Metaphysics is the branch of philosophy that examines the fundamental nature of reality, including the relationship between mind and matter, between substance and attribute, and between potentiality and actuality. The word "metaphysics" comes from two Greek words that, together, literally mean "after or behind or among the natural." Metaphysics studies questions related to what it is for something to exist and what types of existence there are. Metaphysics seeks to answer, in an abstract and fully general manner, the questions: 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.
Metaphysics is considered one of the four main branches of philosophy, along
with epistemology, logic, and ethics. The
metaphysical positions one takes on identity have far-reaching implications on
issues such as the Mind–body problem, personal identity, ethics, and law.
Metaphysical study is conducted using deduction from that which is known a priori. Like foundational mathematics, which is considered a special case of metaphysics applied to the existence of number, it tries to give a coherent account of the structure of the world, capable of explaining our everyday and scientific perception of the world, and being free from contradictions. In mathematics, there are many different ways to define numbers; similarly, in metaphysics, there are many different ways to define objects, properties, concepts, and other entities that are claimed to make up the world.
While metaphysics may, as a special case, study the entities postulated by fundamental science such as atoms and superstrings, its core topic is the set of categories such as object, property and causality which those scientific theories assume. For example: claiming that "electrons have charge" is a scientific theory; while exploring what it means for electrons to be "objects", charge to be a "property", and for both to exist in a topological entity called "space" is the task of metaphysics.
There are two broad stances about what is "the world"
studied by metaphysics. According to metaphysical realism, the objects studied
by metaphysics exist independently of any observer so that the subject is the
most fundamental of all sciences. Metaphysical anti-realism, on the other hand,
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. This position is of more recent origin.
Some philosophers, notably Immanuel Kant, discuss both of these "worlds" and what can be inferred about each one. Some, such as the logical positivists, and many scientists, reject the metaphysical realism 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. Metaphysics itself usually assumes that some stance has been taken on these questions and that it may proceed independently of the choice—the question of which stance to take belongs instead to another branch of philosophy, epistemology.
Identity is a fundamental metaphysical concern. Metaphysicians investigating identity are tasked with the question of what, exactly, it means for something to be identical to itself, or – more controversially – to something else. Issues of identity arise in the context of time: what does it mean for something to be itself across two moments in time? How do we account for this? Another question of identity arises when we ask what our criteria ought to be for determining identity, and how the reality of identity interfaces with linguistic expressions.