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The Problem of Change
Aristotle’s first task in giving a complete, coherent account of particular sensible things and the fact that they seem to be constantly changing, a fact apparent to anyone (though, perplexingly denied by Parmenides), is to confront and overcome Parmenides’s seeming ironclad logic and the Problem of Change he bequeathed to philosophical posterity which not even Plato had adequately, to Aristotle’s mind at least, solved. Bound by logic, Parmenides had been forced to the position that there is in reality no change at all. All change is illusory, unreal, Parmenides insists. Instead, reality is and must be One, and this One, which only is, is unchanging. As he understood the terms of the problem, change is conceptually impossible.
Parmenides had argued that there are only two alternatives, being and non-being. A new being cannot come from non-being since nothing comes from nothing. Nor can a new being come from being since then it would exist already and not come into being.[1]
Being cannot come from non-being, since non-being is nothing, and nothing comes from nothing. Nor can being come from being, for what has being already exists and does not begin to be; being cannot come from being since it is already. Since for Parmenides there is no way for being to come to be, he concludes it does not. Being just is. Nor are some beings distinct from anything else that is (since only non-being would be distinct from what is). Being, therefore, is One, whole and unchanging, immaterial, and eternal.
Aquinas sees Parmenides’s mistake as lying in his failure to distinguish between different senses of being as manifested in the ten categories Aristotle delineated in his logic, that is, the fundamental distinction between substance and accident.
Parmenides assumed false propositions, because he held that what is – namely, being – is said simply (simpliciter), that is, in just one way, when in fact, it is said in many ways. For “being” is said in one way for substance and in another way for accident, and the latter in many ways according to the different genera [categories]. “Being” can even also be taken as common to both substance and accident. Hence, it is clear that the propositions assumed by Parmenides are true in one sense and in another sense they are false.[2]
Aristotle realized that a particular sensible thing, an apple, for instance, is able to change, e.g., from green to red, because it is not a being in just one sense. As he teaches in his logic, “is” is used in various, analogous ways. The apple is a fruit, and it is green. It exists in itself, not in another, as a substance, and the species ‘fruit’ is predicated of it. But it is also green, and this green exists in this apple, but the universal species of color (greenness) is also predicated of the color of this apple. But again, as Aristotle had taught in his logic, this apple is being in the primary sense; it is first or primary substance because without it this green would not exist, nor would the species of fruit, the universal “appleness,” exist. Nor indeed without individual particular substances would universal “greenness” or color or quality, etc. exist. An apple can change color because it is an apple and it is green, but it is not (yet is able to be) red, the color it will become.
When it comes to explaining change, Aristotle thus invokes another distinction in the senses of meaning “being” may have (the distinction, in metaphysical fact, upon which the distinction between substance and the nine categories of accidents is built): actual being and potential being. What changes, in itself, as a substance, may have various accidents, at different times. In itself, the substance is potentially one or another of them. The apple in our example may be either green and red, but not both at the same time (in the same part), and while it is green, it is not actually red, but potentially so.
Aristotle agrees [with Parmenides] that nothing comes from nothing. He argues, however, that there is a sense in which being can come from being if one makes a distinction between being-in-act and being-in-potency. Being-in-act cannot come from being-in-act since it would already exist, but being-in-act can come from being-in-potency. There is no middle ground between being and non-being. But between being-in-act and non-being there is an alternative: being-in-potency. A potency is a capacity for some actuality that one does not yet possess but can acquire.[3]
Whatever comes to be cannot actually already be what it will become, yet it must have a potency to become such. This potency is not simply nothing, for not everything can become everything that it is not. But being a certain type of thing, i.e., a substance of a given sort, it is able to change in certain determinate ways; that is, it is in potency to certain actualizations. A non-red (i.e., green) apple may become red, but a non-grammatical apple may not become grammatical since it does not have the potency to learn grammar. Potency is in a certain sense non-being, but it is not nothing, and whatever is, and can change, is only in potency to a given set or range of actualities.
Three Principles of Change
Thus, in his analysis of change, Aristotle discovered that every change implies duality. It implies a subject in potency which, by the action of some agent, becomes actually different, i.e. receives some new characteristic, a new perfection or actuality. Motion or change (for Aristotle and Aquinas, these terms are practically synonymous (see below)) presupposes the acquisition of something and the loss or corruption of something else by a subject that undergoes the change. The subject of change, however, stays the same through the process, acquiring something new while thereby losing what it previously had. Thus, in order for change to take place, there must be something in potency to a new actuality, and this duality is intrinsic to what changes. Aristotle discovered these concepts of act and potency by observing cases where it is most obvious: when a substance loses an old accident and acquires a new one. These are thus termed accidental changes.
