Substance and accident
As we’ve seen, in any instance of change, there must be both something that changes and something that underlies and persists through the change. In ordinary cases of change, the principle of persistence is a substance and the principle of change is an accident. For example, when a banana changes from yellow to green, its yellowness (an accident) goes from potency to act, and the banana itself (a substance) persists through the change. Likewise, when an object moves spatially, its location (an accident) changes while the object itself (a substance) persists.
What exactly is a substance? To put it in more familiar terms, a substance is an object and its accidents are its properties. A substance is an object that has independent ontological reality, so that accidents (like color, shape, and size) inhere in substances, but substances don’t inhere in anything else. The banana’s yellowness exists in the banana and can’t exist apart from it, but the banana itself exists independently of other things. A substance also has causal powers that are irreducible to the powers of its parts, which distinguishes it from a mere aggregate (like a pile of rocks). This gives us three interlocking principles for identifying a substance:
1. It underlies and persists through ordinary instances of change.
2. It has independent ontological reality.
3. It has powers that are irreducible to those of its parts.
Accidents, on the other hand, are what change during ordinary instances of change, and they don’t have ontological reality apart from the substances in which they inhere.
Using this definition of substance, the fundamental particles are substances. Fundamental particles persist through change (such as change in spatial location), they have ontological reality apart from other particles, and their powers are irreducible, since they have no (physical) parts that their powers could reduce to. Whether other substances exist depends on whether reductionism is true – Aristotelians are almost all anti-reductionists, accepting that (some) wholes are more than the sum of their parts, and that ordinary, natural objects like water, stones, and trees are substances. (I won’t defend this view here, but it’s worth noting that reductionism has been largely abandoned in chemistry and biology; see the SEP articles on these topics for explanation.)
The bundle theory of substances
There is some kind of substance/accident distinction, as even hyper-reductionists must accept about fundamental particles. But how exactly do substances relate to accidents? There are a few different views in modern philosophy. One of the most common views is the ‘bundle theory,’ which originated with David Hume. According to the bundle theory, objects are only the aggregates of all their properties. A banana is the aggregate of its yellowness, its firmness, its shape and size, its spatial location, etc., and nothing over and above those properties.
This view basically amounts to a denial that substances really exist; accidents alone exist, and so-called ‘substances’ are just aggregates of accidents. So does this denial of substance lead to any problems? Yes, there are serious issues with the bundle theory. First, it’s unable to account for persistence through change. The bundle of banana attributes that includes greenness isn’t numerically identical to the bundle that includes yellowness, in the same way that a pile of rocks isn’t numerically identical to the pile with one of the rocks replaced. Mere aggregates don’t persist through the replacement of their parts, without something underlying that ties them together and accounts for identity over time. Therefore, denying the reality of substance leads to the Heraclitean error.
The bundle theory also leads to a few other puzzles. If objects are merely bundles of accidents, why can’t an accident leave the bundle and exist on its own or in another bundle? It’s impossible for this banana’s yellowness to exist in any other object. For that matter, what makes the bundle stay together if there’s no substance underlying it? The banana’s color, shape, and size can’t break away from each other, of course, but it’s difficult for the bundle theory to account for this fact.
Finally, what distinguishes different objects’ accidents from each other? A substance theorist can say that this banana’s yellowness and that banana’s yellowness are distinguished because they inhere in two different substances. A bundle theorist, on the other hand, has to say that they’re distinguished by the bundles to which they belong. But the bundles are themselves defined by their accidents, since there’s nothing else underlying them. Therefore, the bundle theorist’s explanation for how yellowness in general is tied down and distinguished in different objects is circular.
Bare substrata?
These problems with bundle theory show that substances must really exist and underlie their accidents. But if substances by themselves have no accidents, are they nothing more than bare substrata or ‘thin particulars’ with no features? This has often been seen as the alternative to bundle theory. John Locke, who accepted the existence of substances, famously referred to substance as “a something I know not what” because of its apparent featurelessness.
However, this bare substratum theory is also problematic. It leaves no room for an explanation of what a thing is, and why certain accidents are proper to some things but not others. (For example, it’s proper for a human to have arms and legs, but not a banana.) Is it right to say that “Bertrand Russell could have been a fried egg” or that “this banana could have been a dog”? It seems intuitively wrong to say that one and the same thing could have been two very different kinds of things. But according to the bare substratum theory, these statements are true, since the underlying substance is utterly featureless with nothing to determine which accidents are proper to it.
