How to (really) think about metaphysics

Today I was visiting some of the blogs I frequent and I stopped by the blog Atheism and the City, authored by a frequent commenter at my blog who goes by the name “The Thinker”.  He has just posted a review of Chapter 3 of Edward Feser’s book The Last Superstition. There is much to quibble with in Thinker’s post, but I’ll save an attempted refutation of his arguments for another day. My main focus here is regarding his comments on metaphysics and scientism.  I’ve gone on ad nauseum on this blog about the importance of metaphysics, and the falsity and self-refutation of scientism (see here),  but Thinker presents a different spin on this issue—a view he calls “weak scientism”—and therefore I felt the need to point out its falsity, thereby tossing it in the trash-bin with the other failed metaphysical frameworks.

Thinker begins to articulate his thoughts on weak scientism:

 I hold to what is sometimes called "weak scientism." Unlike strong scientism, which says that "the characteristic inductive methods of the natural sciences are the only source of genuine factual knowledge and, in particular, that they alone can yield true knowledge about man and society," weak scientism says that the natural sciences are given a privileged status over metaphysics and logic and all other methods of derived knowledge, but it stops short of saying that science and logic are the "only" ways of yielding true knowledge.

It’s hard to say exactly what Thinker means by the natural sciences entertaining a “privileged status” over metaphysics and logic. Perhaps he means that the natural sciences are more genuine and pure methods for attaining knowledge of reality. It is certainly true that the natural sciences have illumined the majority of beliefs and knowledge that we have about how the natural world behaves. But does this give one warrant to claim that science therefore entertains a privileged status for gaining knowledge about reality, over and above metaphysics? I don’t see how. For, as I’ve argued before, science tells us how the world behaves, but it does not, and cannot, tell us why—only metaphysics can hope to do this. Science might be able to tell us about the four forces of nature, for example, but it cannot tell us why those forces obtain at all as opposed to others. So I fail to see how science should be given a privileged status regarding methods of inquiry above methods like metaphysics.

The Thinker continues:

Furthermore, I apply this privileged status of science mostly when entertaining questions regarding ontology, such as the fundamental nature of reality—for which science is our most reliable epistemology, contrary to what Feser says.

So now Thinker says that science seems to be our most reliable guide for answering questions regarding the fundamental nature of reality. Again, this presents a couple problems. First, science by itself does not have the tools to comment on the fundamental nature of reality. This might seem like an arrogant statement against science, but it’s the truth. Scientific law only describes what already exists and how it behaves. But simply describing what already exists is purely abstract, in that it doesn’t actually enlighten us to the intrinsic nature of what it’s describing. So science simply doesn’t cut deep enough to penetrate the fundamental nature of reality.

Second, the natural sciences necessarily describe only the natural world. Therefore, if there were other aspects of reality, science could not enlighten us one way or another regarding their behavior. In fact, even if there exists no supernatural aspects of reality, science could likewise not comment on its non-behavior. The point is that science is completely silent on the question of what comprises the set of reality and being. To construct my point in another fashion, the description of a set of elements is not sufficient to conclude that only the set exists. Something else would be needed to ground this proposition, and it couldn’t be a mere description of a set’s behavior.

Let’s move on:

No logician could ever derive the physics of quantum mechanics from the laws of logic, or from metaphysics. Only empirical evidence could enlighten us to such phenomena, and the universe is ultimately quantum mechanical in nature.

Thinker continues to make the similar mistakes here. First, I agree that science, and only science, enlightens us to the physics of quantum mechanics and general relativity etc. But nobody ever said that metaphysics was the method of inquiry that should have done this. You see, science uncovers the physical contingencies of the universe that can be repeated and predicted using abstract mathematical equations—equations that could have been different. But, metaphysics enlightens us to the necessities of the universe, and anything that could or would exist. It tells us the ontology of causality, identity, first principles, time and free will etc. So, to once again engage in chest-thumping on behalf of science because of its discoveries of the physical contingencies of the world is simply misplaced here, since metaphysics wasn’t attempting to do this in the first place. It’s like a basketball player praising his dunking ability, while playing soccer. Metaphysics tells us the fundamental nature of reality, and science tells us how this reality happens to behave. The latter is not equivalent to the former.