Accidental Change
Michelangelo is alleged to have said that when he set out to sculpt a statue from a formless block of marble, he sought only to remove the excess marble from the statue that was already there inside the block. The idea that there is something within a substance in potency to becoming actualized by an agent (in this case the artist) lies at the heart of Aristotle’s philosophy of nature, which Aquinas adopts. One can see this also when the subject of the artist’s efforts is clay instead of marble.
An artist, for instance, can shape a block of clay into a statue. How is this possible? Because the clay is endowed with a certain property – the possibility or potentiality for acquiring a new shape. The figure of the statue is in potency in the clay. This potency is real, not with the reality of being-in-act, but with the reality which corresponds to being-in-potency. Potency is not just an idea in the mind of the observer. It is a physical principle of the material object. Some objects possess certain potencies while others do not. Clay has the potency (the “wherewithal”) to be shaped into a statue, but water does not.[4]
Clay has certain potencies, a combination of malleability and firmness, to be shaped by a sculptor and to retain that shape, which water or Jello do not have, which marble also has, but to different degrees. These potencies reside in these different materials and allow them, in their own ways, to become actual statues. Water and Jello, lacking these potencies, cannot be so shaped.
Thus, there are three principles necessary for change to take place. There must be something new that comes to be, something old that passes away (the lack of what is new), and something that stays the same throughout. Aristotle, and Thomas Aquinas after him, call these three principles: form, privation, and matter. At the end of the first chapter of his short early work, On the Principles of Nature, Aquinas explains these three principles of change:
Therefore, three things are required for something to come to be, namely, being in potency, which is matter; not-being in act, which is privation; and that through which something comes to be in act, which is form. For example, when from bronze a statue is made, the bronze which is in potency to the form of the statue is the matter; what is shapeless or undisposed is called privation; and the shape because of which it is called a statue is the form. But not in the sense of substance because the bronze, before receiving the form or shape, has existence in act, and its existence does not depend on that shape. Rather, [the shape] is an accidental form, for all artificial forms are accidental.
In the beginning of the next chapter of his work, Aquinas immediately continues and explains how privation and potency are in the subject of the change, i.e., what is already a complete substance, vis. bronze in his example.
Therefore, there are three principles of nature, namely matter, form, and privation. One of these, form, is that toward which generation tends (id ad quod est generatio); the other two are [found] on the part of that from which there is generation. Hence matter and privation are the same in subject but they differ in idea (ratione); the same thing that is bronze is shapeless before the advent of form, but it is called bronze for one reason, and shapeless for another. Wherefore, privation is not said to be a principle essentially (per se), but rather incidentally so (per accidens), because it coincides with matter.[5]
Form is what comes to be, privation (the lack of that form) is what passes away, and matter is what stays the same throughout the change. In the case of a statue, the shape of the sculpture, Michelangelo’s “David” for instance, is the form that comes to be when a formless block of marble becomes a statue. The formlessness of the block is itself the privation of the statue shape, and while continuing to be marble (and not water or jello), the block is the potency for the statue shape. The marble, first in block shape, later in “David” shape, is what stays the same throughout the change, and so it is matter to the change in shape.
Aquinas also notes in On the Principles of Nature, chapter 2, that matter contains both the privation of form and the potency for form – the former only prior to the change, while it retains the latter (potency) also after the change has been accomplished.
Matter is never without some privation, for insofar as [while] it is under one form it has the privation of another, and conversely. … Because generation does not come about simply from non-being, but from the non-being which is in some subject, and not in just any [non-being] but in a determined subject – because fire does not come to be from just any non-fire, but from such non-fire as is apt to receive the form of fire – therefore, we say that privation is the principle [and not non-being]. Matter is that in which the form and privation are understood, just as in bronze the shape and the shapeless is understood.[6]
As the potency of what comes to be, the matter (bronze or marble for a statue) first has the privation of the form that will come to be, and this is a sort of qualified non-being. The matter also has the capacity to take on that form, and this is not nothing, but it is not actual, either. This is what Aristotle and Aquinas mean by the potency of matter, and the potency for a given form remains even when it has been actualized. As Aquinas says in this same chapter, “Privation differs from the other principles in this: the others are principles both in being and in becoming, but privation is only a principle of becoming.” This is because once the form has actualized the corresponding potency, the matter no longer lacks that form – it loses the relevant privation. But the matter continues to have the capacity for that form; otherwise, it would not be in this new way, actualized by the new form. Marble, once it has been formed into a sculpture of David, loses its shapelessness, but as marble, it continues to retain the capacity to hold the shape. The marble can no longer become a statue, but the marble allows it to be a statue. Thus, at the end of the change, the marble has a new shape, and in general, matter continues to exist as actualized by a new form.
