 It is on the basis of PC and PSR that Wolff proceeds to explicate the fundamental concepts of his ontology. Recall that for Wolff a being in the most general sense is any possible thing. Possible things have essential natures insofar as they are composed of a number of non-contradictory determinations or predicates. The essence of any given possible thing is its principle of being, or principle of individualization. Whereas the essence of a simple being is defined by its essential properties, the essence of a composite being is defined by the manner in which its parts are combined together. In §532 of his Ontologia, Wolff explains: