Wikipedia lists some basic issues of ontology, which are roughly the same question:
"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?"
It then lists 14 more specific question some of which are again another form of another question (like 1 and 10, 7 and 9, etc).
- 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?
Now, can anyone tell which, if any, of these question has found an answer (meaning an answer agreed by [almost] everyone) and who was the author?
P.S.Could you explain what is the level mention in question 9?
the response is generally odd: posing an unanswerable question would be really stupid, like elucubrating about the "angels on a pin" ( whereof one cannot speak....).
But ontology is not concerned with supernatural phenomena, being is something everyone directly experiences, so "what can be said to exist?" "what is a thing" are quite answerable questions, and should be just intelligent and useful definition to be used in further inquiries. They are not much more difficult to answer than "what can be said to live/ when is one alive??" answers may be diverse and apt for different situations.