Metaphysics is a branch of philosophy exploring the fundamental nature of reality. Metaphysics seeks to answer two basic questions:
Ultimately, what is there?
What is it like?
Topics of metaphysical investigation include existence, objects and their properties, space and time, cause and effect, and possibility. A central branch of metaphysics is ontology, the investigation into the basic categories of being and how they relate to one another.
There are two broad conceptions about what "world" is studied by metaphysics. The strong, classical view assumes that the objects studied by metaphysics exist independently of any observer, so that the subject is the most fundamental of all sciences. The weaker, more modern view assumes that the objects studied by metaphysics exist inside the mind of an observer, so the subject becomes a form of introspection and conceptual analysis. Some philosophers, notably Kant, discuss both of these "worlds" and what can be inferred about each one.
Some philosophers and scientists, such as the logical positivists, reject the entire subject of metaphysics as meaningless, while others disagree and think that it is legitimate.