Second, Thinker shoots himself in the foot here with his last statement about the universe being quantum mechanical in nature. How does he know the universe is entirely quantum mechanical in nature? Well quantum mechanics cannot tell him this, since, to reiterate, QM is only a description of sub-atomically existing matter. How does he know this description constitutes the fundamental nature of reality? Even if he has an answer to this question, it will not be given to him by QM. He will have to have some other fulcrum to lay this proposition upon. And this goes for any set of natural sciences you want to erect as the end-all be-all description of reality. A description of a set simply will not be sufficient for concluding that only what’s in the set is what exists, or that the set does not have an aspect of its nature not captured by the original description. The Thinker’s attempt, then, to ground weak scientism seems to have failed.

Naturalism and falsifiability

Metaphysical naturalism is the position that the natural universe is all that exists, and a common corollary of adherence to naturalism is a subsequent adherence to physicalism—the theory that everything that exists is reducible to the material or physical. Such a metaphysical jump seems warranted: if the natural world is all that exists, and the natural world contains only the physical, then everything that exists is physical. These inferences illuminate exactly why naturalists are so enamored with science. For if all that exists is physical, and science studies the physical (empirical) world, then science surely must be the be-all and end-all road to knowledge and inquiry.

However, while the partnership of science and naturalism might seem like a match made in (non-existent) heaven, it quickly becomes riddled with problems. First, remember that science is very strictly committed to the doctrine of falsifiability. That is to say, something is considered to be a valid scientific hypothesis or theory if it could, at least in principle, be proven false. But, how does this view of falsification jive with naturalism? That is to ask, is naturalism itself falsifiable?  Well it might seem at first glance that this is indeed the case. For all one needs is evidence of the supernatural, and naturalism would subsequently be rendered invalid, right? Well, maybe not. Let’s examine a discussion between naturalist Richard Dawkins and Cardinal George Pell that illuminates where I’m going:

Moderator: What would it take [to convince you of the supernatural]?

Dawkins: I used to think that if somehow a great big giant nine-hundred foot high Jesus with a voice like Paul Robeson suddenly strode in and said “I exist, here I am.” Umm, but even that I actually sometimes wonder whether that would…

Pell: I would think you were hallucinating.

Dawkins: Exactly, I agree. I agree.

The point of this exchange is the following. Regardless of the fashion in which the supernatural (if it exists) would expose itself, such exposure could always be explained away in terms consistent with naturalism and physicalism. For instance, if Jesus or God were to appear to me, or to the whole world, this could always be waved away as a hallucination, or a prank by a higher life form in the universe. Now obviously the validity of such theories would be highly implausible, although the naturalist could claim that the opposite—that the supernatural was actually manifest—is equally implausible. Nevertheless, the point remains that naturalism is still salvageable despite apparent evidence to the contrary.

This all means one important thing: naturalism is, at least in principle, not falsifiable. I stress “in principle” because I believe that most logical people would abandon naturalism in practice were the aforementioned events to take place. However, my focal point here is principle. Now, since naturalism is unfalsifiable, then naturalism is unscientific, for remember that science only deals with the falsifiable. But this leads to a perfect irony. For if naturalism and physicalism are true then science and empirical observation are our only source of knowledge. Yet, because of such empirical considerations naturalism itself must be rejected. Thus stated, empiricism and naturalism do not pair together as well as is commonly claimed.

Essentialism and reductionism

Mike D over at The Aunicornist has, once again, devoted a couple of posts to refuting the metaphysics of Thomism. Since I am myself a Thomist I felt the need to deal with such arguments and assertions. I will, for now, be dealing with this post of Mikes. So, let’s dig in.

