 In one respect, simple beings and composite beings are not simply two different species of beings. It is not the case, for example, that within the realm of all possible things simple beings exist separate from, and in addition to, composite beings. More accurately, at the nominal level of reality simples and composites result from an epistemological distinction imposed by a perceiving mind in its analysis of what “exists” (i.e., exists in a nominal sense). Strictly speaking, the only substantial things to exist at any level of reality are simple substances. Simples are defined by their essentialia, and to borrow an expression from Gilson, essentialia are both “compatible and prime” (Gilson 1952: 114). That is to say, the essential properties that define a given simple substance do not contradict one another, or cancel each other out, and they are (themselves) not determined by any other thing and/or property. In this light, Wolff’s notion of substance is perhaps best regarded as a notion of essence, where each simple substance is a different set of compatible and prime essential properties (see Burns 1966: 26 and Gilson, 1952: 115). Furthermore, essential properties should not be viewed as the accidents of substance because, according to Wolff, they are the substance itself. In Wolff’s system, the accidents of substance are the properties that exist by virtue of a thing’s essentialia. And according to Wolff, there are three basic classes of accidents: proper attributes, common attributes, and modes (Ont. §148).