Ontology is the philosophical study of being, existence or reality, as well as the basic categories of being and their relations. Ontology deals with questions relating to entities that exist or can be said to exist, and how such entities can be grouped, related within a hierarchy, and subdivided according to similarities and differences. There are two pillars of thought that stand poles apart within ontology. Realism and Nominalism.
Platonic realism refers to the idea of realism regarding the existence of universals or abstract objects. Universals were considered by Plato to be ideals, and so this stance is also referred to as Platonic Idealism. These abstractions are not spatial, temporal or mental. They cannot be seen or come into sensory contact with people. They in include numbers and geometrical figures (making them a theory of mathematical realism) and the Form of the Good (abstractions are referred to as forms). The Form of the Good makes these ideals a theory of ethical realism as well.
Some contemporary linguistic philosophers construe “Platonism” to mean the proposition that universals exist independently of particulars (a universal is anything that can be predicated of a particular). Similarly, a form of modern Platonism is found in the predominant philosophy of mathematics, especially regarding the foundations of mathematics. The Platonic interpretation of this philosophy includes the thesis that mathematics is not created but discovered.
Universals and Forms are slightly different. Forms refer to paradigms (patterns in nature). An example of Platonic form would be a material triangle contrasted to an ideal triangle. The Platonic form is the ideal triangle — a figure with perfectly drawn lines whose angles add to 180 degrees. Any form of triangle that we experience will be an imperfect representation of the ideal triangle. Regardless of how precise your measuring and drawing tools you will never be able to recreate this perfect shape. Even drawn to the point where our senses cannot perceive a defect, in its essence the shape will still be imperfect; forever unable to match the ideal triangle.
In Platonic realism, forms are related to particulars (instances of objects and properties) in that a particular is regarded as a copy of its form. For example, a particular apple is said to be a copy of the form of Applehood and the apple’s redness is an instance of the form of Redness. Participation is another relationship between forms and particulars. Particulars are said to participate in the forms, and the forms are said to inhere in the particulars.
There are two versions of nominalism: One version denies the existence of universals—things that can be instantiated or exemplified by many particular things (e.g. strength, humanity), the other version specifically denies the existence of abstract objects—objects that do not exist in space and time. General or abstract terms and predicates exist, but universals or abstract objects do not exist. However, some versions of nominalism hold that some particulars are abstract entities (e.g. numbers), while others are concrete entities—entities that do exist in space and time (e.g. tables, chairs).
Principle Questions of Ontology
“What can be said to exist?”
“Into what categories, if any, can we sort existing things?”
“What are the meanings of being?”
“What are the various modes of being of entities?”
“What is existence, i.e. what does it mean for a being to be?”
“Is existence a property?”
“Is existence a genus or general class that is simply divided up by specific differences?”
“Which entities, if any, are fundamental? Are all entities objects?”
“How do the properties of an object relate to the object itself?”
“What features are the essential, as opposed to merely accidental attributes of a given object?”
“How many levels of existence or ontological levels are there? And what constitutes a ‘level’?”
“What constitutes the identity of an object?”
“When does an object go out of existence, as opposed to merely changing?”