Mike states the following:

[W]e all agree that if tomorrow all humans were wiped off the face of the Earth,cats (for example) would still be cats—that is, they would retain the amalgam of physical properties that our brains categorizeas simply “cats”, even there [sic] wouldn’t be anyone around to say, “Hey, that’s a cat!” But does it follow, then, that cats have a property of identity that makes them cats?

Here Mike demonstrates his poor understanding of Thomism by conflating identity with essence. What Mike is talking about here is essence which is just that whereby something is what it is. To grasp a thing’s essence is to grasp all the properties that are essential to that thing remaining that very thing. Or, to take Mike’s example, the essence of a cat is comprised of whatever properties a cat must have for it to remain a cat. Yet, Mike here uses the terms “the property of identity.” But, identity is not the same thing as essence. It is an essence that makes something what it is and not identity. Identity is more broad and more tautological. The property of identity simply means that something is identical to itself and is not identical to what it isn’t. A corollary of this is that all of a thing’s properties are contained in its identity. Yet this is not the same with regards to a thing’s essence.

Let’s use an example to illuminate the distinction between identity and essence. Take two chairs. Each particular chair is identical to itself and is not identical to the other chair even if they both look  exactly the same. Yet, these chairs do share the same essence in that they both provide a seat—here providing a seat is the essence of being a chair. So, two objects can share an essence without being identical. This is because identity takes all of a thing’s properties into account whereas essence only takes essential properties into account. It is essential that a chair provide a seat, but it is not essential for a chair to have four legs. Yet, when contemplating a chair’s identity it is crucial to include the property of how many legs it has—for a chair with four legs is obviously not identical to a chair with three.

All this is important because Mike, in the quote above, is again conflating identity with essence, which wouldn’t have happened if Mike adequately understood the position he’s attempting to refute. So, while none of this, so far, undermines Mike’s arguments, it does demonstrate that Mike does not understand his opponent’s positions the way he thinks he does.

Mike then illustrates his (already confused) argument with an example of a mountain:

Mountains are formed, if I remember grade school geology correctly, when massive tectonic plates press against each other, forcing the earth to slowly rise over eons. I wonder how the Thomist might think about this, then—at what point does the earth have the property of identity of a mountain, versus just being a really big hill or a giant pile of rocks?

Again, Mike should be talking about essence here, and not identity. For a mountain, or hill, at any stage in its formation will still always be identical to itself. And thus, the “property of identity” will always be present. What Mike means to argue here is that if we watched the earth slowly rising over eons then at what point could we say that the earth exhibits the essence of a mountain, as opposed to a hill? The point of this example is to demonstrate that determining the essence of something is not always so clear cut, and that such an endeavor can run into ambiguities and vagueness.

Yet, this argument doesn’t really make the case that Mike thinks it does. First, a Thomist would not claim that determining a thing’s essence is always an easy endeavor. It is in some cases very difficult to determine what the essence of something is, or how such an essence differs from the essence of another. But nothing about this difficulty demonstrates that there are no such things as essences. The real problem then deals with epistemology, and not ontology. Encountering difficulty in determining a thing’s essence is an epistemic problem, while proclaiming that things do not have an essence is an ontological problem, and Mike is conflating the two.  In order for Mike’s example to have any force he would need to demonstrate that difficulty in determining a thing’s essence entails that essences are therefore nonexistent. But Mike has not done this.

Second, Mike’s argument actually makes the case for Thomistic essentialism, and not against. By picking a vague case of determining essence, Mike is implicitly conceding that there are cases that are not vague. In fact, most cases are not vague. One could easily identify the essence of the brain, the heart, water, human rationality etc. The fact is that we can find an abundance of easily identifiable essences all around us. So why should we call essentialism into question because of a few exceptional cases that run into obscurity, instead of affirming essentialism due to the abundance of its applicability? There is no reason. The fact is that we wouldn’t even be able to recognize vague instances of identifying essence if we didn’t already have clear and precise examples.

