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At the Catholic blog Vox Nova, mathematics professor David Cruz-Uribe writes:
I… am currently working through the metaphysics of St. Thomas Aquinas as part of his proofs of the existence of God… [S]ome possibly naive counter-examples from quantum mechanics come to mind. For instance, discussing the principle that nothing can change without being affected externally, I immediately thought of the spontaneous decay of atoms and even of particles (e.g., so-called proton decay).
This might be a very naive question: my knowledge of quantum mechanics is rusty and probably out of date, and I know much, much less about scholastic metaphysics. So can any of our readers point me to some useful references on this specific topic?
I’ve discussed this issue before, and one of Cruz-Uribe’s readers directs him to a blog post of mine in which I responded to a version of this sort of objection raised by physicist Robert Oerter. Unfortunately, the combox discussion that ensues largely consists of a couple of Cruz-Uribe’s readers competing with each other to see who can emit the most squid ink (though Brandon Watson manfully tries to shine some light into the darkness). One reader starts things out by writing:
Feser’s… argument seems to boil down to saying, “Just because we can’t find a cause for quantum phenomena doesn’t mean there isn’t one.” … Thing is, Bell has shown that you can’t have local unknown variables in quantum events. Bohm’s interpretation would give you the possibility of unknown variables (thus taking out the random, seemingly acausal, aspect), but at the price of locality (in short, such variables would be global, and not tied to a specific location; so you lose any predictability, anyway).
As readers of the post on Oerter know, this essentially just repeats the completely point-missing objection from Oerter that was the subject of the post, while ignoring what I said in the post in reply to the objection! The combox discussion goes downhill from there, with so many points missed, questions begged and crucial distinctions blurred that you’d think you were reading Jerry Coyne’s blog.
Cruz-Uribe’s reader accuses me of having a “weak” understanding of the relevant physics, which is why he launches into the mini lesson on Bell and Bohm. But it’s his reading skills that are weak, since I made it clear in the post that I wasn’t in the first place making any claim about the physics of systems of the sort in question, and thus wasn’t saying anything that could be incompatible with what we know from physics. In particular, I wasn’t advocating a “hidden variable theory” or the like, but rather making a purely philosophical point about causality that is entirely independent of such theories.
This is one of many factors that hinder fruitful discussion of these topics even with well-meaning people (like Cruz-Uribe) who know some science but know little philosophy. They constantly translate philosophical claims into the physics terms that they feel more comfortable and familiar with, and proceed to run off at high speed in the wrong direction.
This is why you really can’t address specific issues like radioactive decay without first doing some general philosophical stage-setting. For it’s never really the empirical or scientific details that are doing the work in objections to Scholastic metaphysics like the one at issue. What’s really doing the work is the ton of philosophical baggage that the critics unreflectively bring to bear on the subject -- the assumptions they read into the physics and then read back out again, thinking they’ve raised a “scientific” objection when what they’ve really done is raised a question-begging philosophical objection disguised as a scientific objection.
(I imagine that educated religious people like Cruz-Uribe and his readers aren’t fooled by this kind of sleight of hand in other scientific contexts. For instance, I’d wager that they would be unimpressed by arguments to the effect that neuroscience has shown that free will is an illusion. As I have argued hereand here, neuroscience has shown no such thing, and such claims invariably rest not on science but on tendentious philosophical assumptions that have been read into the scientific findings. But exactlythe same thing is true of claims to the effect that quantum mechanics has falsified the principle of causality, or that Newton or Einstein refuted the Aristotelian analysis of change.)
In what follows, then, I will first prepare the ground by calling attention to some common fallacies committed by critics of Scholastic metaphysics who appeal to modern physics -- fallacies some of which are committed by Cruz-Uribe’s readers in the course of their combox discussion. Anyone wanting to comment intelligently on the subject at hand has to take care to avoid these fallacies. Second, I will make some general remarks about what a philosophical approach to the subject at hand involves, as opposed to the approach taken by physics. (I’ve discussed this issue many times before, and indeed did so in a couple of posts -- hereand here-- that followed up the post on Oerter that Cruz-Uribe and his readers were discussing.) Finally, in light of this background I’ll address the specific issue of radioactive decay and causality.
