  Ontology, while a preface to rational theology, is also necessary for knowledge of the intimate nature of finite being and the world. Rosmini posits three reasons for this. First, although finite being is perceived, perception is limited to very few beings. If the essential conditions of this being are to be known, it is necessary to know the common conditions of all finite beings. This cannot be done without recourse to universal principles which enable us to deduce what is lacking to limited experience. Second, the same principles are needed to provide a notion of finite beings in themselves, divested of the sensory phenomena that accompany their perception. Third, subsistent Being, if known, would be intelligible per se. This is not the case with finite beings, which are known through their participation in intellectual light, and consequently remain unknown without some relationship to concepts.