This case of the coming to be of a statue is an instance of an accidental change; what changes are the accidents of the marble, while what stays the same and endures through and after the change is the substance of the marble.
Aristotle called this principle of potency hyle (matter). He gave the name morphe (form) to the corresponding principle that actualizes the potency. His explanation of the first principles of motion [or change] is therefore given the name hylomorphism (hyle + morphe).[7]
Aristotle and Aquinas thus understand the results of accidental changes as composites of substance (i.e., secondary matter) and accidental forms.
The matter is that which persists through change. The form is that which is acquired or lost by the matter. When a substance gains an accidental form, it changes accidentally; when some matter acquires a new substantial form, a new substance is generated.[8]
Changes to substances are changes of accidents; we will discuss changes of substances, below.
One should remember from our consideration of logic that there are nine categories of accidents, i.e., nine kinds of attributes a substance can have or ways it can be modified while remaining the same substance: quality, quantity, place, time, acting on something, being acted upon, relation, position (or arrangement of parts), and having (shoes, clothes, armor, or weapons on). Technically, a change in any and all of them could be described as the coming to be or receiving of an accidental form by some substance, but usually only the first three – quality, quantity, and location – are so described, and mostly the first. One could say a man receives the form of ‘being a father’ (relation) when he conceives his first child, or a woman actualizes her potency to hammer a nail by receiving the corresponding form (acting on something: hammering), etc. But such ways of speaking are extremely unusual, mostly because all but the first three types of accidents are changes in a substance’s relationship to themselves or other substances, and they are grounded in the accidents of location, quantity or quality. Likewise, they depend on a mind recognizing this change in relationship (these accidents are sometimes called ‘beings of reason’) and not true modifications of the substance itself. Changes in quality, quantity, and location are genuine modifications of the substances that undergo them, and they also admit of degrees or becoming more or less: more or less green or red; more or less tall or heavy; more or less at the end of its journey. As we will see below, Aristotle and Aquinas understand changes in quality, quantity, location as “motions.”
To review, in every change there is, first of all, something that receives a new determination. Before undergoing the change, it lacks the determination it will acquire, and so it has a privation of, and is in potency to a new determination; then under the action of some agent, it receives a new actualization. The marble upon which the sculptor works lacks, yet is in potency to receive, the new determination which the sculptor gives it. This new determination is the form of the statue. Again, this is a case of accidental change, since what comes to be is a new accidental form (the shape) in what is already a substance (the marble). There are also cases of substantial change, that is, cases of new substances coming to be.
Substantial Change
In addition to substances undergoing changes in their accidents, Aristotle saw that substances themselves also change into other ones, a new substance appearing from the destruction of a prior one. This cycle of the coming to be or generation of one kind of thing from the passing away or corruption of another collectively constitutes the ebb and flow of the natural world. That is, not only does an apple change its accidents from green to red or from being in my hand to being in my stomach, but the apple changes from being an apple in my stomach to becoming (part of) me. Or again, not only does wood become darkened and hot by fire, but something new, ashes and smoke, come to be as wood is consumed (corrupted) by fire. Although Aristotle discovered the three principles of change (matter, form, and privation) by analyzing accidental changes, he found that they also explain these more fundamental kinds of changes, changes that constitute the passing away and coming to be of substances. As with accidental changes, substances change in this more radical or absolute way when one substance (wood) becomes another (smoke and ash) through the acquisition of a new form and the loss of a prior one by something that persists through the change, that underlies both forms, and that is potentially both of them. And since these are substances which come to be or cease to be, the kind of form that defines and specifies what each thing is before or after the change is their substantial (as opposed to accidental) form, and the kind of change is substantial change, technically generation and corruption. Likewise, what underlies each substance and what persists through such changes is matter in a more fundamental sense, called prime or primary matter (materia prima).