Fads and fallacies in the name of science
So, let’s consider some of the confusions that are rife in discussions of the relationship between physics on the one hand and philosophy (and in particular Scholastic philosophy) on the other:
A. Conflating empirical and metaphysical issues: Those who know some science but not a lot of philosophy very often assume that when a Scholastic philosopher says something about the nature of causality, or substance, or matter, or the like, then he is making a claim that stands or falls with what physics tells us, or at any rate should stand or fall with what physics tells us. But this is a category mistake. Scholastic metaphysics is not in competition with physics, but approaches the phenomena at a different (and indeed deeper) level of analysis. Its claims do not stand or fall with the findings of physics, any more than the claims of arithmetic stand or fall with the findings of physics. Indeed, like arithmetic, the basic theses of Scholastic metaphysics are (so the Scholastic argues) something any possible physics must presuppose.
Sometimes the critics assume that Scholastic metaphysics is in competition with physics because they are themselves making question-begging metaphysical assumptions. For instance, they might assume that any rationally justifiable claim about the nature of matter simply must be susceptible of formulation in the mathematical language of physics, or must be susceptible of empirical falsification. They are essentially making a metaphysics out of physics. Only physics can tell us anything about the nature of physical reality (so the critic supposes), so any claim about the nature of physical reality is implicitly, even if not explicitly, a claim of physics. As we will see below, this cannot possibly be right. Physics cannot even in principle tell us everythingthere is to know about physical reality (let alone reality more generally). But even if the assumption in question could be right, it simply begs the question against the Scholastic merely to assert it, since the Scholastic rejects this assumption, and on the basis of arguments that need to be answered rather than ignored (arguments I’ll discuss below).
Sometimes the conflation of empirical and metaphysical issues is due less to such large-scale philosophical assumptions than to a simple fallacy of equivocation. Both physicists and Scholastic metaphysicians use terms like “cause,” “matter,” and the like. A superficial reading therefore often leads critics to assume that they are addressing the same issues, when in fact they are very often not using the key terms in the same sense.
Sometimes the conflation is due to sheer intellectual sloppiness. Critics will formulate the issues in ridiculously sweeping terms, making peremptory claims to the effect that “Aristotelianism was refuted by modern science,” for example. In fact, of course, the labels “Aristotelianism” and “modern science” each cover a large number of distinct and logically independent ideas and arguments, and these need carefully to be disentangled before the question of the relationship between Scholastic metaphysics and modern physics can fruitfully be addressed. It is no good to say (for example) that since Aristotle’s geocentrism and theory of natural place have been falsified, “therefore” we should not take seriously his theory of act and potency or the account of causality that rests on it. This is simply a non sequitur. Such issues are completely independent of one another, logically speaking (regardless of the contingent historicalassociation between them).
B. Conflating genus and species: Even when physicists and Scholastic metaphysicians are using terms in the same sense, critics often confuse what is really only a specific instance of the general class named by a term with the general class itself. For example, where the notion of “cause” is concerned, Scholastic metaphysicians distinguish between formal, material, efficient, and final causes. Where efficient causes are concerned, they distinguish between principal and instrumental causes, between series of causes which are essentially ordered and those which are accidentally ordered, and between those which operate simultaneously versus those which are ordered in time. They distinguish between total causes and partial causes, and between proximate and remote causes. They regard causality as primarily a feature of substances and only secondarily as a relation between events. They distinguish between causal powers and the operation of those powers, between active causal power and passive potencies. And so forth. All of these distinctions are backed by arguments, and the Scholastic maintains that they are all necessary in order to capture the complexity of causal relations as they exist in the actual world.
Now, those who criticize Scholastic metaphysics on scientific grounds typically operate with a very narrow understanding of causality. In particular, they often conceive of it as a deterministic relation holding between temporally separated events. They will then argue (for example) that quantum mechanics has undermined causality thus understood, and conclude that it has therefore undermined causality full stop. One problem with this, of course, is that whether quantum mechanics really is incompatible with determinism is a matter of controversy, though as I have said, nothing in the Scholastic position stands or falls with the defensibility of Bohmian hidden variable theories. The deeper point is that it is simply fallacious to suppose that to undermine one kind of causality (and in one kind of context) is to undermine causality as such. Certainly it begs the question against the Scholastic, who denies that all causality reduces to deterministic relations holding between temporally separated events.
The conflation of a general class with a specific kind within the general class is evident too in discussions of motion. Scholastics and other Aristotelians think of motion in general as change, and change as the actualization of potency. Local motion or change with respect to place or location is just one kind of actualization of a potency, and is metaphysically less fundamental than other kinds. When motion is discussed in modern physics, however, it is of course local motion that is exclusively in view.