Mike then articulates one of his central arguments:

Thomists take things like identity, essence, nature, 'prime matter' and potentiality to be literally real properties of the external world, independently of human minds. But at every turn, we can see that we have no reason whatsoever to think that any of these 'metaphysical' properties are anything more than conceptual constructs. There's no reason for us to think that the concept of "cat" is anything more than a useful categorization of our brains for a particular arrangement of matter; we have no reason to think that there exists any such a thing as the identity, essence, or nature of a cat independently of our minds.

Mike is making the argument that our metaphysical concepts are mind-dependent, and that they are simply constructs of our brain that help us order and identify the world we experience. There is no essence of a cat independently of our minds. The essence of a cat is simply something that our brains have constructed in order to arrange the matter that makes up a cat.
While this viewpoint might seem parsimonious, it is rife with problems. First, the world is exactly the way we would expect if there were in fact actual essences and identities of things. One example of this is unity. Let’s illustrate. If I consider all four of my dogs—my Chihuahua, Dachshund, Beagle and Blue Heeler—I can see that all of them share the same essence of “dogness” even though they are not identical. And in that sense they share in a unity between them. They are all individual particulars yet they are in fact related to one another. They are related to one another in a way that they are not related to a cup, a tree, or a lamp. But if Mike is correct, and dogs share no “essence” and we simply construct such things in our brain, then why is it the case that we attribute a shared essence to all these dogs? The only way one can answer this is to say that these dogs, in mind-independent reality, exhibit similar characteristics and have similar properties that allow us to group them into these related “kinds”. But then this simply makes the case for the Thomist! For this is what the Thomist has been arguing all along. For dogs can only be grouped into a specific kind because they have certain dispositions and properties that make them dogs, as opposed to, say, frogs. And it is these dispositions and properties that constitute an essence. So, the fact that the world contains a multitude of particulars yet many of these particulars are unified only makes sense on essentialism.

Second, Mike seems to forget that our concepts are always derived from reality itself. He is so smitten with Model-Dependent Realism and Embodied Realism that he seems to think that our brains literally make this stuff  (like essences) up. (A common example he uses is the way our brain “creates” color. But this is false. Our brains don’t create color. They simply perceive the raw materials of sense datum in a certain way so as to “see’ color. But everything that causes us to see color (e.g. wavelengths and photons ) already exists in the world. So color does exist, it simply exists virtually instead of formally. All this is to say that we perceive color because color exists virtually in reality, and is not a simple construct of our brain. ) But if our concepts are abstracted from reality then this simply isn’t the case. To use an example, I can tell the difference between an apple and an orange. Why? Is it simply because my brain arranges the matter into two different categories? Or is it because there is a real difference between the properties and dispositions of an apple and an orange and my mind is abstracting those differences? Obviously it must be the latter. But if this is true, then there must be something about an apple that makes it different than an orange. And we know this to be true because my understanding of the essence of an orange is different from the essence of an apple. But these essences weren’t simply constructed by my brain, rather they are rooted in the properties of the objects themselves in mind-independent reality.

Mike then states the following:

[W]e can reject the Thomistic metaphysical gobbledygook on the principle of parsimony — the notion that cats have a distinct, non-physical property of 'catness' (their 'essence'), for example, is completely superfluous to our understanding and description of what a cat is. We can have a fully accurate, useful description of the animal simply by recognizing it as an amalgam of physical properties which our brains categorize in a particular way, and nothing more. There is no need to postulate any extra non-physical or 'metaphysical' properties to understand what a cat is, why it behaves as it does, or what it evolved from. Since the assumption of the existence of such things is not essential to our description or understanding of cats, we can discard it. We don't even have to demonstrate its falsity — i.e., somehow 'disprove' the existence of those metaphysical properties — we can simply discard them as superfluous and thus meaningless.