Without prime matter, there could be no substantial change, because there would be no subject of change that persists through the change. There would rather be the complete annihilation of one substance and the creation of another utterly novel substance in its place.[9]
Not only does some new substance replace some prior one, but there must be something that persists and underlies the transition from the prior to the new substance for that transition to be a change, and not merely one thing popping into existence after another popped out.
Many of the more easily recognizable cases of substantial changes occur in the world of living things: the generation or corruption of plants and animals. But non-living substances come to be and cease to be, as well. As we have seen, wood becoming smoke and ash is one example. The log before the advent of fire is actually wood, but potentially ash, and so is a composite of its (one, or as we will see, several) substantial form(s) and prime matter. Once the fire has burnt the log, it is no longer wood; the wood has ceased to be, and in its place, there are now ashes (along with the smoke that has drifted into the air and been blown on the winds). Ash and smoke are new substances, actualized by the fire; new substantial forms for ash and smoke now come to be in the matter that was once wood. Substantial changes, generation and corruption, happen whenever different kinds of things become others, and these occur constantly in the physical world, as when iron rusts, coal becomes diamond under immense pressure, or when hydrogen and oxygen combine to form water, and countless other changes in substances. In all cases, prime matter loses one or more substantial form and acquires new ones as substances undergo corruption and generation.
We know that these are instances of substantial (and not accidental) changes because what comes to be has properties and powers (active and passive potentialities) which define and specify different kinds of things and were not present in the substances that ceased to be and were corrupted. Smoke and ash cannot burn, while wood can (and did). For another example, when hydrogen and oxygen combine to form water, a substantial change takes place: two gases (at room temperature and standard atmospheric pressure) combine to become a liquid, while something that is none of them – prime matter – persists throughout. We know a new substance results from the change since this substance, water, has properties and powers different and distinct from the previous ones: hydrogen is itself flammable, and oxygen is necessary for the combustion of other substances (e.g., wood), while water is not directly flammable, but extinguishes or prevents the combustion of such other substances.
In the compound, the two substances cease to exist as hydrogen and oxygen and exist rather as water, a single substance with its own substantial form. The substance of water can be changed into the substances of hydrogen and oxygen, but this does not mean that hydrogen and oxygen are actually present in the substance of water.[10]
The matter present in hydrogen and oxygen loses those substantial forms when the new form of water arises from the potency of that matter, yet these substantial forms (as oxygen and hydrogen) dispose their matter to become water, and not some other substance (say, a diamond). The prime matter of the gasses (but not the gasses themselves) persists throughout the change, and the prime matter that had the form of oxygen in one body and the form of hydrogen in another body gives up these forms and takes on the form of water. What is different between the beginning and the end of the change is the form that the same prime matter comes to have.
The other principal cases of substantial change occur when plants or animals (clear cases of substances) cease to be what they were or come to be anew. That is, the death of an animal, for example, is a substantial change, wherein the corpse (a collection of substances without any cohesion) comes to be from what previously did have cohesion (a living animal). The substantial form which makes a tree or a dog to be the sort of living thing it is ceases to be in some portion of matter, and in its place, the matter acquires new substantial forms, becoming new substances as a log or a carcass (which are actually collections of substances losing the cohesion (unity and coordinated activity) they enjoyed as parts of a living tree or dog); they then begin to decay and decompose. As we will discuss more fully in the next chapter, Aristotle and Aquinas understand the substantial forms of living things as their souls, the principle that makes them to be, and to act and behave as, the sorts of things they are. When a living thing dies, its matter ceases to be informed by its soul, and new, different substantial forms come to be from the potency of its matter. As Aquinas notes in his Commentary on Aristotle’ On Generation and Corruption, Bk. 1, lect. 8,
When it comes to corruption, there are many intermediate forms [generated] that are incomplete: for the body of an animal is not, as soon as the soul is separated, immediately resolved into the elements; rather this happens through many intermediate corruptions with many imperfect forms succeeding them in the matter.