There is nothing necessarily wrong with this focus, but it would be fallacious to draw, from what modern physics says about “motion” (in the sense of local motion), sweeping conclusions about what Aristotelians say about “motion” (in the sense of the actualization of potency). This would be to confuse what is true of one kind of change for what is true of change as such. Yet this kind of fallacious conflation is very common. Of course, a critic of Scholastic metaphysics might claim that local motion is the only kind of change there really is, but merely to assertthis is simply to beg the question against the Scholastic, who has arguments for the claim that local motion cannotbe the only kind of change there is. (I have addressed this particular issue in detail elsewhere, e.g. hereand here.)
C. Confusing general principles with specific applications of those principles: When a thinker, whether a philosopher or a scientist, puts forward a general principle, he sometimes illustrates it with examples that later turn out to be deficient. But it simply doesn’t follow that the general principle itself is mistaken. For example, people often think of the evolution of the horse as a neat transition from very small animals to ever larger ones, as in the kind of exhibit they might have seen in a natural history museum as a child. It turns out that things aren’t quite so neat. There is no hard and fast correlation between the size of a horse and where it appears in the fossil record. It doesn’t follow, however, that modern horses did not evolve from much smaller animals. That earlier accounts of the evolution of the horse turn out to be mistaken does not entail that the general principle that horses evolved is mistaken. (ID enthusiasts are kindly asked to spare us any frantic comments about evolution. This is not a post about that subject. It’s just an example.)
However, though philosophical naturalists never tire of making this point when Darwinism is in question, they suddenly forget it when Aristotelianism or Scholasticism is what is at issue. For example, Aristotelians defend the reality of final causality -- the idea that natural substances and processes are inherently “directed towards” certain characteristic effects or ranges of effects. In previous centuries, the idea was often illustrated in terms of Aristotle’s view that heavy objects are naturally directed toward the center of the earth as their “natural place.” That turns out to be mistaken. This is often treated as a reason for rejecting the idea of final causality as such, but this simply doesn’t follow. In general, the deficiencies of this or that illustration of some Scholastic metaphysical thesis are simply not grounds for rejecting the thesis itself. (I’ve addressed this issue at greater length before, e.g. here, here, and here.)
The limits of physics
So that’s one set of background considerations that must be kept in mind when addressing topics like the one at issue: the begged questions, blurred distinctions, and missed points which chronically afflict the thinking of those who raise purportedly scientific objections to Scholastic metaphysics. Let’s move on now to the second set of background considerations, viz. the limits in principle to what physics can tell us about physical reality, and the unavoidability of a deeper metaphysical perspective.
As I have emphasized many times, what physics gives us is a description of the mathematical structure of physical reality. It abstracts from any aspect of reality which cannot be captured via its exclusively quantitative methods. One reason that this is crucial to keep in mind is that from the fact that something doesn’t show up in the description physics gives us, it doesn’t follow that it isn’t there in the physical world. This is like concluding from the fact that color doesn’t show up in a black and white pen and ink drawing of a banana that bananas must not really be yellow. It both cases the absence is an artifact of the method employed, and has nothing whatsoever to do with the reality the method is being used to represent. The method of representing an object using black ink on white paper will necessarily leave out color even if it is there, and the method of representing physical reality using exclusively mathematical language will necessarily leave out any aspect of physical reality which is not reducible to the quantitative, even if such aspects are there.
But it’s not just that such aspects might be there. They must be there. The quantitative description physics gives us is essentially a description of mathematical structure. But mathematical structure by itself is a mere abstraction. It cannot be all there is, because structure presupposes something concrete which hasthe structure. Indeed, physics itself tells us that the abstraction cannot be all there is, since it tells us that some abstract mathematical structures do not fit the actual, concrete material world. For example, Einstein is commonly taken to have shown that our world is not really Euclidean. This could only be true if there is some concrete reality that instantiates a non-Euclidean abstract structure rather than a Euclidean abstract structure. So, physics itself implies that there must be more to the world than the abstract structure it captures in its purely mathematical description, but it does not and cannot tell us exactly what this concrete reality is like.
That physics by itself only gives us abstract structure is by no means either a new point or a point emphasized by Scholastics alone. It was made in earlier generations by thinkers like Poincaré, Russell, Eddington, Weyl, and others, and in recent philosophy has been emphasized by Grover Maxwell, Michael Lockwood, Simon Blackburn, David Chalmers, and others.
