2.2 The Riddle of Form and Matter

Watching an embryo develop gives us good reason to reject the little-person version of preformationism, because structures appear that were not there before. But observation alone does not explain where developmental organization comes from. If the organism is not already present in miniature, or otherwise fully specified at the beginning, how does development nonetheless proceed in reliable and recognizable ways? Epigenesis can describe the appearance of new organization, but unless it can say something about how that organization arises, it remains vulnerable to the old charge that it has replaced a preexisting plan with a mysterious formative force.

Consider what an embryo actually asks us to explain. A fertilized frog egg begins as a single cell, without the organs and bodily organization of a tadpole or adult frog. Through repeated cell division, movement, differentiation, and reorganization, structures arise that were not present in recognizable form at the beginning. The resulting tadpole has gills and a tail and lives in water, while the adult that develops later loses its tail, grows legs, and comes to breathe air. The egg, tadpole, and adult are organized in strikingly different ways, yet each belongs to the life cycle of a frog rather than that of a fish or some other animal.

There is therefore a real continuity here, but it cannot be identified with the persistence of one visible form. No one looking at a tadpole would mistake it for an adult frog, and no single outward organization carries unchanged across the life cycle. This makes it tempting to suppose that something less visible does persist, perhaps an essence that makes the organism what it is, or a set of instructions specifying its developmental course. Even if the adult frog is not present in miniature in the egg or the tadpole, is there a frog “form” continuously present in some other way throughout development? If that is the case, what is the nature of this form, and where do we look for it?

Change and the Search for Stability

These questions about continuity and change reach back to some of the earliest debates in Western philosophy. They belonged to a wider argument about whether change or stability is more fundamental, and about how anything can stay identifiable while it is constantly in flux.

Writing around 500 BCE, Heraclitus emphasized the fluidity of life and the ubiquity of change. He has been associated with expressions such as “change is the only constant” and “you cannot step into the same river twice.” He may not have written those particular phrases, but they capture the process-oriented view attributed to his teachings (van Geert & de Ruiter, 2022, Chapter 1). A river appears to be a stable thing that we can identify and name, but the water that makes it up is continually moving, so that what we call the river may be better understood as an ongoing process than as a fixed entity.

Around a century later, Plato also recognized that the world available to our senses is characterized by continual change, but he proposed that rational understanding required a stable foundation that was not itself caught up in this flux. For Plato, this foundation was provided by abstract and unchanging “Forms”, which were grasped through reason rather than perceived through the senses. A circle drawn on paper will always be imperfect, but it can be understood as an approximation of the ideal Form of a circle. In the same way, particular things can be understood in relation to stable Forms that are not themselves subject to the changes undergone by material things.

Applied to development, this way of thinking suggests that the developing organism realizes or manifests an organization that is located in a Form outside the developmental process itself. The individual organism changes, while its Form does not, so that development can be understood as matter coming to realize an organization that is already given.

A thing-based account and a process account differ in what they take for granted. A thing-based account takes the organism’s persistence largely as given and asks what changes happen to it, whereas a process account treats that persistence as an achievement and asks how it is continually produced.

Paul van Geert and Naomi de Ruiter (2022) place Aristotle at an interesting point in the history of these ideas, imagining him rowing just before a river forks into substance-based and process-based accounts of the world. Most of psychology rowed down the substance branch, treating persons and psychological characteristics as relatively stable things. Not many stepped into the other branch, which van Geert and de Ruiter call Heraclitus’ river. Aristotle’s thinking may be useful partly because he stands at that fork.

Aristotle, who had studied with Plato, rejected the proposal that Forms existed in a separate realm. He wanted an account more closely grounded in our experience of the world, including our experience of living things as organized beings that continually change. He was also an unusually careful observer of biological phenomena, dissecting animals, comparing their structures and ways of living, and opening fertilized chicken eggs at successive stages of incubation to observe the appearance of the heart, blood vessels, eyes, and other structures that had not previously been visible (Lennox, 2026).

