The author analyses the concept of existence in Christian Wolff's 'Philosophia prima sive ontologia' and confronts it with his concept of being. This distinction helps him to highlight the difference between ontology and more specific parts of metaphysics such as psychology, theology or transcendental cosmology. Wolff's ontology contains only a modal explication of being. No a priori ontological analysis can answer the question why this and not some other being has been brought about to exist. Contingent beings cannot exist without a sufficient reason, thus their existence can be explained only by specific sciences.