This problem demonstrates the need for the Aristotelian notion of essence, the quiddity or “what it is” of a thing. Substances aren’t utterly featureless substrata; they have essences intrinsic to them, that determine which accidents are proper to them. One and the same substance can’t be a human or a fried egg, a banana or a dog. It could only be one of these things because it has just one essence. In order to preserve the substance/accident distinction (and avoid bundle theory), essences can’t be accidents or bundles of accidents; instead, essences are things from which certain accidents (proper accidents, or essential properties) flow.
Matter and form
The reality of change and persistence, in ordinary instances of change, show that there must be a real distinction between substance and accident. But what about substantial changes where one substance changes into another? In this case, what underlies the change can’t itself be a substance; it must be something that isn’t a substance, but is capable of taking on new accidents and powers. Aristotle referred to this as “matter” (hylē). And what changes can’t be an accident, since entirely new powers are taken on during substantial change; it must be something in virtue of which matter comes to have new accidents and powers. Aristotle referred to this as “form” (morphē).
If substantial change occurs, then physical objects must be composites of matter and form. As it turns out, even hyper-reductionists must accept the reality of substantial change (and thus of matter and form), since fundamental particles can decay into other types of fundamental particles. For example, a muon decays into an electron and two neutrinos. Therefore, even if fundamental particles are the only real substances, they must be composed of matter and form.
Later Aristotelian philosophers further distinguished between prime matter and secondary matter, and between substantial form and accidental form. Prime matter is utterly featureless, pure potentiality, which can’t exist apart from form; substantial form is what informs prime matter with an essence. These are the principles of persistence and change, respectively, in cases of substantial form. Secondary matter is matter that is already informed by some substantial form; accidental form is what informs secondary matter with its accidental properties, which can change without changing the substance itself.
Let’s return to our banana example. In this case, the banana itself is a composite of prime matter and substantial form; in other words, it’s made up of secondary matter. When it changes from green to yellow, the accidental form of the banana changes, but its substantial form remains the same (i.e., it continues to be a banana). Or take the example of a statue. The clay that makes up the statue is a composite of prime matter and substantial form. The statue itself, on the other hand, is composed of secondary matter (i.e., the clay) and an accidental form (i.e., the shape of the statue).
Forms as universals
We’ve already seen, in three different contexts, the need for essence/substantial form in metaphysics. But are essences particulars or universals? That is, does every individual substance have its own unique essence, or is there such a thing as an essence like humanity that’s shared by every human? We can ask the same question about accidental forms. For example, does each ‘red’ thing have its own unique accidental form that we call ‘redness,’ or is there really such a thing as redness that’s shared by every red thing?
Some philosophers deny the reality of universals, arguing that even though we call different things by the same name (e.g., ‘human’ or ‘red’), each thing is really completely unique. This position is called nominalism. One of the most prominent nominalists is William of Ockham, who used his famous razor to ‘shave off’ universals from his ontology, since he thought that they were superfluous and didn’t actually explain anything.
However, there are a few serious problems with nominalism. For one thing, it seems to be incompatible with the pursuit of logic, mathematics, and science. Science relies on our ability to generalize the results of our experiments. For example, if I perform an experiment on the group dynamics of a population of fish, I have to assume the results can be generalized (at least) to the whole species, not just that population, or else my experiment is basically pointless. Likewise, when someone discovers a new valid argument form, or a new mathematical proof, we assume that the results can be generalized to multiple situations, or else the discovery is basically useless.
Furthermore, nominalism has a problem with explaining resemblance between things. According to nominalists, red things don’t instantiate the universal redness, but merely resemble each other in such a way that we call them ‘red.’ But as Bertrand Russell noted, in this case resemblance is also a universal, which is instantiated in multiple relationships between things. The nominalist could respond that we call these relationships ‘resemblance,’ but it’s not mind-independent; we only call these instances by the same name because of some other relationship between them, let’s call it super-resemblance. But this would also be a universal. So if nominalism is true, the resemblance between things is either a vicious infinite regress, or (if we stop the regress at an arbitrary point) a brute fact.
Finally, the nominalist says that the universal redness doesn’t exist, we just use the word ‘red’ to describe different things. But words are also universals! If nominalism is true, then each time someone uses the word ‘red,’ it’s not actually the same word, just different words that resemble each other. But communication relies on words having determinate meaning; if they didn’t, then we couldn’t really know what other people mean by a word. This makes nominalism self-undermining, since no one else could truly understand what the nominalist’s arguments mean.
These problems show that (at least some) universals must be real. Even if redness and other colors aren’t universals – suppose that they’re reducible to the wavelengths of photons – even hyper-reductionists have to agree that some universals exist. (The laws of nature depend on universals being real; for example, Coulomb’s law claims to be generalizable to all charged objects, which relies on the reality of negative charge and positive charge as universals.)
