Metaphysics is the philosophical investigation of reality, including the nature of the world and the entities it contains. While such an investigation inevitably overlaps with natural philosophy—the sciences, especially physics and the natural sciences—it can be contrasted in two important respects.
First, the topics of metaphysics are, in a sense, more fundamental than those of the sciences because they are concerned with the presuppositions made within these topics. For example, our scientific theories are often held to discover the laws of nature, but they do not say what it means to be a law of nature—whether a particularly robust regularity that just happens to hold between different entities, a special kind of relationship that holds between certain properties, or ultimately a claim about what happens in other possible worlds. Similarly, while the natural sciences study and manipulate various causal relationships, it is a task of metaphysics to investigate the nature of causation itself, to question (for instance) whether an alleged cause is an objective feature of reality or, rather, a function of the way in which we perceive reality.
Second, the scope of metaphysics is broader than that of the natural sciences in that it is also concerned with the existence of nonphysical entities. This, in fact, is the origin of the name metaphysics—it is literally beyond, after, or outside of (meta-) physics and the physical. The nonphysical objects of metaphysical contemplation may include (among other things) irreducibly mental entities, abstract objects (such as mathematical objects), and even the existence of other possible worlds.
Metaphysics is also concerned with whether certain properties are essential to an entity (as opposed to merely accidental) and if properties can be multiply instantiated among different entities (or if everything in the world is an individual). In the early half of the twentieth century, various philosophical schools of thought maintained that metaphysics should be abandoned as a philosophical discipline, either on the grounds that it is just bad science or that its statements are unverifiable (and thus meaningless); traditional metaphysical questions were, therefore, reduced to logical questions about the use of language. In the 1970s, however, Saul Kripke’s (1940-) work on reference in the philosophy of language
sparked renewed interest in substantive metaphysics and has produced ongoing research into questions concerning the nature of necessity, essential properties, and the identity conditions of objects across different possible worlds.
