I am writing this in response to a reply to my previous post. I am posting the response as a post in its own right, because it has grown too long for a simple reply. First of all I will publish the comment, which is well worth reading, and then follow up with my response.
In my article, I said that Thomistic metaphysics was static and unchanging, whereas modern science demands a metaphysical view that keeps pace with our evolving scientific understanding of the reality of the world. Nothing in Bonnette's article gave me reason to think otherwise. As you alluded, his stance (as well as that of many Thomists') doesn't seem to be in complete agreement your own. But now I have to step back and re-assess what I said. Despite what I think I have heard from so many Thomists, perhaps there is room for change in Thomistic metaphysics.
But this brings up another issue. How much can Thomism be adapted to modern science before it can no longer be considered to be Thomism - the philosophy of Aquinas? Would he have agreed with you that a down-quark is the efficient cause of the W boson and the up-quark? It seems to me that this is something of a stretch. It still happens spontaneously, with no apparent triggering mechanism, which is what most of us would regard as an efficient cause.
And it seems to me that you are stretching more than just causality. In dismissing my criticism of essentialism, you side-step the main issue (dare I say the essential issue?). The point I raised is that there is not always clear distinction between the essence of one form and another. Your example is of a case where it is clearly defined, but my examples show that the distinction isn't always so sharp. I asked when an ape gives birth to a man. In evolution (didn't figure into Aquinas' calculus) there is no such distinction. Any dividing line you may wish to draw between one form and the other is arbitrary, and obviously subject to disagreement. But that casts doubt on one of the pillars of Thomistic philosophy.
And of course, there is still the matter of act and potency. You claim that this metaphysical concept is presupposed in all modern physics. Really? It's strange that I never heard any such thing (and I am not altogether uneducated in physics). It's not an underlying concept upon which other metaphysical concepts are based. There are no such dependencies, whether stated or otherwise. It's not something that can be quantified. It adds nothing to our understanding of any system of mechanics. And if you want to claim that it does, then you have to water it down so severely that it wouldn't be recognized by Aquinas.
If I'm not mistaken, his system of metaphysics was teleological. God was presumed, and things were purpose-driven, which is antithetical to any modern scientific metaphysical view. God was said to be pure act. Potency was the ability or tendency to move toward the the final end, which was seen as God's purpose for something in nature. This is what we understood as final causation. Now, you are telling us that these concepts are still presumed in modern science? I think you are mistaken. Or you have changed Thomistic philosophy by drastically that it wouldn't be recognized by Aquinas himself.
Thanks very much for your reply. I'll try to keep this brief, but almost certainly won't succeed.
I should note that am not a philosopher, nor a historian of philosophy, just somebody who has read the original sources and interpreted them as best I can. So I am writing this quickly based on a copy of Aristotle's works in front of me, an index; the text on a computer with a search function, and my vague memories of reading the text a little while ago. I don't claim to have an expert's breadth of knowledge at my fingertips, and thus it is possible that I haven't presented Aristotle completely fairly. If I have made a misrepresentation, I apologise. But I hope that I have done enough to show that my interpretation of Aristotle can at least be defended.
My intention has been to investigate whether an Aristotelian/Thomist interpretation of contemporary physics can be defended, and if so, promote a reformulation of Aristotelian philosophy that can be used as the basis of a philosophy of quantum physics. You will appreciate that this is not an easy task, if we want to do it properly. It is made harder because we are the cultural heirs of a 500 year old intellectual tradition which has generally misrepresented classical philosophy, and in particular changed a lot of the definitions. Contemporary Aristotelian philosophers claim that a lot of what is taught by other philosophers about Aristotle is mistaken. The only way to settle this is to lay aside any conceptions we have, and go back to the original sources.
This leaves us with a four step process.
- Find and read the original sources.
- Extract Aristotle's philosophy from his physics.
- Translate that philosophy into a more modern terminology.
- Consider how that philosophy might be applied in the circumstances relevant to today's physics, and whether it leads to a something consistent with modern physics.
The first step is easy. Translations of all the important works are just a quick google away.
