Metaphysics is one of the core branches of philosophy and it is among the oldest ones. Metaphysics centers on the study of the most fundamental categories of reality and their relations, such as god, person, thing, event, property, causation. The question typical of this branch is, hence: “What is there, really?”
Since at least Aristotle, a central category in metaphysics has been the one of individual, at times also referred to as thing or particular or individual substance. Most of us nowadays also represent the world as populated of individual things, including individual people, animals, plants, mountains, rocks, and commodities such as cars, books, or computers. The concept of the individual is most easily coupled with oneness: individuals are the basic unities of reality; of each individual there is one and only one, necessarily. Thus, for example, there may be a lot of people who resemble you; you may even had an identical twin or you may fancy of having some clones of yourself; still, no one else is you. There is only one of you and in this lies your individuality. Western philosophy, and more generally Western civilization, seems to be centered on the concept of the individual, unlike other traditions in which the collectivity or the whole is privileged.
A property is that which qualifies what an individual is like: tall, charged, red, clean, healthy, fruity, and so on. The landmark of a property is its being an entity which can occur more than once (although in need not to), unlike individuals that are unique. Thus, two individual cans of soda may have exactly the same color, or the same weight: that color and that weight will be two properties that the cans have in common. Properties are divided into different types, some of which are opposing pairs, such as dispositional vs categorical, sparse vs abundant, essential vs accidental, intrinsic vs extrinsic, natural vs conventional.
Events deserve special consideration in the inventory of the major categories. With the exception of achievements, events are entities whose existence spurs across time. For example, volleyball game can last for three hours and an art exhibit for a month. Some hold that events are individuals, whose peculiarity consists in extending through time: each individual volleyball game is hence a different entity. Other philosophers instead believe that events can occur multiple times, just like properties: for example, the showing of a movie typically occurs multiple times.
Metaphysicians study also fundamental relations among individuals, or among properties, or among individuals and properties. One such relation is causation. Over the centuries multiple theories of causation have been offered. Some philosophers see causation as a tie between different individuals; others as relating different properties; others still, as relating different events. Regardless of the dispute, causation involves some sort of necessary link between entities: to say that A caused B is to say that B’s occurrence was necessitated by A’s occurrence. To spell out such necessitation relation is one of the hardest metaphysical issues to date. Causation is often discussed in connection with laws of nature: if there are laws in nature (such as gravitational laws) then they are displayed in the causal relationships among entities.
In most philosophical traditions it is common to find metaphysical systems that include a special being, having such properties as immutability, indivisibility, perfection, infinite knowledge, infinite power, or infinite goodness. Clearly this is the case of monotheistic religions. Although ancient Greek philosophers seldom explicitly make reference to “god” or “gods” in metaphysical treatises, it is common to find an appeal to a special being, such as Aristotle’s “prime mover” or Parmenides’s “one.” In the medieval and early modern periods, the philosophical discourse on God played a central role; see for example Descartes's appeal to God's existence in the Meditations