  Most traditional category systems, such as Aristotle’s, lay out a single dimension of categories supposed to be mutually exclusive and exhaustive. Ingarden, by contrast, develops a multi-dimensional category scheme by dividing ontology into three parts: formal, material and existential ontologies, corresponding to three distinct aspects that may be discerned in any entity (its formal structure, material nature, and mode of being respectively). These different formal, material and existential aspects of the object, studied by the different types of ontology, may thus be used to classify an object in any of three interpenetrating dimensions (although not all combinations among formal, material and existential modes are possible).