The second step is harder. Firstly, Aristotle is not an easy read, particularly for those of us trained in a different intellectual tradition. Secondly, Aristotle's physics is certainly wrong. He brings in other assumptions beyond his metaphysics, such as his cosmology and the four element theory. Unfortunately, his philosophy is illustrated with examples taken from his physics, and those examples are at best irrelevant and at worst badly misguided. Reading Aristotle is often frustrating and annoying. The questions he addresses tend to be slightly different from the ones we would ask today. We have to piece things together from what he would say. And, of course, he would make some things much easier for himself if he expressed them mathematically. To see if there is anything we can salvage from his work, we need to extract the fundamental principles away from these images. Modern commentators help a lot here, but unfortunately still frequently fall back to Aristotelian examples. So I have a lot of sympathy for people who give up at this stage. However, it is possible to do this.
The third step is necessary, but dangerous. There is always the risk that I introduce some ideas of my own when doing the translation. But, if we compare Aristotelian philosophy against modern physics, we need to use a common language and notation. Since I am more committed to physics than I am to the philosophy, better to leave the physics as it is and re-express the philosophy. And it is easier to bring Aristotle up to date, than express physics in fourth century BC language.
Finally, the comparison. For me, this is the easiest step, though I know that many others have stumbled at this stage.
The reply contained four points. I will discuss each of these in turn, though not in the same order. My replies to several of these points are related to matters of terminology. So I will begin by quoting the classical sources, to see what Aquinas and Aristotle meant by the term. I will then re-express that meaning in my own words. And finally, show how it fits into modern physics. [I note that I took a few short cuts in my last section, so didn't apply this methodology there. Apologies for that.]
Efficient causality. You ask whether St Thomas would have accepted my statement that the down quark is the efficient cause of the W- Boson. At one level, of course, St Thomas wasn't aware of up quarks and down quarks, and the mathematical framework used to describe the decay would have been alien to him. But I think that he would have had more sympathy with my description of efficient causality than you might realise. You state that most people today would regard some sort of triggering mechanism as the efficient cause. I would agree that most people think that. I used to be the same. But the question is not what people today think, but what Aristotle and Aquinas thought. Since Aquinas took Aristotle's views on causality, act, potency etc. as written (he developed other areas of Aristotle's thought, but not this), I will primary use Aristotle as my source here.
Aristotle's definition of the efficient cause, as referenced in his physics (II.3), was
Again (3) the primary source of the change or coming to rest; e.g. the man who gave advice is a cause, the father is cause of the child, and generally what makes of what is made and what causes change of what is changed.
Now it is clear that each of the examples that Aristotle used for an efficient cause is a substance. We have a man, a father, a maker. Aristotle describes the cause as an object rather than a triggering mechanism. The term "efficient cause" isn't found in Aristotle as far as I can see; it's a name that was later applied to his concept. Instead he tends to use the terminology "mover" or "moving cause". For example, in Metaphysics XII.3
For everything that changes is something and is changed by something and into something. That by which it is changed is the immediate mover; that which is changed, the matter; that into which it is changed, the form.
Note, next, that each substance comes into being out of something that shares its name. (Natural objects and other things both rank as substances.) For things come into being either by art or by nature or by luck or by spontaneity. Now art is a principle of movement in something other than the thing moved, nature is a principle in the thing itself (for man begets man), and the other causes are privations of these two.
The moving causes exist as things preceding the effects, but causes in the sense of definitions are simultaneous with their effects.
In each of these quotes, Aristotle describes what can only be his efficient cause as a thing, i.e. some substance rather than a mechanism.
While Aristotle claimed that everything except his first mover has an efficient cause, he did not object to spontaneous generation. Metaphysics VII.9
The question might be raised, why some things are produced spontaneously as well as by art, e.g. health, while others are not, e.g. a house. The reason is that in some cases the matter which governs the production in the making and producing of any work of art, and in which a part of the product is present, some matter is such as to be set in motion by itself and some is not of this nature, and of the former kind some can move itself in the particular way required, while other matter is incapable of this; for many things can be set in motion by themselves but not in some particular way.
