“You Can’t Step Into the Same River Twice”: What Is Process Metaphysics?

What does metaphysics study? How do the metaphysical assumptions we all hold shape our worldview and our lives? And is it really true that “you can’t step into the same river twice”? Let’s explore why seeing the world as a simple collection of things is incomplete, what we gain by moving beyond a view of reality as a set of static, disconnected objects, how process metaphysics leads us to rethink other key philosophical concepts like identity, and how this relates to understanding life—not as a quest to find your “true self,” but as an expansion of your boundaries.

Metaphysics is an attempt to understand how existence works by studying the components of reality, the differences between mental and physical entities, and fundamental questions of being and reality. But metaphysics isn’t just a mysterious branch of philosophy: people use metaphysical assumptions to navigate the world. Our beliefs about what exists and what is fundamental have a powerful impact on our lives. In fact, the less aware we are of our metaphysical assumptions, the more we are controlled by them.

Western metaphysics tends to rely on the paradigm of substances. We often see the world as a world of things—made up of atoms, natural kinds, galaxies. Objects are the paradigmatic way of existing, the basic building blocks of the universe. What exists, exists as an object. This means things belong to certain classes, have specific qualities, and clearly defined spatial and temporal boundaries. For example, Fido is my dog, he’s gray, and was born a year ago. (It’s worth noting that even such a simple statement sparks a host of metaphysical disagreements within substance metaphysics: realists believe in universals like the natural kind “dog,” while nominalists see them as mere intellectual abstractions.)

Although substance metaphysics seems to underlie the Western idea of “common sense,” I believe it’s mistaken. To see why, consider the cliché about a glass of water: is it half empty or half full? This question assumes a static organization of things, which supports either an optimistic or pessimistic interpretation. We can endlessly debate the correct description of the physical state or the validity of psychological judgments. But what if the very construct of “a glass of water” doesn’t give us the information we need? Each of us would prefer a glass that’s being filled over one that’s being emptied, even if both are at the same level. Any analysis without information about change loses its meaning—and this is exactly the information missing from substance metaphysics.

Meanwhile, process philosophers argue that we should move beyond seeing the world as a set of static, disconnected objects, and instead study the processes that make up the world. Processes, not objects, are fundamental.

Heraclitus and the River: The Roots of Process Metaphysics

The ancient Greek pre-Socratic philosopher Heraclitus created the most famous image of process metaphysics. He said, “You can’t step into the same river twice,” because existence depends on change; the river you step into the second time is different from the one you entered the first time (and you have changed as well). While substance philosophers seek the smallest constituent objects to define the most fundamental elements of reality, process philosophers argue that this isn’t enough. Modern physicists agree. Electrons are now understood as bundles of energy in a field, and quantum vacuum fluctuations show that fields can exist without bundles, but not bundles without fields. Things, it seems, can be reduced to processes, not the other way around. (As philosopher Alfred North Whitehead put it, we should think in terms of “happenings,” not “things.”)

Universals, Evolution, and the Limits of Substance Metaphysics

Change often poses a problem for substance metaphysics. Universals have traditionally been a popular way to get around this. These static entities are hard to define precisely, but can be seen as “hyper-things” embodied in many different particular things. Universals are what’s common to particular things; they include genera, species, and relations. Universals are fundamentally different from particulars: Aristotle, for example, claimed that particular things like my dog Fido are subject to coming into being and passing away, while species are eternal universals. But science seems to favor process metaphysics here as well. The theory of evolution disproved Aristotle’s idea that species are unchanging and eternal. Species evolve. They change. Dogs descended from wolves and are now a separate species. Again, we’re better off using the paradigm of change rather than substances.

Process Metaphysics and the Concept of Identity

Process metaphysics leads us to rethink other important philosophical concepts. Take identity. To explain why things can change without losing their identity, substance philosophers have to postulate some core essence that remains the same through change. As the Ship of Theseus paradox shows, it’s hard to say what that core could be. The ship goes on a long journey and needs major repairs along the way: old planks are replaced with new ones, rotten oars swapped for sturdy ones, and so on, until, before the ship even returns to port, not a single part remains from when it set out. Is it still the same ship, even if it’s materially completely different? For substance philosophers, this is a paradox; for process philosophers, it’s part of identity. Of course it’s the same ship. Identity is no longer static equivalence of a thing with itself. Without repairs, the ship would have lost its function. Instead of static equivalence, as German philosopher Nicholas Rescher argues in his book Ideas in Process (2009), identity is actually programmatic development. That is, the identity of a process is the structural identity of its program. All else being equal, every puppy will become a dog. (This program doesn’t have to be deterministic. According to Rescher, the interaction of processes allows for variation.)

The Heap Paradox and the Nature of Processes

As the heap paradox shows, processes aren’t just intervals between two different states of affairs or two objects. Take a heap of sand and remove one grain. It’s still a heap; one grain doesn’t change that. But if you repeat this enough times, eventually only one grain remains. Clearly, that’s not a heap. When did it stop being a heap? By looking at the process, not just the end states, you realize it’s impossible to pinpoint the difference between a heap and a non-heap. (Similarly, no single animal was the decisive turning point in the evolution from wolves to dogs.) At the very least, this warns us about the subtle abstraction behind dividing things into natural kinds. Process philosophers like Henri Bergson stop at this negative conclusion and believe that processes can’t be understood, only experienced. Despite this, Danish philosopher Johanna Seibt notes that focusing on processes may simply require a completely new way of seeing.

The Practical Value of Process Philosophy

Seeing the world as a network of interconnected processes has scientific and philosophical advantages, but there are also more down-to-earth benefits. Process philosophy encourages us to consider longer timeframes, blurred boundaries, and interconnections. Identity as a programmatic but non-deterministic process fosters innovation through small, periodic changes. According to these metaphysical assumptions, a meaningful life is less about finding your “true self” and more about expanding your boundaries.