It can be difficult to debate Thomists, precisely because they adopt a highly esoteric lexicon and apparently idiosyncratic definitions of their key terms that are not always obvious. For example, a Thomist might tell you that you cannot so much as think about a thing without thinking about its identity — the very thing which allows us to distinguish one thing from another. And surely, we all agree that our brains can and do require us to categorize things and make distinctions between them in order to coherently think about them. But what the interlocutor may miss is that the Thomist is saying more than that — the Thomist is claiming not just that our brains organize patterns into discrete categories and objects, but that objects have, independently of the brain, the property of identity.
This is a subtle but critical distinction. Because surely, again, we 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 categorize as simply "cats", even there 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?
The silliness of such a proposition can possibly be illuminated by looking at things in evolutionary terms. Let's take 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? If we were to watch a computer simulation of mountain formation, sped up so something like Everest formed in a few minutes, would the Thomist be able to pause the video at the exact right moment and say "There! Now it has the property of identity of a mountain!" Where might this property of identity have come from? Did it evolve from some lower-order property, or did God just put it there?
This line of thinking extends to other terms integral to the Thomistic view, like essence and nature. From my reading, the distinction between identity and essence seems a bit fuzzy, but to use the example of the cat again, we can say that the 'essence of cat' is whatever it is that makes it a cat. A cat has some sort of 'catness' — its material form and the properties that arise from that material form. Try to wrap your head around this convoluted explanation:
The essence of material substances is composed of substantial form and prime matter. Substantial form is the source of the specific identity or identity as a species, as a human being, as a dog, etc. Prime matter is pure potentiality to be specified, determined, activated by the form. It is the principle of individuation: it multiplies the form and accounts for diversity within the unity of the form or species by receiving and restricting the form to “this” material subject–the possibilities of the species are not exhausted by an individual. For example, “humanity”is multiplied into “many human individuals” by matter.This illustrates why conversing with Thomists is often confusing; the term 'prime matter' here is not being used to refer to 'matter' in the physical sense (that's the 'substantial form', or the 'form' — a cat, in this instance — that the substance takes). The IEP clarifies it a bit:
If we think of matter as without any form, we come to the notion of prime matter, and this is a type of matter that is totally unformed, pure materiality itselfThis is the point when any rational person should want to say Whoa, hold up. "Pure materiality"? "Matter that is totally unformed"? What does that even mean? Suffice to say that no person has ever observed unformed matter. The IEP goes on to explain... sort of:
prime matter, as pure potency, cannot in fact express any concrete mode of being, since as pure potency is does not exist except as potency. Thus, prime matter is not a thing actually existing, since it has no principle of act rendering it actually existing.Here we get to the central confusion that Thomists trip over — they inadvertently blur the distinction between conceptual abstractions and literally real objects. By the description here in IEP, 'prime matter' by definition doesn't exist; it's just an abstraction of the concept of matter. And yet somehow, on the Thomistic view, this 'pure potentiality', which doesn't actually exist, is somehow able to be acted upon — specified, determined, and activated by the 'form' of things that do exist. To which I have only one reaction:
How does something non-existent interact with existent things, or vice versa? Well, that's a whole other pile of confusion because, according to the Thomist, existence is just a property that something does or doesn't have. The SEP clarifies:
While [material substances] exist, their existing is not what they are. Thomas accepts from Boethius that it is self-evident that what a thing is and its existing differ (diversum est esse et id quod est).Yes, they differ, in our minds. In reality, things cannot have properties if they do not exist, and things especially cannot interact with other things or be acted upon if they do not exist. Cognitive abstractions do not have properties, but abstractions of properties. This is how we know, for example, that unicorns are imaginary and not real. Unicorns don't have the property of being equines, of having horns, of being able to traverse rainbows, etc., because unicorns are fucking imaginary. Rather, unicorns have the conceptual properties of being equines, having horns, etc.
And that's the rub with Thomism. 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.
To reiterate, this is of course not to say that cats don't have distinct properties, or that cats would cease to be cats if humans suddenly disappeared. But we 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.
In fact, the whole proposition of metaphysical properties just complicates our understanding of the physical world. We know, for example, that the domestic dog evolved from wild wolves. Yes, even Binky the Pekingese is an evolutionary descendent of the mighty wolf, and even science-denying IDers and creationists will not dispute this fact. So at what point does the 'essence of wolf' become 'essence of domestic dog'? This was a gradual process, taking tens of thousands of years as wolves lived on the edges of human settlements; more docile wolves were artificially selected by humans and, over the millennia, became the companion dogs we know and love today. Where does the essence of one species end, and the other begin? How do these essences and substances interact with one another, particularly if they are (in the case of prime matter) non-existent? At what point does a population of wolves take on the metaphysical property of 'dogness'? None of this terminology illuminates anything at all about what these creatures are or the process of evolution itself; it unnecessarily complicates the issue by layering on superfluous and unjustified assumptions.
Aquinas, and his forebearer Aristotle, did not know about the evolution of species nor about the eons over which the earth slowly changed. They had no concept of quantum superposition or indeterminacy. The idea of emergent properties was foreign to them. They seemed to think that something was either this or that, clearly and completely distinct and precisely the way God made it to be. Similarly, they did not understand the mind like we do today; they did not understand that concepts are neural structures in the brain, and not necessarily representative of actual things. And that is why their metaphysical musings are irrelevant; they were a failed science, an ambitious but hopelessly hamstrung attempt to make sense of the world around them and, more importantly, backwards-rationalize the existence of the God they assumed exists. But we no longer have any reason to take their claims about reality seriously, much less take seriously any conclusions about the divine that they hoped to gleam from their metaphysics. Thomism can go to the dustbin, with the rest of metaphysics.