While we rightly laugh at his examples, Aristotle had no objection to something coming into being without a triggering mechanism. I could also cite his belief that maggots might arise spontaneously in meat. His science was wrong, but it demonstrates that his philosophy is consistent with the idea that triggering mechanisms are not a necessary part of the chain of explanations. Yet he believed that everything had an efficient cause. Thus he could not have believed that efficient cause could be reduced to a triggering mechanism.
Aristotle had no difficulties with things being the cause of their own motion (or change). Physics VIII.4Of things to which the motion is essential some derive their motion from themselves, others from something else: and in some cases their motion is natural, in others violent and unnatural.
Finally (in my treatment of Aristotle), when he does discuss a "triggering mechanism", he makes it clear that it is distinct from both the mover (the efficient cause) and the moved. Physics VIII.1
But at any rate all things that are capable respectively of affecting and being affected, or of causing motion and being moved, are capable of it not under all conditions, but only when they are in a particular condition and approach one another: so it is on the approach of one thing to another that the one causes motion and the other is moved, and when they are present under such conditions as rendered the one motive and the other movable.
Thus the approach is needed to make the mover or efficient cause affective, but it is not in itself part of the description of the efficient cause, which is the mover by itself. I can't think of any place in Aristotle's work where he relates a triggering mechanism to an efficient cause. Instead, as far as I am aware always refers to an agent that institutes the process of change. Of course, I could be mistaken in saying that nowhere in Aristotle does he use a different understanding of efficient causality -- I am not an expert, and can't bring all of his work to mind -- but certainly this is the primary sense in which he describes efficient causes. He stated that in some cases circumstances need to align for the efficient cause to bring about its effect. But he also allowed for things to happen spontaneously without needing a particular set of circumstances. This is what we see in quantum physics. An electron may emit a photon and drop down an energy level (which is spontaneous), or absorb a photon and move up an energy level (which depends on circumstances). In my terminology, the efficient causes of the electron in the higher energy state are the photon and the electron in the lower energy state.
I also note that Aquinas' second way, the cosmological argument based on efficient causality, depends on the version of causality that links one substance with another rather than via "triggering mechanisms".
Thus my picture of the down quark decaying is an accurate implementation of Aristotle's idea of efficient causality. It contains the notions of causality linking substances, things containing the cause of their own motion within themselves, and the spontaneous generation of the W- Boson out of the down quark.
The conservation of energy and momentum is respected in all quantum field theories. Every particle carries positive energy. Therefore (at least this side of a theory of quantum gravity), we can say that particles cannot come from nothing; a new particle can only emerge from the decay of another particle. Obviously, in some theories of quantum gravity, the negative energy of the gravitational field can cancel out the positive energy of the newly created mass. I don't believe that such theories are proved to be correct, but even if they were, the gravitational field would count as the efficient cause. However, a discussion of that goes beyond both the scope of this discussion and known physics.
Thus, I think that my point that, while quantum physics destroys most formulations of causality, Aristotle's is consistent with it stands up well to scrutiny. The main stumbling block is that Aristotle meant a different thing by "efficient cause" than the definition used by most people since the Renaissance. So Aristotle's causality is often misrepresented and misunderstood.
Teleology or final causality. Once again, there is a lot of misunderstanding over this concept. I personally blame the Renaissance (but again, I'm no historian of philosophy). At some point, the meaning of telos changed from end to purpose. I'll again begin with Aristotle's definition from Physics
Again (4) in the sense of end or 'that for the sake of which' a thing is done, e.g. health is the cause of walking about. ('Why is he walking about?' we say. 'To be healthy', and, having said that, we think we have assigned the cause.)
At first sight, this doesn't seem to help me much, with Aristotle's example from medicine not really applicable to physics. But when Aristotle turns to physical processes, it becomes clear that in this case he has something else in mind. Physics II.2, II.8
Again, 'that for the sake of which', or the end, belongs to the same department of knowledge as the means. But the nature is the end or 'that for the sake of which'. For if a thing undergoes a continuous change and there is a stage which is last, this stage is the end or 'that for the sake of which'.
