Most Western philosophy since Aristotle has been built on a metaphysics of substance: the world is fundamentally composed of things, persistent, identifiable entities with properties. A rock is a substance. A human being is a substance. God, for Descartes and Spinoza, is the ultimate substance. Even atoms, electrons, and quarks in modern physics are typically conceived as tiny things that persist through time and have properties.
But think about what you actually encounter when you pay close attention to experience. Not static things, but events: a flame burning, a thought arising and dissolving, a conversation unfolding, a storm passing through. Even what we call "things" are better described as relatively stable processes: a mountain is a very slow geological event. A human body is a cascade of biochemical processes maintaining a temporary coherence. A rock is a crystal lattice of molecular vibrations that happen to persist for a long time. Things are stable patterns in a flow, not the other way around.
Alfred North Whitehead (1861–1947), mathematician, co-author with Bertrand Russell of Principia Mathematica, and late-career philosopher who produced Process and Reality (1929), built his entire metaphysics on this insight. The world is not fundamentally made of substances (things that persist and have properties) but of events, momentary occasions of experience he calls actual occasions (or actual entities). Each actual occasion is a brief, complete event of becoming, "a throb of experience", that comes into being, achieves its fullness, and then perishes, leaving its influence as data for the next generation of occasions.
The analogy Whitehead himself suggests: think of a musical note. A note is not a thing; it is an event, it comes into being, sounds through a duration, and ceases. Its identity is inseparable from its becoming. If you freeze a note, you have destroyed what it is. Now imagine that every entity in the universe is like this, not a persistent object but a brief, complete event of becoming that comes-to-be, achieves its character, and ceases. That is Whitehead's ontology.
Each actual entity is conceived as an act of experience arising out of data. It is a process of 'feeling' the many data, so as to absorb them into the unity of one individual satisfaction.
— Whitehead, Process and Reality (1929); quoted in BRLSI 'A.N. Whitehead's Process and Reality'
This is not merely a description of living organisms or of human consciousness. For Whitehead, every entity at every scale, from subatomic events to cells to thoughts to galaxies, is an actual occasion of this kind. The universe is, at bottom, composed not of particles or fields or substances but of experiences, brief, relational, creative events of feeling and becoming. This sounds extravagant. but Whitehead argues it is the only way to avoid the bifurcation of nature If experience is real (which it obviously is, you are having it right now), then either experience is a fundamental feature of the universe or we have no explanation of how it arises. Whitehead opts for the first.
This doctrine is sometimes called panexperientialism (not panpsychism in the naive sense that rocks have thoughts, but the view that experience in some form, however simple, is a feature of all actual occasions, not just brains). A rock is not conscious in any meaningful sense, but the subatomic events that constitute a rock involve rudimentary forms of feeling and response that, in vastly more complex organizations, give rise to what we call consciousness.
Quick reflection
Whitehead says that when you freeze a note, you destroy what it is — because a note's identity is its becoming. Can you think of other things in your experience where this is true: where the identity is the process, not the static state? What happens to the substance metaphysics model when you focus on those?