Aristotle did not hold a modern theory of biological development: some of his empirical claims were wrong, and his view of the living world as divided into relatively fixed natural kinds does not fit easily with an evolutionary account. Nevertheless, his work provides a vocabulary for approaching the problem left to us by the preceding reading: how can matter become organized as a living thing without appealing either to a miniature organism hidden in the starting materials or to an ideal Form existing separately from them?

Form and Matter

A key idea in Aristotle’s philosophy is that any living or nonliving thing can be understood in terms of form and matter. Matter answers the question “What is it made of?” Form answers something closer to “What is it?” or even “How is this matter organized as this particular thing?” Form means more than outward shape or appearance: it is the organization through which material components constitute a recognizable unity (Ainsworth, 2024).

Aristotle often illustrated this distinction using nonliving things. A house may be made from bricks, stones, and timber, but those materials do not constitute a house unless they are organized in a particular way. A bronze statue and the bronze from which it is made can likewise be distinguished without imagining that the statue’s form exists as a second object alongside its matter.

The Aristotelian view of the relation between form and matter is known as hylomorphism, from the Greek hyle, meaning matter, and morphe, meaning form. The same root appears in metamorphosis, which means a change of form. This is especially appropriate for the example of the frog, since metamorphosis makes clear that form cannot simply mean one outward arrangement that remains fixed throughout the organism’s existence.

The example of a house also reveals an important limitation of comparisons between artifacts and organisms. A house may be represented in a plan before it is built, with a builder then organizing materials in accordance with that plan. Aristotle does not deny that a house can exist as a plan first. His point is that the actual house is the materials organized as a house, not matter on one side plus a second substance called form on the other.

Living things make the problem more difficult because their organization is not imposed in the same way by a builder standing outside them. A living organism helps produce its own organization and keep it going, even as that organization changes.

We could simply say that the world contains many different kinds of things, including frogs, birds, trees, and houses, and for many purposes that is enough. The developmental problem begins when we ask how something comes to be one of those kinds. Calling a frog a frog identifies a recognizable organization, but it does not explain how a single cell develops through the whole of a frog life cycle. Form names something real about the organization of the living thing, while still leaving us to explain how that organization arises and persists through change.

We can distinguish form from matter in thought without treating them as separate ingredients of the thing itself. The form of a living being is realized through its material organization, while those materials constitute this living being only through the relations into which they are organized.

This inseparability makes form difficult to pin down. If form is not a separate thing, what kind of reality does it have? We cannot simply get rid of the concept and say that matter is all there is, because we would still need to explain why matter is organized in the particular ways we observe. To say that an organism consists of cells, proteins, water, and other material components does not explain how those materials constitute a unified living being, just as listing the materials in a house does not explain how they have come to constitute somewhere that can be inhabited.

All this may seem a long way from psychology, but the relation between form and matter bears directly on how psychology and developmental science frame their oldest problems: body and mind, nature and nurture, biology and culture, individual and environment. Once we treat these as separate sources of influence, we are left asking how much each contributes or how they interact. Interaction may be important, but it does not by itself overcome the initial separation. It still begins with two independently defined things and then asks how one affects the other.

Hylomorphism suggests another possibility, in which distinctions can be made without turning the distinguished aspects into separate entities. Form and matter are different, but like two sides of a coin, neither can be understood without the other. Instead of looking for form as something that stands apart from development and organizes it, we can begin to look for organization in the activity of the developing system itself.

Form as a Way of Life

We still need to say what form actually is, given that Aristotle did not use the term simply for what something looks like. And we cannot point to form in the way we can point to an animal, organ, cell or molecule. One way forward is to connect form to the characteristic way of life of an organism. If form answers the question “What is it?”, then for a living being the question might become “What is its way of life?” or “What are its characteristic activities?”

The form of a bird, for example, involves more than the visible arrangement of its wings, feathers, talons, eyes and beak. Those structures are organized in relation to activities such as flying, feeding, perceiving, nesting, regulating body temperature and reproducing. The organization of the body cannot be separated from the activities it makes possible, and those activities cannot be separated from the environments in which they happen. A wing is part of an organized capacity for a particular way of moving through the world.