A few responses here. First, nobody has argued that postulating things like essences and identities are pragmatic. So why would Mike be looking for pragmatism here? Why? Because Mike has an empiricist presupposition, and anything that is not scientifically observable, predictable, and measurable is, to him, not pragmatic and therefore “superfluous.” But in order for Mike’s position to be vindicated, he would need to demonstrate that science is the only path to knowledge. But Mike knows that this is self-undermining. So why, then, should we believe that metaphysical concepts like essence and identity can be thrown out simply because science has no use of them? The answer is that there is no reason.

 Moreover, let it not be forgotten that science is only interested in the quantitative aspects of the world and not the qualitative. Yet, metaphysical concepts like identity and essence are not quantitative, and therefore the fact that they might not be pragmatic in scientific investigation is completely irrelevant, since science isn’t looking for them to begin with!

Second, Mike’s argument above implicitly affirms reductionism. That is, Mike believes that any entity or object is nothing more than an amalgam of its physical parts and components. But reductionism runs into problems. For example, if water is nothing but hydrogen and oxygen composed together, then water should not have any dispositions and properties different from its constituents. But surely water does have properties that differ from hydrogen and oxygen. So then water cannot be nothing more than its physical parts. At this point one might say that we should expect water to have different properties than its parts alone, because water isn’t simply hydrogen and oxygen, but hydrogen and oxygen bonded together. So of course hydrogen and oxygen bonded together will behave different than hydrogen and oxygen by themselves. But this is exactly the anti-reductionist’s point. If water behaves differently than its constituents, then there is something about water as a substance in itself that amounts to more than its parts. Thus stated, if a substance has properties that its physical parts do not have by themselves, then there is something over and above these parts alone that make up the dispositions of said substance, and therefore a substance cannot be said to be nothing but an amalgam of its parts.

Hence we see that Mike D has failed to make his case. First, he has shown that he doesn’t even correctly comprehend the position he is arguing against. He constantly rebukes those who recommend Scholastic literature to him, yet he’s shown that his research into this very area is minimal and is lacking. So why not at least buy a book that gives a formal defense of such a position? That would be my recommendation. Second, he has tried to demonstrate why metaphysical concepts like essence and identity are meaningless and superfluous, yet his arguments are rife with problems. Third, he’s attempted to affirm metaphysical theories like reductionism which, when considered, only lead back to the very metaphysical concepts he was attempting to overthrow. I maintain, then, that nowhere have we seen good reason to overthrow Scholastic metaphysics, let alone metaphysics themselves.

 

 

Death to metaphysics?

Logical positivism was a philosophical school of thought that was hot during the 1920s. The bedrock of this philosophy was the explicit rejection of metaphysics as a valid mode of inquiry into the nature of reality. The positivists believed that metaphysics was essentially meaningless, and that the only statements that harbored meaning were statements that could be, at least in principle, empirically verifiable. The first fruits of this philosophy could even be seen in the writings of David Hume in his Enquiry Concerning Human Understanding:

If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, "Does it contain any abstract reasoning concerning quantity or number?" No. "Does it contain any experimental reasoning concerning matter of fact and existence?" No. Commit it then to the flames: for it can contain nothing but sophistry and illusion.

While logical positivism as an organized philosophy has died out, there still remain many who, along with Hume, would affirm a death sentence to metaphysics. This contemporary school of thought still takes metaphysics as essentially ambiguous and superfluous.

 I myself, as a Scholastic, would disagree with this. The Scholastics—a school of philosophy which is actually increasing in adherents—would claim that not only should metaphysics be utilized, but that it must. It would be claimed by them that metaphysical concepts like identity, essence, form, substance, and act/potency etc., describe fundamental properties of reality, and therefore that such concepts are necessary for a sound framework regarding the nature of reality in the first place.

 However, the contemporary positivist scoffs at such assertions. They would claim that concepts like the ones mentioned above are, even if remotely accurate—of which the positivist would most likely claim they are not—completely superfluous for describing reality. Physics already tells us how one event can, and does, follow another, and thus the need for a description riddled with the act/potency distinction is not necessary. Likewise, physics and chemistry already enlighten us regarding what physically composes and makes up a certain empirical object, so that to add descriptive concepts like identity and essence on top of these physical explanations and descriptions is to upset the principle of parsimony. Science, it is seen by contemporary positivists, already tells us (or will eventually tell us) all we need to know about the objects and phenomena that make up our observable universe. Hence, the need for any metaphysical description is seen as completely superfluous.

