information:filtering, information:classification, structural:summarization
Particulars and universals

A central distinction in ontology is between particular and universal entities. Particulars, also called individuals, are unique, non-repeatable entities, like Socrates, the Taj Mahal, and Mars. Universals are general, repeatable entities, like the color green, the form circularity, and the virtue courage. Universals express aspects or features shared by particulars. For example, Mount Everest and Mount Fuji are particulars characterized by the universal mountain.

Universals can take the form of properties or relations. Properties express what entities are like. They are features or qualities possessed by an entity. Properties are often divided into essential and accidental properties. A property is essential if an entity must have it; it is accidental if the entity can exist without it. For instance, having three sides is an essential property of a triangle while being red is an accidental property. Relations are ways how two or more entities stand to one another. Unlike properties, they apply to several entities and characterize them as a group. For example, being a city is a property while being east of is a relation, as in " Kathmandu is a city" and "Kathmandu is east of New Delhi ". Relations are often divided into internal and external relations. Internal relations depend only on the properties of the objects they connect, like the relation of resemblance. External relations express characteristics that go beyond what the connected objects are like, such as spatial relations.

Substances play an important role in the history of ontology as the particular entities that underlie and support properties and relations. They are often considered the fundamental building blocks of reality that can exist on their own, while entities like properties and relations cannot exist without substances. Substances persist through changes as they acquire or lose properties. For example, when a tomato ripens, it loses the property green and acquires the property red.

States of affairs are complex particular entities that have several other entities as their components. The state of affairs "Socrates is wise" has two components: the individual Socrates and the property wise. States of affairs that correspond to reality are called facts. Facts are truthmakers of statements, meaning that whether a statement is true or false depends on the underlying facts.

Events are particular entities that occur in time, like the fall of the Berlin Wall and the first moon landing. They usually involve some kind of change, like the lawn becoming dry. In some cases, no change occurs, like the lawn staying wet. Complex events, also called processes, are composed of a sequence of events.