Denis Walsh and Kayla Wiebe describe this reciprocal relation between form and matter in terms of the organism’s pursuit of its distinctive way of life:

“For Aristotle, the being of living beings consists in the process by which the form of the organism synthesises, alters, and orchestrates its constituent matter in ways that equip the organism for the pursuit of its distinctive way of life. Reciprocally, the matter of an organism confers on it the capacity to pursue those vital processes appropriate to its form.” (Walsh & Wiebe, 2020, p. 122)

The organism’s form is expressed through processes that alter and organize its material parts, while the material organization of the body supplies the capacities through which those processes can happen. Neither form nor matter stands alone as the cause of the other. Form is embodied in the matter of the organism as it lives, and the organism’s activity keeps producing and maintaining the organization of that matter.

This changes how we understand continuity across the life cycle. The phases of a frog’s life share neither an outward form nor a set of activities, but each belongs to the same life cycle. What persists is an organized developmental process, through which new structures and activities arise at each phase of a frog’s way of life.

Capacities and Their Use

In a living thing, the form of the body is what Aristotle called its soul. Form is broader than life, because Aristotle also used the term for the organization of houses and statues, but only living things have souls. Even scholars of Aristotle have a hard time saying exactly what he meant by the soul, but he plainly did not mean a separate, immaterial entity living inside the body. The soul is the body’s organized capacity to live a particular kind of life. James Lennox makes the point with an American Bald Eagle. Its soul is what it is to be an eagle: to soar, see prey from a great distance, dive, grasp fish in its talons and build enormous nests. Those capacities are what its wings, eyes, beak and talons are for (Lennox, 2021, p. 232).

Aristotle also distinguished potentiality from actuality: potentiality concerns what something can become or what it is capable of doing, while actuality concerns what it is or what it is doing at a given moment. Aristotle used dunamis for potentiality, a word that can also mean power and is related to the English word dynamic, and energeia for actuality, a word that carries the sense of being at work (van Geert & de Ruiter, 2022, Chapter 1).

Within actuality, Aristotle distinguished having a capacity from using it, and compared the difference to that between sleeping and waking. Someone asleep still knows what they know, even though they are not using that knowledge.

To work through all of this, I sometimes think about bats. A hibernating bat has the capacities (e.g. using its wings and ears) to fly and hunt insects, though it is using neither. Aristotle located the soul at this level, defining it as the first actuality of a natural organized body, meaning the capacities themselves, whether or not they are in use (Aristotle, De Anima II.1, 412a27–b6). A dead bat has lost those capacities, even though its wings and ears are still there. Aristotle said that if an eye were an animal, sight would be its soul, and an eye that can no longer see is an eye in name only, like the eye of a statue or a painted figure (412b18–22). The dead bat’s wings are wings in the same way.

Because energeia carries the sense of being at work, it ties what a living thing is to what it does. That might seem to sit awkwardly with the hibernating bat, which is neither flying nor hunting. But staying alive is itself something the bat does: through the winter it keeps breathing, very slowly, and burns through the fat it stored in the autumn. The most basic capacity Aristotle gave to every living thing, that of maintaining itself through nourishment, is still at work, so the bat’s unused capacities belong to a body that is still active. The frog likewise does not first possess frog-ness as an internal property and then perform frog activities because of it. What it is to be a frog is realized through the activities that make up a frog way of life, even though those activities change dramatically across the life cycle.

A way of life also cannot be located entirely inside the organism, because the frog’s life includes water, food, temperature, seasons, predators, other frogs and the wider ecological conditions that make frog activities possible. What this means for how an organism encounters its world is the subject of the companion resource, Enacting Life.