 While such a position might seem rational, especially since science has been extremely successful in a pragmatic and predictive sense, I maintain that said position is not only false, but demonstrably so--and for multiple reasons at that. First, the picture that science provides of the material world can only be seen as comprehensively descriptive if one assumes ahead of time that reality can only be described in scientific terms and concepts. Theologian David Bentley Hart, in his amazing book The Experience of God, articulates such fallacious thinking:

And naturalism’s claim that, by confining itself to purely material explanations for all things, it adheres to the only sure path of verifiable knowledge is nothing but a feat of sublimely circular thinking: physics explains everything, which we know because anything physics cannot explain does not exist, which we know because whatever exists must be explicable by physics, which we know because physics explains everything. (p. 77)

The problem here should be obvious. Moreover, the claim that science gives us an exhaustive portrait of reality is itself a claim that only metaphysics, and not science, can make. So, even ignoring the question begging nature of positivism alluded to above, the positivist still has the problem of utilizing the very method they’re attempting to overthrow. They’re basically claiming the following: all metaphysics is meaningless, except for the metaphysical statement I’m making currently.

 The second problem that positivism runs into—as if being subject to question-begging and self-refutation weren’t bad enough—is that the scientific method already a priori rules out the kinds of descriptions that could be considered metaphysical in the first place. You see, the objective of science was to be able to control and predict the world around us using the language of mathematics. This means, and this is crucial here, that science is, already at the outset, only looking for a quantitative description of reality—thereby paying no attention to anything qualitative or metaphysical. This is to say, the scientific method already stipulates what kind of answers and descriptions it’s looking for. It’s not as if science, looking for the quantitative, will somehow stumble upon the metaphysical, because it’s not looking for the metaphysical. To use an example, science will not stumble upon the ‘essence’ of a substance, because science is not looking for ‘essences’ in the first place--precisely because a thing’s essence cannot be modeled by the language of mathematics and prediction. Philosopher Edward Feser, in his (also) amazing book Scholastic Metaphysics, further articulates my point:

The reason qualitative features don’t show up is not that the method has allowed us to discover that they aren’t there but rather that the method has essentially stipulated that they be left out of the description whether they are there or not. (p. 14)

And again, David Bentley Hart reinforces the point:

If we look exclusively for material and efficient processes, then indeed we find them, precisely where everyone, of nearly any metaphysical persuasion, expects them to be found. All this shows is that we can coherently describe physical events in mechanical terms, at least for certain limited practical purposes; it certaintly does not prove that they cannot also be described otherwise with as much or more accuracy. To paraphrase Heisenberg, the sorts of answers that nature provides are determined by the sorts of questions we pose of her. (p.65)

Moreover, there is a third problem for positivism. You see, science, and physics most importantly, only enlightens us regarding how nature tends to behave, or how certain events produce or tend to be produced by others. Atheist Bertrand Russell articulates:

[Physics] lays down certain fundamental equations which enable it to deal with the logical structure of events, while leaving it completely unknown what is the intrinsic character of the events that have the structure[…] All that physics gives us is certain equations giving abstract properties of their changes. But as to what it is that changes, and what it changes from and to—as to this, physics is silent.

