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Existence and categories of being

Metaphysicians often see existence or being as one of the most basic and general concepts. To exist means to form part of reality and existence marks the difference between real entities and imaginary ones. According to the orthodox view, existence is a second-order property or a property of properties: if an entity exists then its properties are instantiated. A different position states that existence is a first-order property, meaning that it is similar to other properties of entities, such as shape or size. It is controversial whether all entities have this property. According to Alexius Meinong, there are nonexistent objects, including merely possible objects like Santa Claus and Pegasus. A related question is whether existence is the same for all entities or whether there are different modes or degrees of existence. For instance, Plato held that Platonic forms, which are perfect and immutable ideas, have a higher degree of existence than matter, which is only able to imperfectly mirror Platonic forms.

Another key concern in metaphysics is the division of entities into different groups based on underlying features they have in common. Theories of categories provide a system of the most fundamental kinds or the highest genera of being by establishing a comprehensive inventory of everything. One of the earliest theories of categories was provided by Aristotle, who proposed a system of 10 categories. Substances (e.g. man and horse), are the most important category since all other categories like quantity (e.g. four), quality (e.g. white), and place (e.g. in Athens) are said of substances and depend on them. Kant understood categories as fundamental principles underlying human understanding and developed a system of 12 categories, which are divided into the four classes quantity, quality, relation, and modality. More recent theories of categories were proposed by C. S. Peirce, Edmund Husserl, Samuel Alexander, Roderick Chisholm, and E. J. Lowe. Many philosophers rely on the contrast between concrete and abstract objects. According to a common view, concrete objects, like rocks, trees, and human beings, exist in space and time, undergo changes, and impact each other as cause and effect, while abstract objects, like numbers and sets, exist outside space and time, are immutable, and do not enter into causal relations.