Further, where a series has a completion, all the preceding steps are for the sake of that. Now surely as in intelligent action, so in nature; and as in nature, so it is in each action, if nothing interferes. Now intelligent action is for the sake of an end; therefore the nature of things also is so. Thus if a house, e.g. had been a thing made by nature, it would have been made in the same way as it is now by art; and if things made by nature were made also by art, they would come to be in the same way as by nature. Each step then in the series is for the sake of the next; and generally art partly completes what nature cannot bring to a finish, and partly imitates her. If, therefore, artificial products are for the sake of an end, so clearly also are natural products. The relation of the later to the earlier terms of the series is the same in both.
So here, we are discussing a series of physical processes, similar to what we consider in particle physics. Now the "stage that is last" is a physical state, having the same standing as the stage that was first, meaning that final causality once again links one substance with another. So I took this idea in my adaptation of Aristotelian philosophy to quantum physics. The only thing I have added to this is the idea of indeterminacy (and I'm not sure that it is an addition to Thomistic thought): that a single object can have many possible ends. If the end marks the end effects of a possible physical process, then clearly the ends or final causes of a down quark include the up quark, electron and anti-neutrino.
We can thus translate final causality to mean a list of the possible final states of a physical process acting on a given initial state.
The opposition of teleology in physics basically states that physics can be described without reference to an (intellectual) purpose. [This doesn't mean that there is no intellectual purpose present, just that it is not part of the description provided by physics. I personally view the indeterminacy of quantum physics as a sign that physics isn't a complete description of reality; but that is a topic for another time.] But this just refers to the definition of final causality adopted during the Renaissance from (I think, but I could be mistaken) the writings of Cicero. It does not exclude the Aristotelian definition of final causality. Indeed, when particle physicists discuss "decay channels" today, they are just describing the possible end states of an unstable particle, precisely what Aristotle meant by final cause.
The relevance of teleology in the Aristotelian sense is abundantly clear in quantum field theory. Take, for example, the Hamiltonian operator (which describes change in time) for a fermion in quantum electrodynamics.
[Obviously we still need to renormalise everything; that doesn't affect my conclusions.] The creation and annihilation operators are usually used to describe the physical creation and annihilation of particles. But their role when we perform computations in QFT is slightly different. The annihilation operator basically scans for initial states. This term in the Hamiltonian seeks out a fermion at location x. The creation operators describes final states. So what this equation states is that if we start out with a fermion, then the possible final states are either a fermion in the same state (the mass term), or a fermion that has either emitted or absorbed a photon (the photon term), or a fermion in a neighbouring location (the derivative term). In other words, the central object in QFT describes the possible end states of a given initial state. And this is Aristotelian teleology, at the centre of our modern description of physics.
When you say that "God was presumed" in Aquinas' teleology, I think you have things the wrong way round. Aquinas took his teleology from Aristotle pretty much unchanged, who maintained that motion and ends were wholly natural. The only place where God entered Aristotle's thought was as the terminus of the cosmological argument (the series of movers). So the presumption in Aquinas' thought was that things had natural ends, in the sense that I have described above. The fifth way was an attempt to demonstrate the existence of God from the premise that final causes exist in nature. You might not think much of Aquinas, but he clearly wasn't so much of an idiot that he would take as his premise something that presumed his conclusion. The existence of God was not an assumption of Aquinas' teleology, but a conclusion which Aquinas (but not Aristotle, who shared the same teleology) drew from it.
I am not claiming that (intellectual) purpose in the universe has a place in modern science (although I believe that it can be inferred from the best scientific theory, but that is a philosophical argument building on a scientific theory which began by assuming no purpose). But I am claiming that the idea that an initial state can lead to certain final states but not others, Aristotle's understanding of teleology, is implicit in all forms of modern science from Newton to the present day.
Act and Potency. I claim that this concept is presupposed in all modern physics. I would fully agree with you that the words "Act" and "Potency" won't be found in any physics textbook since the middle ages. But my claim is that the concepts are still there, just called by a different name (or sometimes presumed without being stated).
I am going primarily to have to describe in my own language the concept as I understand it, since I can't find a single brief quote in the source work that expresses concisely what I want to say. I think my interpretation of this can be exegeted out of the sources, but were I to quote explicit passages as I did in the previous examples, there would be too much to filter out. Aristotle gave five possible definitions of potency in his metaphysics. The main one refers to a principle that allows for change. Both the actual and the potential refer to physical objects. Aristotle uses the example of a statue that could be carved out of a block of marble. Currently it is a block of marble actually, while the statue exists potentially. Thus a potency points to something which could be an actual object.