Moreover, we know there must be more to causality specifically than physics does or could tell us about. The early Russell once argued that causation must not be a real feature of the world precisely because it does not show up in the description of the world physics gives us. For physics, says Russell, describes the world in terms of differential equations describing functional relations between events, and these equations make no reference to causes. “In the motions of mutually gravitating bodies, there is nothing that can be called a cause, and nothing that can be called an effect; there is merely a formula” (“On the Notion of Cause,” pp. 173-74). Russell’s position has been the subject of a fair bit of attention in recent philosophy (e.g. here).
Now, I don’t myself think it is quite right to say that physics makes no use of causal notions, since I think that physics tells us something about the dispositional features of fundamental particles, and dispositionality is a causal notion. Still, as other philosophers have argued, higher-level causal features -- such as the causation we take ourselves to experience continuously in everyday life, in the behavior of tables, chairs, rocks, trees, and other ordinary objects -- are more difficult to cash out in terms of what is going on at the micro level described by physics. Hilary Putnam is one contemporary philosopher who has addressed this problem, as I noted in a post from a few years ago. Trenton Merricks is another, and argues that at least macro-level inanimate objects are unreal, since (he claims) they play no causal role in the world over and above the causal role played by their microphysical parts.
Merricks thinks living things are real, and certainly a Russell-style across-the-board denial of causation would be incoherent, for a reason implicit in a fact that the later Russell himself emphasized. Our perceptual experiences give us knowledge of the external physical world only because they are causally related to that world. To deny causality in the name of science would therefore be to undermine the very empirical foundations of science.
Now, if there must be causality at the macro level (at the very least in the case of the causal relations between the external world and our perceptual experiences of it), and this causality is not captured in the description of the world that physics itself gives us, then it follows that there is more to causality than physics can tell us. And even if you dispute the views of Russell, Putnam, Merricks, et al., physics itself is not going to settle the matter. For it is not an empirical matter, but a philosophical dispute about how to interpret the empirical evidence.
(Nor will it do to dismiss such disputes on the grounds that the competing views about them are “unfalsifiable.” It may be that there is no human being more comically clueless than the New Atheist combox troll who thinks he can dismiss philosophy on grounds of falsificationism -- a thesis put forward by a philosopher, Karl Popper. As Popper himself realized, falsificationism is not itself a scientific thesis but a meta-level claim about science.)
If physics in general raises philosophical questions it cannot answer, the same is if anything even more clearly true of quantum mechanics in particular. Feynman’s famous remark that nobody understands quantum mechanics is an overstatement, but it is certainly by no means obvious how to interpret some of the theory’s stranger aspects. Quantum mechanics has been claimed to “show” all sorts of things -- that the law of excluded middle is false, that scientific realism is false, that idealism is true, etc. By itself it shows none of these things. In each case, certain philosophical assumptions are first read intoquantum mechanics and then read out again. But the same thing is true of claims to the effect that quantum mechanics undermines causality. By itself it does not, and could not, show such a thing either. Here as in the other cases, it is the metaphysical background assumptions we bring to bear on quantum mechanics that determine how we interpret it. This is as true of philosophical naturalists, atheists, et al. as it is of Scholastics.
Now, the Scholastic metaphysician argues, on grounds entirely independent of questions about how to interpret quantum mechanics, that there are a number of metaphysical theses that any possible empirical science is going to have to presuppose. Most fundamentally, there is the Aristotelian theory of act and potency, according to which we cannot make sense of change as a real feature of the world unless we recognize that there is, in addition to what is actual on the one hand, and sheer nothingnesson the other, a middle ground of potentiality. Change is the actualization of a potentiality, and unless we affirm this we will be stuck with a static Parmenidean conception of the world. And that is not an option, because the existence of change cannot coherently be denied. Even to work through the steps of an argument for the non-existence of change is itself an instance of change. Sensory experience – and thus the observation and experiment on which empirical science rests – presupposes real change. (Hence it is incoherent to suggest, as is sometimes done, that relativity shows that change is illusory, since the evidence for relativity presupposes sensory experience and thus change.)
Now, the main concepts of the Aristotelian-Scholastic metaphysical apparatus – substantial form and prime matter, final causality and efficient causality, and so forth – are essentially an outworking of the theory of act and potency. You can argue about whether this or that object truly has a substantial form or is merely an aggregate, about whether we have correctly identified and characterized the teleological features of such-and-such a natural process, and so on. What cannot be denied is that substantial form, teleology, etc. are bedrock features of the natural order and will inevitably feature in a complete picture of the physical world at somelevel of analysis. All of that follows from a consistent application of the theory of act and potency. It also cannot be denied that any potential that is actualized is actualized by something already actual. That is the core of the “principle of causality,” and It follows from the principle of sufficient reason -- a principle which, rightly understood, also cannot coherently be denied.
