and that like objects are endow’d with like powers’ (91).
Suppose that powers were allowed in the sense that the production of an object B by object A implies a power in A. It is still the case that the past exercisings of A’s power to produce B are not enough to prove that A necessarily has the power to cause B, that is A causes B simpliciter. The reason is that past experience ‘can only prove, that that very object, which produc’d any other, was at that very instant endow’d with such a power; but can never prove, that the same power must continue in the same object or collection of sensible qualities; much less, that a like power is always conjoin’d with like sensible qualities.’ So, unless a variant of PUN is assumed, the conclusion that A causes B does not follow. But this PUNlike principle raises the further question of its own provenance, and the standard Humean objection is repeated: ‘why from this experience we form any conclusion beyond those past instances, of which we have had experience’? This would lead to regress ‘which clearly proves, that the fore going reasoning had no just foundation’ (91). Hume’s point, then, is that powers cannot be necessitymakers either on metaphysical or epistemic grounds.
1.4. From Powers to Laws
Hume’s critique of powers did not persuade Reid to stop positing powers. He spoke freely of active powers and took it that:
(a) the very concept of power is simple and undefinable;
(b) power is not something we either perceive via the senses or we are aware of in our consciousness (we are conscious only of the operation of power and not of the power itself);
(c) power is something whose existence we infer by means of reason based on its operation/manifestation;
(d) power is distinct from its exertion in that there may be unexerted powers;
(e) the idea we have of power is relative, viz. as the conception of something that produces or brings about certain effects;
(f) power always requires a subject to which it belongs: it is always the power of something; the power that something has; and
(g) causation is the production of change by the exercise of power.
Exercising a power requires agency and since there is no agency in nature, strictly speaking, there is no causation in nature. Indeed, Reid (2011, I.6, 41–7)
insisted that, properly understood, active powers require subjects that have intelligence and will to exercise them. Inanimate matter then can be no such subject. Only God—who is an ‘offstage agent’—can be the cause possibly by means of secondary causes.
But Reid wrote after Newton and thought that the universe was lawgoverned. Yet, subsumption under laws of nature does not constitute causation; nor does it amount to causal explanation. For him, ‘the laws of nature are the rules according to which the effects are produced; but there must be a cause which operates according to these rules. The rules of navigation never steered a ship. The rules of architecture never built a house’ (I.6, 47) Hence, a cause is something that has the power to bring about an effect in accordance with the law; but knowing the laws does not amount to knowing the causes. From all this, he drew the rather pessimistic conclusion that, in spite of the fact that scientists have discovered a number of laws of nature, ‘they have never discovered the efficient cause of any one phenomenon’ (I.6, 47). Which, for Reid, is just as well since those scientists who understand what science is about and what the laws of nature are do not claim that science discovers (or aims to discover) causes. For Reid causation is tied to agency and laws of nature are not agents. As he (IV.3, 288) put it, laws of nature ‘are not endowed with active power, and therefore cannot be causes in the proper sense. They are only the rules according to which the unknown cause acts.’
John Stuart Mill’s critique of sui generis powers (or qualities, or virtues) consolidated the move from powers to laws of nature. Besides, Mill was among the first to secularize the conception of natural laws. Mill’s key argument against powers is that they are the products of a double vision. Echoing Hume, but not referring to him directly, Mill argued that there is nothing more to the power of an object to produce Φ than actually producing Φ. So, when, for instance, snow is endowed with whiteness, being the power to produce in us a certain sensation of white, what is really asserted is that ‘the presence of snow produces in us that sensation’. Positing the power (‘something in virtue of which the object produces the sensation’) is redundant. As Mill noted, this hypostatization of powers (or qualities) ‘Moliere so happily ridiculed when he made one of his pedantic physicians account for the fact that opium produces sleep by the maxim, Because it has a soporific virtue’ (Mill 1911, 58). According to Mill, stating the ‘soporific virtue’ ‘merely asserted over again, the fact that it produces sleep’. More generally, when it is asserted that object A has the power P to produce Φ, it is asserted that A produces Φ.
With this ‘reductive’ understanding of powers in mind, Mill had no problem with talking about powers, as required by common opinion and common