The thing with the greatest degree of potency is pure matter, because this could be turned into everything. Thus all the possible substances which could be formed out of that matter exist potentially within that matter.
Another important sense in which Aristotle uses potentiality is to describe the parts of a being. So, if one has a composite being, then one can't say that the parts themselves exist actually (since they are combined within a being). But instead they do exist potentially. I would use the example of a string (one of Aristotle's examples applied to a physical object). One could in principle cut the string in the middle and turn it into two short strings. Those short strings don't exist actually until the string is cut, but they are nonetheless in some sense contained within it. Aristotle states that they exist potentially, in the sense that the longer string has the capacity of being turned into two short strings.
We can also apply the concept of potency to the ability to rearrange matter to give it a different shape. Aquinas discusses this in his commentary on Aristotle's metaphysics, part 5.
And there are some things to which both of these apply, because in a sense the position of their parts accounts for their differences; and of these we use both terms—all and whole. And these are the things in which, when the parts are interchanged, the matter remains the same but not the form or shape. This is clear, for example, in the case of wax; for no matter how its parts are interchanged the wax still remains, but it does not have the same shape. The same is true of a garment and of all things which have like parts and take on a different shape. For even though liquids have like parts, they cannot have a shape of their own, because they are not limited by their own boundaries but by those of other things. Hence when their parts are interchanged no change occurs in anything that is proper to them. The reason for this difference is that the term all is distributive and therefore requires an actual multitude or one in proximate potency to act; and because those things have like parts, they are divided into parts entirely similar to the whole, and in that manner multiplication of the whole takes place. For if every part of water is water, then in each part of water there are many waters, although they are present potentially.
We can go a bit beyond Aquinas' picture here, and consider the difference between some still water and water containing waves. In some sense, they are the same substance. The difference is merely a rearrangement of the parts. The difference is that at any one moment, one of these possibilities is actual, and the others remain potential. Thus potency also applies when we rearrange or move matter, such as in the example of the wax candle.
In his commentary on metaphysics, book 7, Aquinas wrote,
For in these sensible bodies, which all men admit to be substances, there are certain attributes such as the affections of bodies, for example, hot and cold and the like, which are evidently not substances. And in these bodies there are also “certain activities,” i.e., processes of generation and corruption and motions, which are clearly not substances. And in them there are also potencies, which are the principles of these activities and motions, i.e., potencies of acting and being acted upon, which are present in things; and it is also clear that these are not substances, but that they rather belong to the genus of quality.
Here Aquinas states that there are attributes within substances. For example, something might be either hot or cold. But equally there is the action of moving from cold to hot. This change is obviously, to Aristotle and Aquinas, described in terms of act and potency. The being is actually in a state where it is cold, and potentially in a state where it could be hot. But Aquinas also states that because the motion can happen, then within the actually cold substance, there must be potencies. Since a potency always refers to a thing, this means that the same being heated resides or exists potentially within the cold being.
There is one further application of potency which I need to discuss, and that is the presence of potency in a universal (which in this context I mean the description of beings of the same type -- the concept of universals is often applied to properties, but that's not what I am describing here). The discussion I want to focus on here is in Aristotle's first book of his physics, where he discusses act and potency as a rival to the other theories of motion and change. Aquinas' commentary on this states,
That universals are confused is clear. For universals contain in themselves their species in potency. And whoever knows something in the universal knows it indistinctly. The knowledge, however, becomes distinct when each of the things which are contained in potency in the universal is known in act. For he who knows animal does not know the rational except in potency. Thus knowing something in potency is prior to knowing it in act. Therefore, according to this order of learning, in which we proceed from potency to act, we know animal before we know man.
It would seem, however, that this is contrary to what the Philosopher says in Posterior Analytics, I:2, namely, that singulars are better known to us, whereas the universals are better known by nature or simply. But it must be understood that there he takes as singulars the individual sensible things themselves, which are better known to us because the knowledge of sense, which is of singulars, does precede in us the knowledge of the intellect, which is of universals. And because intellectual knowledge is more perfect, and because the universals are intelligible in act, whereas the singulars are not (since they are material), the universals are better known simply and according to nature.
