Aristotle’s Fourfold Division of Being

Aristotle’s fourfold division of being, found in his work Categories, classifies entities based on whether they are “Said of” a subject and whether they are “Present in” a subject. The four categories obtained by these distinctions are: Accidental Particulars (not Said of but Present in), Accidental Universals (Said of and Present in), Primary Substances (or Substantial Particulars) (neither Said of nor Present in), and Essential Universals (or Substantial Universals) (Said of but not Present in) (also called Secondary Substances).

	Present in	Not Present in
Said of	Accidental Universals	Essential Universals
Not Said of	Accidental Particulars	Primary Substances

Or, looking at these four categories another way, with explanation:

	Said of?	Present in?
Essential Universals	Yes	No	These are species or genera that are “Said of” particular substances but are not “Present in” any single subject. (i.e. the species of “humans”)
Accidental Particulars	No	Yes	These are non-substantial particulars that are “Present in” a subject but are not “Said of” any other subject. (i.e. the wisdom of Socrates)
Accidental Universals	Yes	Yes	These are universals that are both “Said of” and “Present in” subjects. (i.e. the wisdom that can be in some humans, such as philosophers)
Primary Substances	No	No	These are individual, particular things that can exist on their own, such as an individual man or other type of entity. (i.e. Socrates)

Note the original term “accidental” can now be thought of as merely “non-substantial”. But Aristotle’s fourfold division wasn’t discussed at great length in the Categories, as pride of place was devoted to his ten-fold ontology, which we won’t be covering here.

The recent philosopher E. J. Lowe wrote much on essentially this same fourfold classification, which he called the Four Category Ontology. Lowe’s diagram of this, which he called “The Ontological Square”, also describes several other relationships between these four categories: Instantiation (between Kinds and Objects, and Attributes and Modes), Characterization (between Kinds and Attributes, and Objects and Modes), and Exemplification (between Attributes and Objects).

	Non-substances	Substances
Universals	Non-substantial Universals, i.e. Attributes	Substantial Universals, i.e. Kinds
Particulars	Non-substantial Particulars, i.e. Modes	Substantial Particulars, i.e. Objects