 Proper and common attributes of substance follow from and are determined by a thing’s essentialia. Proper attributes are the properties of a thing that are determined by all the essentialia taken together, and common attributes are the properties of a thing that are determined by only some, but not all, its essentialia. Attributes (as such) are perhaps best understood as necessary accidents, since they are determined by and necessarily follow from a thing’s essentialia. Modes, in contrast, are only contingent accidents of substance. They are the properties of a thing that may or may not be present, and if actually present, they are causally the result of some contingent state of affairs. More precisely, the possible presence of any given mode follows from a substance’s essentialia, but the actual presence of a given mode is the result of something outside the substance’s essence that is causally responsible for its presence in a being. At the nominal level of reality, composite beings exist insofar as the accidents of a certain simple substance, or set of simple substances, are linked and/or arranged together in a certain sort of way. In §789 of his Ontologia, Wolff writes: 