Here Aquinas is distinguishing between a universal, the species described by that universal, and singulars (that is individual members of the species). When he says that universals are confused, he means that that they don't describe one singular member of the species, but the species as a whole, with all the differences and distinctions that entails. So, for example, we may take the universal that describes the hydrogen atom. The singular would be one particular hydrogen atom in one particular state. All hydrogen atoms would be part of the species. The universal is (in part) a description of the set of every possible state of a hydrogen atom. When Aquinas discusses that the universals contain in themselves their species in potency, he seems to me to be saying that once we have awareness of the universal, from that concept we understand the set of states that could potentially exist. The potentia are thus the set of possible states of that being. We observe one of these states when that particular potency is actualised. But just as the singular state still reflects the universal, the other possible states of the being are contained within it as potency.
So the potencies of a being can be defined as the set of possible actual states of that being. If this being exists in reality, then one of those states will be observed as actual, and the others in potency. If it doesn't exist in reality, then all the states exist as potencies. And this I will take as the primary definition of act, potency and potentia.
Now you will rightly ask what any of this has got to do with modern physics. And at first sight, the answer is indeed not very much. But let us look deeper. Since the topic is quantum physics, I'll use an example from quantum physics, the hydrogen atom.
We know that in Schroedinger quantum mechanics (the same fundamental principle carries forward into field theory, but since I understand this example better in the QM description, I will use that), the electron in the hydrogen atom is described by Schroedinger's equation with a Coulomb potential energy. This is a differential equation acting on the wavefunction or probability amplitude of the electron, which in turn describes the likelihood that we will find the electron at any given place. Now, the Schroedinger equation strongly restricts the possible solutions which the wavefunction could have. These are distinguished by three integers, the positive n which indicates the energy level, and two which describe the angular momentum of the electron. The energy of each of these solutions is (baring some corrections from magnetic and other interactions which I am neglecting here) proportional to 1/n^2.
It is customary to discuss these solutions as different states of the system. They are, as discussed, discrete, characterised by various different integers. One cannot get from one to the other via a continuous change. At any one moment, when we take a measurement, we will find the system in one of these states. But it could, and does, transit to one of the other states.
The mathematical description of the atom describes all these states. That alone cannot say which one we will find the electron in when we take a measurement. In terms of the actual physical object which we observe only one of these states exists at a given time (I discussed superposition briefly in my original post). But equally the other states still play a role, since the electron can only transit to them. We can't say that those other states don't exist, since they are part of the mathematical description. Equally, we can't say that they do exist in the same sense that we say that the state which is currently occupied exists, because they are not tangible. We thus need a third option to describe hydrogen atoms existing in states which could be occupied but which happen not to be occupied.
And this is where the connection with Aristotle comes in. He uses very different language, but what he describes in the concept of potential existence is precisely that middle ground between tangible existence and non-existence which we need to describe the full set of states of the Hydrogen atom. The state of the atom that is currently occupied is actual; the other states exist in potency. In Aristotle, the potencies describe the possible substances that could arise after a change. In quantum mechanics, the non-occupied other states of the atom describe the states which the atom could change into. In Aristotle, actuality describes the being as it is in tangible existence. In quantum mechanics, we speak of one of the particle being measured to be in one particular state, or that that state is occupied.
I perhaps go a little bit beyond Aristotle and Aquinas in defining the potentia of a system as any way in which it could possibly exist. In quantum mechanics, this would refer to the eigenfunctions of the Schroedinger equation; in field theory it refers to the Fock states associated with the renormalised creation and annihilation operators. In this, I follow the terminology of Heisenberg in his Philosophy and Physics. One of these states is in act (again, the concept of superposition complicates matters, but I discussed that in my previous post), with all the properties that Aristotle attributes to act. The others states are in potency, with all the attributes that Aristotle assigned to potency.
