Metaphysics is a branch of philosophy concerned with explaining the fundamental nature of being and the world that encompasses it. Metaphysics seeks to address two two propositions ... what exists and what is it. In that pursuit it has various branches.
A particularly interesting branch of Metaphysics is Ontology. The central questions of Ontology include:
"What can be said to exist?"
"What is a thing?"
"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?"
Some philosophers have provided different answers to these questions. One approach involves dividing the subjects and predicates into groups called categories. Such lists of categories differ widely from one another. It is through the co-ordination of different categorical schemes that ontology relates to such fields as library science and artificial intelligence. Such an understanding of ontological categories, may however be simply taxonomic. Aristotle's categories are the ways in which a being may be addressed simply as a being, such as:
what it is (its 'whatness')how it is (its 'howness' or qualitativeness)how much it is (quantitativeness)where it is, its relatedness to other beings
Further examples of ontological questions include:
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?
Do physical properties actually exist?
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 is a physical object?
Can one give an account of what it means to say that a physical object exists?
Can one give an account of what it means to say that a non-physical entity exists?
What constitutes the identity of an object?
When does an object go out of existence, as opposed to merely changing?
Do beings exist other than in the modes of objectivity and subjectivity, i.e. is the subject/object split of modern philosophy inevitable?