 Now, the reader might be inclined to part ways with Russell here. “Surely” one might claim, “physics does tell us the intrinsic nature of things. Physics give us comprehensive knowledge of the physical characteristics of certain objects, and describes how it is possible for these objects to behave the way they do.” But this is only half true. No matter what explanations and descriptions that physics gives of certain characteristics of an object or event, these descriptions themselves will still only boil down to abstract mathematical equations and functions. For example, for physics to describe the behavior of object (x), physics will posit the description that A events follow B events, of which object (x) is a part of. But, A events and B events will also only be subject to further descriptions of certain other objects following certain other sequential events.  So, no matter how many layers of physics one traverses to explain a certain object or event, one will invariably run into nothing but abstract behavioral descriptions dancing to the tune of mathematics—and this, again, only gives structure, and does not enlighten us regarding any sort of intrinsic nature that these objects or events have themselves. I’ll pass the baton once more to Feser:

By the very fact that physics tells us that an abstract structure of such-and-such a mathematically describable character exists, then, physics implies that there is more to reality than structure itself, and thus more to reality than what physics can reveal. (p.18)

These three problems (there are others) articulated above suffice to show that positivism is, quite frankly, vacuous. Nowhere have we uncovered any cogent reason to believe that metaphysical descriptions are superfluous, nor have we seen any reason to believe that science gives an exhaustive description of reality. On the contrary, we’ve seen that metaphysical explanations and concepts are completely necessary, and are required, even in light of, and because of, science. Metaphysics, then, is here to stay. Thus stated, we shouldn’t be worried about utilizing metaphysical inquiry, but rather we should be worried about utilizing it fallaciously, and I think the positivist position is a good example of how this can be done.

 

Metaphysics vs Science? A dialogue with The A-Unicornist

Mike D, over at The A-Unicornist, and I have been engaging in a spirited discussion regarding the validity of metaphysical reasoning, which began as a discussion regarding the validity of the Scholastic concepts of actuality and potentiality (it began in the comment section here). Upon writing my most recent response to Mike, I felt that the length, as well as the topics covered in said response, deserved a post here on my blog. That being said, I also feel that Mike is one the most well-read and intelligent skeptics here on the blogosphere, and I believe that his perspective is founded upon well-thought out arguments. Therefore, I feel that a discussion such as the following is very fruitful and illuminating discourse. 

What follows (beginning with a quote from Mike) is my response to Mike's well articulated points:

It's trivially true that metaphysics are distinct from science – that one can't justify evidentialism with evidence, and so forth. All the empirical evidence in the world can't "disprove" idealism, and so forth.

I’m not really sure why you think this is “trivially true”. Is it only trivially true that biology and physics are distinct? If one’s epistemology asserts that method (a) is the only valid method for attaining knowledge about reality, then to admit of another method (b) to justify (a) would be to pull the rug out from that epistemology. This is exactly what happens to empiricism, and evidentialism as well.  

“However, consider that any metaphysical proposition is essentially a set of assumptions about what reality is.”

Not at all. You seem to think that metaphysicians simply wake up one morning and say “I’m going to postulate a concept called causation, let’s see how far I can go with it.” Metaphysicians begin with an observation of reality, and then derive their concepts. For example, a philosopher will see a billiard smash into another and, perhaps, abstract the concept of causation. Then, the philosopher will ask questions such as the following:

(1) Is this an example of event causation, or agent causation?

(2) Is causation simply the constant conjunction of events?

(3) Is causation transitive?

(4) Can causation be defined in more simplistic concepts, or is it irreducible and primitive?

Nothing in this endeavor is pure assumption. It is an abstraction of concepts from experience, followed by an inquiry into the nature and relations of these concepts. This is not something science can do, and it’s something that science relies on itself.

“So, unless we just want to engage in endless mental masturbation, metaphysical assumptions must be consistent with reality as we actually observe it.”

Maybe, or maybe not. It is arguable that there are metaphysical propositions so certain that no amount of empirical investigation can overturn. For example, we know that mathematics is based off of set theory which is based off of experience. (Just like the questions above are based on the concept of causation which has been abstracted from experience.) Some mathematics informs us of things we can be absolutely certain about, like 1+1=2. Now, what if we experienced a phenomenon such that every time we put two sticks together a third stick would spontaneously appear beside them? Would we then need to call into question our proposition that 1+1=2, and subsequently replace it with 1+1=3? Surely not.