Likewise, when living things come to be, a new substantial form actualizes the matter of some prior substances that, while provided by individual plants or animals of the same species, are not actually complete plants or animals, but literally the building material for complete organisms. (We will have more to say about animal and human generation in the next chapter dealing with living or animate substances.) So too, when living things consume nutrients, the matter that makes up the food or nourishment (water, oxygen, sugar, protein, minerals, etc.) becomes the living organism, and comes to be actualized by the soul, i.e., the substantial form of that living thing. In hylomorphism, substantial changes are occurring all the time in nature, as new substances come to be by generation, and old substances pass away, by being eaten, or simply by dying and decomposing.
Substantial form and prime matter, thus, are principles of substantial change, and together they constitute material things as substantial units, singular substances (of given species or kinds). Yet, in general, form cannot exist without matter, nor matter without some form. Substantial forms (with the notable exception of human souls (which we will consider in considerable detail in the next chapter)) only exist by actualizing prime matter, and prime matter cannot exist unless some substantial form or other actualizes it. So, neither can exist without the other (in general). But these are real principles corresponding to the real actuality and potentiality inherent in material things, and one can know them as real, yet distinct, features of the things that exhibit them even if they cannot exist as distinct.
We will have much more to say about prime matter and substantial form (also known as material cause and formal cause) when we examine them in terms of their being two of Aristotle’s Four Causes.
Thus, in developing his explanation of change, Aristotle continues to call the principle of stability, identity, knowledge, and now actuality, “form” but insists it is “in,” or inherent in material things and co-relative to matter, a principle of potentiality or passivity. This use of “form” coincides with what, in relation to knowledge and logic, he termed “species,” “essence,” and “quiddity.” Correspondingly, when he elaborates on the coming to be or generation of substances, Aristotle refers to this formal principle as “nature,” as what accounts for natural, physical changes that constitute birth and growth (phusis). And so, substances which share the ‘look’ (eidos) of a given kind belong to the same species (being born of like parents) due to “what they were to be” (to ti en einai), i.e., their physical nature or being (ousia), quiddity (whatness) or essence (beingness). Thus, things are what they are due to their intrinsic essential nature.
[A]s Aristotle understands the material and formal causes of a thing, they imply far more than the obvious fact that the ordinary objects of our experience are made up of some kind of stuff or other organized in a certain way. Aristotle’s entire metaphysical scheme as we’ve considered it thus far – moderate realism, hylomorphism, the whole ball of wax – is implicated as well. A thing’s formal cause is, at the deepest level, its substantial form or essence; its material cause entails that it has certain potentialities and lacks others; its formal cause, being its substantial form or essence, is shared by other things and known by the intellect via abstraction from experience; and so forth.[11]
[9] Edward Feser, Aristotle’s Revenge: The Metaphysical Foundations of Physical and Biological Science (Editiones Scholasticae 2019), Kindle Edition, p. 30.
[11] Edward Feser, The Last Superstition: A Refutation of the New Atheism. (St. Augustine’s Press, 2008) p. 93-4.
Accidental Change
Michelangelo is alleged to have said that when he set out to sculpt a statue from a formless block of marble, he sought only to remove the excess marble from the statue that was already there inside the block. This sentiment expresses what Aristotle discovered to be necessarily true for all change.
Aristotle discovered the concept of potency by observing accidental changes. He observed, for instance, that a sculptor can make a statue from a block of marble. This is possible only because the block of marble is endowed with a certain property – the possibility and capacity of being transformed. The figure of the statue is in potency in the block of marble. This potency is not nothing, it is not non-being. It is real; not with the reality of being-in-act, but with the reality which corresponds to being-in- potency.
The first principles of motion can be discovered but they cannot be demonstrated. In order to demonstrate them, we would have to assume that they are the result of other principles, in which case they would not be the first principles. (Posterior Analytics I, 3) These principles are not demonstrated but discovered by analyzing substantial changes.
In his analysis of change, Aristotle discovered that every change implies duality. It implies a subject in potency which, by the action of some agent, passes into act, i.e. receives some new perfection or actuality. Motion presupposes the acquisition of something and the corruption of something else. The subject of change is what stays the same through the change. However, through the change, it acquires something new and loses what it previously had. Motion implies a passive principle and an active principle, intrinsic to the thing that changes.
Thus, there are three principles necessary for change to take place. There must be something new that comes to be, something old that passes away, and something that stays the same throughout. In the Aristotelian tradition, these principles receive the names form, privation and matter. Form is what comes to be, privation is what passes away and matter is what stays the same throughout the change. In the case of a statue, the shape of the sculpture, Michelangelo’s “David” for instance, is the form that comes to be when a formless block of marble becomes a statue. The formlessness of the block is itself the privation of the statue shape, and the potency for the statue shape. The marble, first in block shape, later in “David” shape, is what stays the same throughout the change. The case of the coming to be of a statue is an instance of an accidental change; what changes are the accidents of the marble. What stays the same is the substance of the marble.