So when I say that the theory of act and potency underlies quantum physics, this is what I mean. Quantum physics presupposes a set of Hilbert states, the eigenstates of the Hamiltonian operator. [I had better define what I mean by that. One form of the Schroedinger equation states that the Hamiltonian operator acting on the wavefunction is equal to the energy times the wavefunction. It is basically a mathematical operator that takes in one function and spews out what is usually a different function. Only in a few limited examples do you get the output as something proportional to the input. The eigenstates are the possible solutions to this equation. The Schroedinger equation doesn't continue in this form into quantum field theory, but the concepts of the Hamiltonian operator and eigenstates do, which is why I express things in this more general language.] You might quibble about whether this is a presupposition or a conclusion of quantum mechanics, but the mathematical structure that field theory depends on certainly presupposes it. But these states have precisely the same role in quantum physics as the actual and potential beings do in Aristotle's philosophy. Thus we can identify the eigenstates with the Aristotelian potentia. Since that physics can be described in terms of such states is presupposed by quantum physics, we can say that the theory of act and potency is presupposed by quantum physics. The words are not used by quantum physicists because a different language has been adopted. But the same concept is there. Once you have these states, then you can start calculating things such as the likelihood that a particle will transition to one state to another, or decay, or whatever.
In my book, I also express classical mechanics in the same language of states. My motivation there is as part of an introduction to quantum physics, and I don't discuss the philosophical implications. But that classical mechanics can be expressed in such a language means that that formulation of it at least also depends on the concepts of act and potency.
Essentialism. I gave the example of an ethanol molecule to argue that essence is a real thing. One ethanol molecule is the same as any other, which is what we mean when we talk about essences. You gave the example of the sand dune and the transitional form between a monkey and a man to say that essentialism isn't a real concept.
Now I could retort that the topic of this discussion is quantum physics, and the ethanol molecule is considerably closer to an object of study of quantum physics than a sand dune, and thus the more pertinent example. But I don't think that will satisfy you, so I ought to elaborate.
There are two different ways in which we can consider the essence of some substance. We can group things together by identity, or by similarity. The two senses of essence are merely analogous to each other.
In fundamental physics and chemistry, the essences are described by identity. One electron is indistinguishable from any other electron. They all have the same mass, coupling to the photon, the same energy levels and so on. The Pauli exclusion principle (which only applies to identical particles) makes it clear that this is not an accidental similarity, but a definite part of nature. The same applies to composite particles such as protons, and going further afield to the atoms and molecules that are studied by chemistry. The example I gave was of this class. One ethanol molecule is the same as any other ethanol molecule (baring that in one of them one state might be actual, and in the other some different state might be actual, but the two molecules have the same potentia, and the essence describes the set of all potentia, not which one happens to be actual at any given moment).
Biological species are at the other extreme. Here we group animals together not by identity, but on the basis of similarity. One monkey is not identical to another monkey of the same species, but only similar in certain respects. Because they are marked by similarity rather than identity, there is ambiguity about where we draw the lines between different species. However, this doesn't deny essentialism in physics. Each monkey is made of various molecules and polymers, and these are identical to any other molecule and polymer of the same type. Thus physical essentialism still applies at a more fundamental level even when we discuss biological species. Thus ambiguity in finding essences at the biological level doesn't really influence the discussion at the level of physics and chemistry. When we talk about the essence of a biological species and the essence of a protein, we are discussing different things.
So the question is whether it is useful to discuss essences at a biological level. Generally, it is easy to draw boundaries between objects where the difference between one and the other is discontinuous. That's how we can distinguish ethanol from methanol: there isn't an intermediate state between them. Clearly, at one level we can do this in biology. Not every species is connected by small changes. For example, insects have 6 limbs, mammals four. That's a discontinuous change, and it is easy to draw a line between them. Different species have different numbers of chromosomes. Obviously, evolution can still explain how a mutation can allow something with twenty chromosomes to give birth to something with 21, but the mutation which does this still leads to a clear discontinuity in the structure of the animal. So not every biological change is continuous. At a crude level, those discontinuities allow us to draw lines around groups of organisms even though they evolved from each other.