I spell out the reasons for all of this in detail, and also discuss the inherent limitations of empirical science, in Scholastic Metaphysics. The point to emphasize for present purposes is that the Scholastic holds that there a number of general metaphysical truths which we can know completely independently of particular disputes within physics or any other empirical science, precisely because they rest on what any possible empirical science must itself presuppose. (One of Cruz-Uribe’s readers insinuates that in resting its key theses on something other than empirical science, Scholastic metaphysics undermines the possibility of any common ground with its critics. But this is precisely the reverse of the truth and once again completely misses the point. Since Scholastic metaphysical arguments begin with what empirical science presupposes -- for example, the possibility of sensory experience, and the possibility of at least partial explanations -- they thereby begin precisely with what the critics already accept, not with what they reject.)
Radioactive decay
So, here is where we are before we even get to the issue of radioactive decay: Purportedly physics-based objections to Scholastic metaphysics – including objections to Scholastic claims about causality -- are, as a matter of course, poorly thought out. They commonly blur the distinction between empirical and philosophical claims, confuse what is really only one notion of causality with causality as such, and confuse mere illustrations or applications of general metaphysical principles with the principles themselves. Meanwhile, we know on independent grounds that physics, of its very nature, cannot in principle tell us everything there is to know about physical reality, including especially the causal features of physical reality. Its exclusively mathematical conceptual apparatus necessarily leaves out whatever cannot be captured in quantitative terms. Physics also implies that there must be something more to physical reality than what it captures, since mathematical structure is of itself a mere abstraction and there must be some concrete reality which has the structure.
We also know that quantum mechanics in particular raises all sorts of puzzling metaphysical questions (not merely about causality) that it cannot answer. And, the Scholastic argues, we know on independent grounds – grounds that any possible empirical science must presuppose – that there are a number of metaphysical truths that we must bring to bear on our understanding of the world whatever the specific empirical facts turn out to be, including the truth that causality must be a real feature of the world.
So, when critics glibly allege that radioactive decay or other quantum phenomena undermine causality, the trouble is that they are making a charge that doesn’t even rise to the level of being well thought out. It is preposterous to pretend that the burden of proof is on the Scholastic to show that quantum mechanics is compatible with Scholastic claims about causality. The burden of proof is rather on the critic to show that there really is any incompatibility. (Few people would claim that the burden of proof is on anyone to prove that quantum mechanics doesn’t establish idealism, or doesn’tundermine the law of excluded middle, or doesn’t refute scientific realism. It is generally realized that the claims in question here are very large ones that go well beyond anything quantum mechanics itself can be said to establish, so that the burden of proof is on anyone who wants to claim quantum mechanics has such sweeping implications. So why is the burden of proof on the Scholastic to show that quantum mechanics doesn’t undermine causality?)
In particular, the critic owes us an account of why, since physics cannot in principle capture all there is to physical reality in the first place -- and in particular arguably fails entirely (as Russell held) to capture causality in general -- we should regard it as especially noteworthy if it fails to capture causality in one particular case. If the critic, like the early Russell, denies that there is any causality at all, he owes us an account of how he can coherently take such a position, and in particular how he can account for our knowledge of the world physics tells us about if we have no causal contact with it. If the critic says instead that genuine causality does exist in some parts of nature but not in the particular cases he thinks quantum mechanics casts doubt on, he owes us an account of why we should draw the line where he says we should, and how there could besuch a line. (As we had reason to note recently with respect to PSR, it is difficult to see how it could be coherent to think that things are in principle explicable in some cases while denying that they are in general explicable in principle. Yet to affirm the principle of causality in some cases and deny it in others seems similarly incoherent.)
In short, anyone who claims that quantum mechanics undermines Scholastic metaphysical claims about causality owes us an alternative worked-out metaphysical picture before we should take him seriously (just as anyone who would claim that quantum mechanics undermines the law of excluded middle owes us an alternative system of logic if we are to take him seriously). And if he gives us one, it would really be that metaphysical system itself, rather than quantum mechanics per se, that is doing the heavy lifting.
