 Despite the lack of helpful definitions of these two concepts, there is a fairly straightforward, though certainly not uncontroversial, characterization of them that many scholars have adopted. By focusing on Aristotle’s illustrations, most scholars conclude that beings that are said-of others are universals, while those that are not said-of others are particulars. Beings that are present-in others are accidental, while those that are not present-in others are non-accidental. Now, non-accidental beings that are universals are most naturally described as essential, while non-accidental beings that are particulars are best described simply as non-accidental. If we put these possibilities together, we arrive at the following four-fold system of classification: (1) accidental universals; (2) essential universals; (3) accidental particulars; (4) non-accidental particulars, or what Aristotle calls primary substances. This system maps readily onto Aristotle’s own terminology, given at 1a20: (1) Said-of and present-in: accidental universals; (2) Said-of and not present-in: essential universals; (3) Not said-of and present-in: accidental particulars; and (4) Not said-of and not present-in: primary substances. A brief discussion of each of these classes should suffice to bring out their general character.