Aristotle defined humanity as a rational animal. Rationality in this case means (or contains the meaning) of being able to form abstract representations of objects, either in thought, language, mathematics, art and so on. This is a discontinuous change in the evolution of species. Either you can do this, or you can't. Admittedly some people are better at doing it than others, but even if you can't do it well, you still can do it. The claim (I'm not up to date with biological research in this area, so can't comment on how accurate this claim is) is that this is something that we as a species can do, but other animals including our evolutionary ancestors can't. [I recognise that some birds and primates are able to use tools for some tasks, but one can do that without abstract thinking in the sense that I mean.] There was thus along the evolutionary chain, at least one point where a mutation led to a non-rational animal giving birth to a rational animal. This is a discontinuous change, and can be used to distinguish between the species [species here is used in the Aristotelian rather than modern definition].
At some point, we had a common ancestor, and the evolutionary chain split, with one branch leading to modern chimpanzees, another to gorillas, and the other to modern humans, and so on. Mankind is certainly rational. If chimpanzees, gorillas, etc. aren't capable of abstract representation, then the break occurred some point or points along the chain from that common ancestor to ourselves. The hominid on one side of the change was a non-rational animal, the hominid on the other side was a rational animal and thus (by the Aristotelian definition) human. If it turns out that other primates are capable of abstract thought, then the break occurred somewhere on the path between the monkeys and the primates.
I expect you will object that I using Aristotelian definitions of species instead of more modern biological ones, dividing things in terms of animal and rational animal rather than distinguishing between snow leopards and African leopards, for example. But what I am trying to do is to show that discussing essences in this fashion is consistent with evolution. I don't want to make any further claims here than consistency; I leave it to Thomists who specialise in biology to draw things further than that, if they can.
Another possibility is to define ideal archetypes which are that individual or those groups of individuals which are best suited to their environment (in the sense required by natural selection), and define a species [in the modern sense of the word] as those which are capable of interbreeding with those ideals to produce fertile offspring. This leaves the question of what to do with those individual animals capable of interbreeding with two disjointed archetypes (I would then class all of those beings into one species). Then we have those possible individual animals which aren't capable of interbreeding with any archetype. These I would class into a separate species of transitional forms. I am not a biologist, so I don't know whether this idea would work in practice. But if it plausible, then it remains a possible way to distinguish essences of species.
But I would be better turning this question to those Thomists who have more of an interest in biology than I do. I am, however, convinced that the problems with essentialism in biology don't create difficulties for essentialism in physics, which is my main concern. The word "Essence" means slightly different things in biology and physics.
With regards to sand dunes, the issue is a different one. We first of all have to consider the grain of sand itself. This I take to be a small speck of a mineral crystal, and thus it falls under the purview of the physical essentialism. Of course, different grains of sand might have different mineral compositions and thus belong to different essences, but that doesn't change the overall picture.
Then we come to sand as a whole. The essence of sand is that it is a collection of grains of sand (we have the ambiguity here, of course, of what counts as a sand grain; how big it has to be -- I'll leave this to one side). Once again, essence is used analogously with how we describe the essence of an ethanol molecule. So difficulties in defining essences here don't necessarily mean that there are difficulties at the level of physics. The constituents of an ethanol molecule are tightly bound together into a single substance. The constituents of sand are only very minimally bound, if at all. In terms of the metaphysics of the thing, that's a big difference.
But let's ignore those caveats, and proceed. Sand is the substance. That sand can exist in numerous different shapes and structures. So sand might be actually flat and potentially a sand dune, or actually a sand dune and potentially flat. So when you refer to sand dunes, you are drawing a line, not around a substance as a whole, but around a set of potentia within that substance, and arbitrarily separating them off from other potentia. Difficulties in defining what is and what isn't a sand dune arise because you have artificially drawn a line through a substance. Essence applies to substances, not potentia within a substance. Thus, in Aristotelian thought, a sand grain has an essence. Sand, if the caveats I raised before can be overcome, has an essence. But a sand dune doesn't, because it refers to only some of the potentia of a form. This in no way destroys essentialism (not least because the fundamental constituents of sand have essences, so it is not as though we have described a system without any reference to essence): it just means that we have to be careful what we apply it to.
Thus, while I can see how your example might be a problem for those other forms of essentialism which don't allow for an essence to have different potentia, I don't see how it is a difficulty for a strictly Aristotelian essentialism. And particularly not when we are discussing essentialism in theoretical physics.
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