Therefore, substantial forms (like humanity and electron-ness) and accidental forms (like negative charge) are universals. This further demonstrates the need for matter (in the Aristotelian sense), as a limiting principle of form. The substantial form banana-ness doesn’t exist in any spatial location, nor is it identical to any particular individual; it requires matter to tie it down to a specific location and individual, making a concrete banana out of abstract banana-ness.
Material and formal causes
This understanding of matter and form allows us to round out the Aristotelian theory of causation. In Aristotle’s thought, a cause is anything that explains “why”: why a thing is the way that it is, and changes the way that it does. We’ve already seen his concept of efficient cause, which is what we usually mean by “cause,” and final cause, which is a thing’s disposition to act and change in a particular way. There’s also material cause, which refers to the matter that makes up a thing, and formal cause, which refers to the form of a thing.
All four of these types of causes interact to explain why a thing is the way that it is. Take the example of a statue. The efficient cause of the statue is the artist who sculpted it. The final cause of the statue is its end purpose, which could be decoration. The material cause of the statue is the clay from which it is made. The formal cause of the statue is its shape. Each of these things partially explains the “why” of the statue, and all four together give a complete explanation of its “why.”
Essence and existence
So far, we’ve examined cases of accidental and substantial change, but what about the most extreme cases of change: coming into existence (creation) and going out of existence (annihilation)? Aristotle didn’t investigate such instances of change, but later Aristotelians did, like Ibn Sina (Avicenna) and Thomas Aquinas. In cases of creation and annihilation, the principle of change must be existence itself, since a thing goes from existing to not existing (or vice versa). What persists through the change is the (universal) essence of the thing. For example, if our banana ceases to exist – even if all bananas cease to exist – then banana-ness in the abstract will persist.
There are a few ways to avoid this conclusion. You could simply deny that anything is really created or annihilated. The first law of thermodynamics – that energy can’t be created nor destroyed – might support this view. However, that law only tells us what is physically impossible, not that creation and annihilation are metaphysically impossible. Furthermore, it says nothing about the creation or annihilation of things other than energy. Science tells us that there was a time when no humans existed, when no liquid water existed, even when no atoms existed... Since these things are kinds of substances (in the Aristotelian sense), the essence/existence distinction would apply to them.
But why should we believe that anything persists through the annihilation of a thing? Simply put, because what it is to be that thing (i.e., its essence) would remain. Even though (non-avian) dinosaurs have long been extinct, dinosaur-ness persists (otherwise we couldn’t study them). Likewise, even though unicorns don’t exist (and have never existed), we can grasp unicorn-ness. Even if there were no countable objects in the world (e.g., if only God existed), mathematical truths like 2 + 2 = 4 would still be true. And if all bananas suddenly went extinct, it would be possible for a banana to exist again in the future (for example, through genetic engineering), which shows that banana-ness persists.
Acts of existence
We’ve now seen that essence and existence must be distinct, but what exactly is existence? We can run through the same possibilities that we did about the laws of nature: either existence refers to something extrinsic to a thing, something neither extrinsic nor intrinsic, or intrinsic to a thing.
If existence refers to something extrinsic, it could be something personal outside the natural order (i.e., God) or something impersonal outside the natural order (i.e., a Platonic form). The first option directly leads to pantheism, since it means that the existence of anything just refers to God existing. Most people won’t want to accept this huge theoretical cost. The second option says that things exist by participating in some Platonic Form of Existence. But this leads to a vicious circle: the Form of Existence exists, which means that it exists by participating in itself, which means that it’s explanatorily prior to itself.
This shows that a thing’s existence can’t merely refer to something outside of itself. But what if existence is simply a fact, not any ontological reality extrinsic or intrinsic to a thing? This is similar to the Frege-Russell-Quine view held by most analytic philosophers, who see the statement that “xs exist” as meaning nothing more than “there are xs,” not analyzable as anything more than that. But this view doesn’t explain how some possible kinds of things exist and others don’t, it simply asserts that they do. (In fact, on this view, statements like “unicorns don’t exist” are contradictions, since they boil down to “the existing unicorns don’t exist.”) This boils down to the Parmenidean error of denying that potentials exist, since it denies that we can make true statements about anything that doesn’t actually exist. (For further problems with this view, see Nevitt 2018.)