Now, no one expects a logician to launch into a mini treatise on quantum mechanics before setting forth a textbook exposition of classical logic, law of excluded middle and all. The reason is that it is widely understood that it is just false to say flatly that “Quantum mechanics has undermined classical logic.” Quantum mechanics has done no such thing. Rather, some people have been led by their metaphysical speculations about quantum mechanics to wonder whether logic might be rewritten without the law of excluded middle. Logicians who have independent grounds to think that the law of excluded middle cannotbe false have no reason to take these speculations very seriously or respond in detail to them when going about their ordinary work.
Similarly, there is no reason why a Scholastic metaphysician should be expected to launch into a detailed discussion of quantum mechanics before deploying the principle of causality in a general metaphysical context, or when giving an argument for the existence of God. For it is also simply false to say that “Quantum mechanics has undermined the principle of causality.” It has done no such thing. The most that one can say is that some people have been led by their metaphysical speculations about quantum mechanics to wonder whether metaphysics might be rewritten in a way that does without the principle of causality. But metaphysicians who have independent grounds to think that the principle of causality cannot be false have no reason to take these speculations very seriously or to respond in detail to them when going about their ordinary work.
Of course, logicians have examined proposed non-classical systems of logic, and classical logicians have put forward criticisms of these alternative systems. The point is that their doing so is not a prerequisite of their being rationally justified in using classical logic. Similarly, a Scholastic metaphysician, especially if he is interested in questions about philosophy of nature and philosophy of physics, can and should address questions about how to interpret various puzzling aspects of quantum mechanics. But the point is that doing so is not a prerequisite to his being rationally justified in appealing to the principle of causality in general metaphysics or in presenting a First Cause argument for the existence of God.
But how might a Scholastic interpret phenomena like radioactive decay? I hinted at one possible approach in the post on Oerter linked to above, an approach which is suggested by the way some Scholastic philosophers have thought about local motion. Some of these thinkers, and Aquinas in particular, take the view that a substance can manifest certain dispositions in a “spontaneous” way in the sense that these manifestations simply follow from its nature or substantial form. A thing’s natural tendencies vis-à-vis local motion would be an example. These motions simply follow from the thing’s substantial form and do not require a continuously conjoined external mover. Now, that is not to say that the motion in question does not have an efficient cause. But the efficient cause is just whatever generated the substance and thus gave it the substantial form that accounts (qua formal cause) for its natural local motion. (It is commonly but erroneously thought that medieval Aristotelians in general thought that all local motion as such required a continuously conjoined cause. In fact that was true only of some of these thinkers, not all of them. For detailed discussion of this issue, see James Weisheipl’s book Nature and Motion in the Middle Ages, from which I borrow the language of “spontaneity.” I also discuss these issues in more detail here.)
Now, Aquinas himself elaborated on this idea in conjunction with the thesis that the “natural place” toward which heavy objects are inclined to move is the center of the earth, and he supposed also that projectilemotions required a conjoined mover insofar as he regarded them as “violent” rather than natural. Both of these suppositions are outmoded, but the more general thesis summarized in the preceding paragraph is logically independent of them and can easily be disentangled from them. One can consistently affirm (a) that a substance will tend toward a certain kind of local motion simply because of its substantial form, while rejecting the claim that (b) this local motion involves movement toward a certain specific place, such as the center of the earth. (This is a point missed by one of the more clueless commentators in Cruz-Uribe’s combox, whose capacity for grasping obvious distinctions is not much better than his reading ability. He ridicules the distinction I make here without offering the slightest explanation of what exactly is wrong with it.)
Indeed, some contemporary Aristotelians have proposed that affirming (a) while rejecting (b) is the right way to think about inertialmotion: Newton’s principle of inertia, on this view, is a description of the way a physical object will tend to behave vis-à-vis local motion given its nature or substantial form. (Again, see this article for discussion of the relevant literature.) The point for present purposes, though, is that the idea just described also provides a model -- I don’t say it is the only model, just a model -- for understanding what is going on metaphysically with phenomena like radioactive decay.
The idea would be this. Let’s borrow an example from philosopher of science Phil Dowe’s book Physical Causation, since I’ll have reason to return to the use he makes of it in a moment. Dowe writes:
Suppose that we have an unstable lead atom, say Pb210. Such an atom may decay, without outside interference, by α-decay into the mercury atom Hg206. Suppose the probability that the atom will decay in the next minute is x. Then
P(E|C) = x
where C is the existence of the lead atom at a certain time t1, and E is the production of the mercury atom within the minute immediately following t1. (pp. 22-23)
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