 Finally, formal ontologies have been proposed as systematic theories of what there is, with some restrictions. Such systematic theories hope to give one formal theory for all there is, or at least a good part of it. Hardly anyone would claim that there can be a simple formal theory that correctly states what concrete physical objects there are. There does not seem to be a simple principle that determines whether there are an even or odd number of mice at a particular time. But maybe this apparent randomness only holds for concrete physical objects. It might not hold for abstract objects, which according to many exist not contingently, but necessarily if at all. Maybe a systematic, simple formal theory is possible of all abstract objects. Such a systematic formal ontology will most commonly have one kind of entities which are the primary subject of the theory, and a variety of different notions of reduction that specify how other (abstract) objects really are entities of this special kind. A simple view of this kind would be one according to which all abstract objects are sets, and numbers, properties, etc. are really special kinds of sets. However, more sophisticated versions of systematic formal ontologies have been developed. An ambitious systematic formal ontology can be found in (Zalta 1983) and (Zalta 1999, in the Other Internet Resources).