For Aristotle, motion is the technical name for changes in accidents. There are three kinds of motion for Aristotle: a change in quality (which he calls alteration), a change in quantity, size (called growth or diminution), and a change in place (called local motion). In all cases, motion, as such, is defined as the act of a being in potency insofar as it is in potency. Motion is the process that a substance goes through in which it loses one accidental form or actuality and gains another. As such, motion is an actuality, but an imperfect one. Hence, the definition includes the qualification “insofar as it is in potency.” Motion is the act of something that does not yet have, but is acquiring, the full act of a new accidental determination, a new quality, size or position. While the motion is taking place, the new determination is neither fully actual (for then the motion would be over) nor fully potential (for then the motion would not have begun.) The fact that motion is an imperfect act implies, for Aquinas, that for every true motion, there must be a cause sustaining that motion. This is the basis for Aquinas’ First Way of proving the Existence of God in the Summa Theologiae(S.T. Ia, 2, 3), a point often overlooked by his interpreters and critics.
Substantial Change
Aristotle discovered these principles of nature (matter, form and privation) by analyzing accidental changes. He found that they could also explain the more fundamental kinds of changes, changes that involve the passing away and coming to be of substances.
In order to find an example of a substantial change, i.e. a change that involves the coming to be or passing away of a substance, one first has to admit that there are substances of different kinds. For example, if one admits that sodium and chlorine are different substances (and they certainly appear different – one is a white metal, the other a green gas), and that they are each different from salt (also apparently so), then one can see that the change from sodium and chlorine to salt is a substantial change. The Aristotelian principles used to explain the change are the analogous to those used in explaining accidental changes: matter, form and privation. What comes to be is a new form in the matter, i.e. in what persists through the change. This new form comes to be in what previous lacked that form, i.e. in what had the privation of the form. Thus, the form salt comes to be in the matter of chlorine and sodium. Form and matter, however, make up a substantial unity; one cannot have form without matter, nor matter without some form. But, one can still distinguish these principles, and also understand that these principles are real features of the things that exhibit them.
For more on form and matter, be sure to check out the Four Causes.
Modern science has pretty well confirmed atomic theory as an explanation of chemical reactions. Thus, it is sometimes thought that this theory supersedes Aristotle’s hylomorphic theory. Material things, then, are thought to be fully explained as the collection of atoms, united into molecules of varying size and complexity. Macroscopic objects, like trees and animals and planets, are thus seen to be the collection of so many parts, much like a machine, that work together in a kind of harmony. If Aristotle’s thought is to be assimilable to modern atomic theory at all, it is sometimes proposed that his notion of form simply designates the arrangement of smaller parts, say atoms or molecules.
However, to say that the form is the configuration of parts, does not capture all that Aristotle means by form. While it is true that there can be no form without matter, and in a certain sense, form is realized in matter in a certain configuration, the matter all by itself does not account for its configuration. A favorite example of Aristotle’s is the case of a house made out of bricks. The bricks are the matter of the house, but bricks all by themselves do not account for the house, as opposed to a pile of bricks. The form is a cause in the sense of that it is constitutive of the thing it is the form of, just as the matter is constitutive of the thing. But form has a certain priority and explanatory value because the form accounts for the matter being in a certain configuration while in that configuration, something that matter cannot do.
Thus, to claim that atomic theory explains all of the phenomena of observation is simply to miss some observations, or to suppose that more is explained than actually is. Richard Connell in Substance and Modern Science (Houston: Center for Thomistic Studies, 1988) puts the point well.
Reflecting on the theory, we see that the representation of atoms as aggregates does account for the union and separation of atoms, but that it does not explain the disappearance of some properties and the coming to be of others, except for adding the mass (mass-energy), which is conserved. (We might add the that total charge is conserved too.) So given that changes have occurred in almost all the properties, we know that something more than a mere uniting must occur, even though the nature of the additional activities or interactions is left in the dark. That some sort of interaction must occur cannot be doubted, if observation is to mean anything, but precisely what the character of the interaction is we have no way of determining.
p. 86.
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