Looking for an Essence

Human beings appear to have a strong tendency to assume that members of a natural category share some hidden property that makes them what they are. Developmental psychologist Susan Gelman (2003) has described this tendency as psychological essentialism. Even young children often reason as though there is something inside an animal that determines what kind of animal it really is, although they may be unable to say what that hidden property consists of. In a classic study, Rheta DeVries (1969) showed children a cat named Maynard and then fitted him with a realistic dog mask. Younger children were sometimes persuaded that Maynard had become a dog. Older children insisted he was still a cat, as if something deeper than his appearance settled what he was.

There is something useful in the intuition that members of a kind share a characteristic organization. Frogs look recognizably like frogs and engage in frog activities, and our ability to identify such regularities is part of how we make sense of the living world. We might even call this organization an essence, provided that the term is being used to name what characterizes a frog rather than to posit a separate thing hidden inside it.

The difficulty arises when that regularity is made to explain itself. To say that a frog develops as a frog because it possesses an internal frog essence simply relocates the organization we are trying to explain. The same problem appears if DNA is treated as the material location of that essence. Identifying an organism through its DNA is not the same as explaining how its form develops, since DNA participates within cells and systems whose conditions and relations are themselves changing.

Frog form is real, so the question is where that reality lies and what kind of explanation it needs. The alternative suggested here is that form belongs to the organized living system and is realized through the relations and activities by which that system develops and maintains itself. That still leaves the question of how new organization arises.

The Space of Possibilities

If form is not already present as a plan, where does it come from? Perhaps it preexists in some abstract sense, ready to shape matter as development proceeds. But can a way of life preexist? If it can, then development again begins to look like the unfolding of something specified in advance, and we have returned to preformationism in a less visible form.

The alternative is an epigenetic view in which the forms that arise through development are not already fully present or specified at the beginning. Here epigenetic is being used in its older and broader sense, relating to epigenesis as the progressive formation of new organization. The distinction from epigenetics in its narrower modern sense was drawn in Made or Given?

Developmental psychobiologist Gilbert Gottlieb (2007) used the term probabilistic epigenesis for an account in which developmental outcomes arise through reciprocal relations among processes operating at several levels, rather than being determined by a path specified at the beginning.

To say that a frog egg has the potential to become a frog does not mean that an adult frog is already represented inside it. Potentiality can instead be understood in terms of the possibilities available to the developing system at a particular point.

A fertilized frog egg can develop in more than one way, but it will not become just anything. It will not become a fish, for example. At the same time, we do not need to posit a static frog essence that remains unchanged while the egg divides, folds, and reorganizes into an embryo with a nervous system, a beating heart, a head, and a tail. New structures arise as the system develops, and those structures alter what the system can do next.

Putting Aristotle’s potentiality together with probabilistic epigenesis suggests the idea of a changing developmental space of possibilities. Neither Aristotle nor Gottlieb put it quite this way. At any particular moment, the current organization of the system makes some changes possible, makes others more likely, and excludes still others. The relations and conditions that restrict or channel these possibilities are constraints on the system.

Constraints are often thought of only as limitations, but this misses an important point. By closing off some possibilities, constraints give structure to those that remain, whereas a completely unconstrained system would have no organized developmental future and no reliable pathways at all. The organization of the developing system does not specify one complete future, but it does give shape to what can happen from the point the system has reached.

The space of possibilities changes as the system changes. The path already taken influences what new paths become available and which of them are more likely to be followed. Developmental history is important because each new organization becomes part of the conditions for what can happen next, so that the path appears as the organism moves through the space rather than existing beforehand as a complete route waiting to be traveled.

This general idea overlaps with what developmental psychologists describe as developmental cascades, in which a change in one process or domain alters the conditions for later changes, sometimes spreading into other areas of development (Masten & Cicchetti, 2010). A cascade is one way in which the path already taken can reshape what becomes possible next. Cascade accounts can themselves span biological and social processes across different timescales. But cascades are only one route. Physical constraints, cellular organization and ecological conditions also reshape what the system can do next, and some paths close without any single cascade to blame.

Nor is the developing organism simply moved through this space by forces acting upon it. Its own activity is among the processes that continually alter the conditions of development. As cells divide and move, they change the conditions around them. Later, the organism’s movements change what it encounters and can also alter its surroundings. The organism does not choose its developmental future in advance, but neither is it a passive object traveling along a path constructed elsewhere. Its activity helps make the path as development proceeds.

