Process philosophy (also ontology of becoming, processism, or philosophy of organism) identifies metaphysical reality with change and development. Since the time of Plato and Aristotle, philosophers have posited true reality as "timeless", based on permanent substances, while processes are denied or subordinated to timeless substances. If Socrates changes, becoming sick, Socrates is still the same (the substance of Socrates being the same), and change (his sickness) only glides over his substance: change is accidental, whereas the substance is essential. Therefore, classic ontology denies any full reality to change, which is conceived as only accidental and not essential. This classical ontology is what made knowledge and a theory of knowledge possible, as it was thought that a science of something in becoming was an impossible feat to achieve.
In opposition to the classical model of change as accidental (as argued by Aristotle) or illusory, process philosophy regards change as the cornerstone of reality—the cornerstone of Being thought of as Becoming. Philosophers who appeal to process rather than substance include Heraclitus, Karl Marx,Friedrich Nietzsche, Henri Bergson, Martin Heidegger, Charles Sanders Peirce, Alfred North Whitehead, William James, R. G. Collingwood, Alan Watts, Robert M. Pirsig, Charles Hartshorne, Arran Gare, Nicholas Rescher, Colin Wilson, and Gilles Deleuze. In physics Ilya Prigogine distinguishes between the "physics of being" and the "physics of becoming". Process philosophy covers not just scientific intuitions and experiences, but can be used as a conceptual bridge to facilitate discussions among religion, philosophy, and science.