Process philosophy (or ontology of becoming) 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.