What Shapes the Space

What else constrains this space? It is tempting to look for one privileged source, but constraints operate across the explanatory frames introduced in The Big Picture of Development. The developing frog embryo is constrained by the physical and chemical relations among its cells in the present moment, which can be considered within the microgenetic frame, and by the developmental history through which its current organization arose, which belongs to the ontogenetic frame.

It is also constrained by the more recent history of the population and habitat to which it belongs, including changes in the pond in which its eggs were laid, which belongs to the diachronic frame, and by life-cycle patterns stabilized over evolutionary time, which belong to the phylogenetic frame. All of these act on the developing embryo at once.

The frog life cycle is therefore not invented from nothing in each generation. It is reliably reconstructed through materials, relations, activities, and ecological circumstances that are themselves products of previous developmental and evolutionary histories. These conditions recur with considerable reliability, which helps explain why development is patterned without requiring a blueprint that describes the outcome in advance. At the same time, the reconstruction is never perfectly identical, leaving room for developmental variability and, across generations, for evolutionary change.

The ancient Greeks do not provide us with a finished theory of development, but we still face their question of how materials become organized as a living thing. What has changed is how we ask it. Instead of asking where form is stored, we can ask how organized relations arise and hold together through change.

The organization of a living thing is not stored as a complete plan anywhere, so looking more closely at the starting materials will not tell us where form is kept. DNA is the place where this claim meets the greatest resistance, because no molecule has been asked more often to contain the plan for the organism.

Sources

Ainsworth, T. (2024). Form vs. matter. In E. N. Zalta & U. Nodelman (Eds.), The Stanford Encyclopedia of Philosophy. https://plato.stanford.edu/entries/form-matter/

Aristotle. (n.d.). On the soul (De anima) (J. A. Smith, Trans.). Classics in the History of Psychology. https://psychclassics.yorku.ca/Aristotle/De-anima/de-anima2.htm (Original work published ca. 350 B.C.E.)

DeVries, R. (1969). Constancy of generic identity in the years three to six. Monographs of the Society for Research in Child Development, 34(3), 1–67.

Gelman, S. A. (2003). The essential child: Origins of essentialism in everyday thought. Oxford University Press. https://doi.org/10.1093/acprof:oso/9780195154061.001.0001

Gottlieb, G. (2007). Probabilistic epigenesis. Developmental Science, 10(1), 1–11. https://doi.org/10.1111/j.1467-7687.2007.00556.x

Lennox, J. G. (2021). Form as cause and the formal cause: Aristotle’s answer. In L. Jansen & P. Sandstad (Eds.), Neo-Aristotelian perspectives on formal causation (pp. 225–237). Routledge. https://doi.org/10.4324/9780429329821-15

Lennox, J. G. (2026). Aristotle’s biology. In E. N. Zalta & U. Nodelman (Eds.), The Stanford Encyclopedia of Philosophy. https://plato.stanford.edu/entries/aristotle-biology/

Masten, A. S., & Cicchetti, D. (2010). Developmental cascades. Development and Psychopathology, 22(3), 491–495. https://doi.org/10.1017/S0954579410000222

van Geert, P., & de Ruiter, N. (2022). Toward a process approach in psychology: Stepping into Heraclitus’ river. Cambridge University Press. https://doi.org/10.1017/9781108859189

Walsh, D. M., & Wiebe, K. (2020). The being of living beings: Foundationalist materialism versus hylomorphism. In A. S. Meincke & J. Dupré (Eds.), Biological identity: Perspectives from metaphysics and the philosophy of biology (pp. 107–127). Routledge. https://doi.org/10.4324/9781351066389-6

Explore Further

Ainsworth’s Form vs. matter offers a fuller introduction to hylomorphism and the debates surrounding it.

Lennox’s Aristotle’s biology places form, function, and explanation within Aristotle’s study of living things.