If existence isn’t something extrinsic to a thing, and it isn’t neither extrinsic nor intrinsic, then it must be intrinsic to a thing. This leads to the Aristotelian notion of existence as an act which is intrinsic to a thing. For the reasons described earlier, a thing’s act of existence must be really distinct from its essence (at least, in ordinary things). Nor can a thing’s existence flow from its essence, because then every possible essence would exist, which isn’t the case. Its existence must be prior to, and actualize, its essence. (This naturally leads to the question of what unites a thing’s essence to its act of existence, but that’s outside the scope of this post.)
We can arrive at the same conclusion if we consider what accounts for the existence of a material substance (matter-form composite). Form by itself can’t account for the substance’s existence, since then every possible form would actually exist as a substance. Matter by itself (i.e., prime matter) is pure potentiality, so it can’t account for the actual existence of anything. The matter-form composite can’t account for its own existence, since this would be a vicious explanatory circle. Therefore, there must be something in addition to and prior to both matter and form, which accounts for the substance’s existence: this is its act of existence.
The unity and simplicity of existence
In his work De Ente et Essentia, Aquinas offers another argument for the real distinction between essence and existence. He tries to show that, if there were a being whose essence is its existence, there could only be one such being and it would be simple (without parts). Since none of the things we ordinarily observe are like that, essence and existence must be really distinct in all those things.
First, consider that this thing in which essence and existence aren’t distinct must have a perfectly simple essence. Existence couldn’t merely be a part of its essence, because existence would have to be prior to the other part(s) of its essence and actualize them (see above). There would still be a real distinction between its existence and this other part(s), so the other part(s) would be the real essence of the thing, and it would still be an essence-existence composite. Something that has no essence/existence distinction would have to have an essence that is pure existence with no composition.
It follows that this thing would also not be a composite of matter and form, since its essence (substantial form) would be to exist, and it would subsist on its own without entering into composition with matter. It wouldn’t be a composite of substance and accidents, since it would be a self-subsisting substantial form, and accidental forms inhere in material substances – not substantial forms. Since it isn’t composed in any of these ways, it would be immutable, unable to undergo any of the types of change we’ve examined, and therefore timeless. Furthermore, it would necessarily exist, and would be unable to come into existence or go out of existence, because its essence is to exist.
Now consider the following principle:
Identity of Necessary Indiscernibles (INI): it is impossible for there to be separate entities that have all of their properties, including possible properties, in common.
Note that this is a weaker principle than the Identity of Indiscernibles, which is the above principle minus the “including possible properties” caveat. There are several proposed counterexamples to II, but those don’t apply to INI. INI says that if it’s not even possible to distinguish between two things (i.e., they necessarily share the same properties), then they’re one and the same thing.
If there were two beings of pure existence, they would both be an individual, self-subsisting substantial form which just is pure existence. They couldn’t be distinguished by something over and above existence in their essence, since their essence is pure existence. They couldn’t be distinguished by different parcels of matter, like we normally distinguish different material substances of the same kind, since they have no matter. They couldn’t be distinguished by different accidents, since accidental forms inhere in material substances, not in other (substantial) forms. Since it’s not even possible to distinguish two beings of pure existence, there could only be one such being.
This shows that a being whose essence and existence aren’t distinct would be utterly unique, simple, immaterial, immutable, timeless, and impossible to come into or go out of existence. Such a being, if it exists, could only be God. Since none of the natural objects we ordinarily observe are like this, existence and essence must be really distinct in all those things.
Conclusion
Understanding the reality of change and persistence allows us to carve reality at its joints and find the metaphysical realities that underlie the physical world, in line with Aristotelian metaphysics. From these principles we can deduce the reality of (and distinction between) substance and accident, matter and form, and essence and existence. All the natural things we see in the world are metaphysical composites of these underlying realities.
Why is this important? For one, even though we don’t often think about these things, they’re necessary for us to truly discover anything about the world. Philosophers and scientists who deny the reality of any of these principles inevitably fall into the Parmenidean error of denying change or the Heraclitean error of denying persistence, which undermines their own pursuit of truth. Furthermore, carving reality at its joints allows us to get at the most basic level of reality, beyond what science can discover. This furthers our knowledge of the world, which is valuable in its own right.
Finally, Aristotelian metaphysics, better than any other metaphysical framework, helps us discover the existence of God. Aristotle himself, even though he lived in a pagan context, deduced from his own metaphysics the existence of a transcendent, unique, personal, intrinsically relational Being at the most fundamental level of reality. He effectively developed the first philosophical argument for God’s existence, and his arguments were further developed and improved upon by his followers (like Thomas Aquinas). I didn’t really get into arguments for God in this post, but I might in the future.