“What I do know is that empiricism produces models of reality that reliably comport with independently verifiable observation. To paraphrase Stephen Hawking, science wins because it works. Empirical science has given us the ability to predict and manipulate the physical world.”

First, you seem to be pitting science and metaphysics against each other—Science wins because it works?--as if they were competing for a comprehensive view of reality. Science wins at describing the physical, metaphysics wins at describing the metaphysical, and that’s all there is to it—they are, contrary to what you say, complementary. The only out here is to claim that reality cannot be described by metaphysics. But this is, once again, self-refuting. You seem to want to assert that physics gives us as best and comprehensive a view of reality as we can obtain. But, to repeat ad nauseum, once you make a claim like that, you have ventured into metaphysics and ontology, and have (1) opened the door for metaphysical inquiry, and (2) put into question the very proposition that physics is our best lens into reality.

Second, of course science has given us the ability to predict and manipulate the physical world, that’s exactly what it was designed for. It was designed for pragmatic utility. But to claim that this allows us to declare science the victor over metaphysics is only to make a category mistake. Metaphysics is not in the game of utility, it is occupied with the fundamental structure of reality, and we have seen that, on pain of logical contradiction, science cannot take us this far.

“So when I'm presented with concepts like "potentiality" and "actuality", all I have to do is ask myself why I should believe they are descriptive of a fundamental reality, and not merely abstractions ascribed to the physical world by human minds. Is there any phenomenon, such as "change", that is either better explained or necessarily explained by potentiality and actuality? No.”

Personal incredulity is not enough to argue that concepts like act/potency are superfluous. The problem is that you’re trying to find the utility in utilizing these metaphysical concepts. And you think that since you can describe causation or change without reference to these concepts then they are unnecessary. But, you’re looking at it from the wrong perspective. It is not claimed that when describing change or causation one needs to use act/potency. Rather, it is claimed that the concepts of change and causation rely on these concepts in order to be intelligible. It must be true that the intelligibility of these concepts and concepts like them, of which I mentioned earlier, will not be illuminated by scientific inquiry. Science can say things like “event (a) exhibits causation,” but it cannot answer the question of what causation would need to entail for event (a) to participate in it.

“The laws of physics govern the interaction of objects in the physical universe, and the conservation of energy eliminates the need for a supernatural force to constantly intervene in or somehow govern the changes of energy and matter.”

The simple concepts of act/potency require no prima facie need for a supernatural force. By actuality one simply means the state whereby a substance is actual or existing, and by potentiality one means the potential realizations a substance can actualize. There is no need, at this level, for talk of supernatural forces. Act/potency are pure metaphysical concepts, and if one wants to utilize them for natural theology then we can determine whether this is valid or not, but the concepts themselves don’t necessitate such an endeavor.

“Potentiality and actuality simply add nothing useful whatsoever to our understanding of reality.”

You’re only begging the question here. As articulated above, they might not be necessary for an empirical description of reality, but they are quite necessary for a metaphysical description of reality. And to claim that the latter description is invalid is (1) self-refuting and (2) question-begging.

“The only reason you seem to assume their existence as fundamentally descriptive states of being is because Aquinas' argument from motion is meaningless drivel without that assumption.”

On the contrary, act/potency are metaphysical concepts just as foundational as identity, event, substance, composition etc. To claim that I only assume their existence for my own theological agenda is simply to beg the question.

“Occam's Razor tells us not to multiply assumptions beyond necessity. And I simply see no reason whatsoever to make the assumption that potentiality and actuality add any necessary descriptive value to our understanding of reality.”

You’re once again conflating the metaphysical with the physical. I concur that metaphysical concepts might not be necessary in describing events in physics—though I might not even go that far. But metaphysical concepts operate on a different plain than physical science.

Moreover, to claim that act/potency add no value to a description of reality is already to assume that reality can only be described in empirical terms. It is to beg the question in favor of empiricism. Act/potency surely add to metaphysical descriptions, and there is no logical way to deny the validity of metaphysical descriptions.