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 is the "world" 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 weak, 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, such as the logical positivists, and many scientists, reject the strong view of metaphysics as meaningless and unverifiable. Others reply that this criticism also applies to any type of knowledge, including hard science, which claims to describe anything other than the contents of human perception, and thus that the world of perception is the objective world in some sense.
The word "metaphysics" derives from the Greek words μετά (metá, "beyond", "upon" or "after") and φυσικά (physiká, "physics"). It was first used as the title for several of Aristotle's works, because they were usually anthologized after the works on physics in complete editions. The prefix meta- ("after") indicates that these works come "after" the chapters on physics. However, Aristotle himself did not call the subject of these books metaphysics: he referred to it as "first philosophy." The editor of Aristotle's works, Andronicus of Rhodes, is thought to have placed the books on first philosophy right after another work, Physics, and called them τὰ μετὰ τὰ φυσικὰ βιβλία (tà metà tà physikà biblía) or "the books after the physics". This was misread by Latin scholiasts, who thought it meant "the science of what is beyond the physical".
However, once the name was given, the commentators sought to find intrinsic reasons for its appropriateness. For instance, it was understood to mean "the science of the world beyond nature" (φύσις - phýsis in Greek), that is, the science of the immaterial. Again, it was understood to refer to the chronological or pedagogical order among our philosophical studies, so that the "metaphysical sciences" would mean "those that we study after having mastered the sciences that deal with the physical world" (St. Thomas Aquinas, Expositio in librum Boethii De hebdomadibus, V, 1).