Metaphysics



Metaphysics is the branch of philosophy that examines the basic structure of reality. It is often characterized as first philosophy, implying that it is more fundamental than other forms of philosophical inquiry. Metaphysics is traditionally seen as the study of mind-independent features of the world, but some modern theorists understand it as an inquiry into the conceptual schemes that underlie human thought and experience.

Many general and abstract topics belong to the subject of metaphysics. It investigates the nature of existence, the features all entities have in common, and their division into categories of being. An influential contrast is between particulars, which are individual unique entities, like a specific apple, and universals, which are general repeatable entities that characterize particulars, like the color red. Modal metaphysics examines what it means for something to be possible or necessary. The nature of space, time, and change is also discussed by metaphysicians. A closely related issue concerns the essence of causality and its relation to the laws of nature. Other topics include how mind and matter are related, whether everything in the world is predetermined, and whether there is free will.

Metaphysicians employ various methods to conduct their inquiry. Traditionally, they rely on rational intuitions and abstract reasoning but have more recently also included empirical approaches associated with scientific theories. Due to the abstract nature of its topic, metaphysics has received criticisms questioning the reliability of its methods and the meaningfulness of its theories. Metaphysics is relevant to many fields of inquiry that often implicitly rely on metaphysical concepts and assumptions.

The origin of metaphysics lies in antiquity with speculations about the nature of reality and the universe, like those found in the Upanishads in ancient India, Daoism in ancient China, and pre-Socratic philosophy in ancient Greece. The subsequent medieval period in the West discussed the nature of universals as shaped by ancient Greek philosophers Plato and Aristotle. The modern period saw the emergence of various comprehensive systems of metaphysics, many of which embraced idealism. In the 20th century and contemporary period, a "revolt against idealism" was started, and metaphysics was once declared meaningless, then revived with various criticisms of earlier theories and new approaches to metaphysical inquiry.

Definition
Metaphysics is the study of the most general features of reality, including existence, objects and their properties, possibility and necessity, space and time, change, causation, and the relation between matter and mind. It is one of the oldest branches of philosophy.

The precise nature of metaphysics is disputed and its characterization has changed in the course of history. Some approaches see metaphysics as a unified field and give a wide-sweeping definition by understanding it as the study of "fundamental questions about the nature of reality" or as an inquiry into the essences of things. Another approach doubts that the different areas of metaphysics share a set of underlying features and provides instead a fine-grained characterization by listing all the main topics investigated by metaphysicians. Some definitions are descriptive by providing an account of what metaphysicians do while others are normative and prescribe what metaphysicians ought to do.

Two historically influential definitions in ancient and medieval philosophy understand metaphysics as the science of the first causes and as the study of being qua being, that is, the topic of what all beings have in common and to what fundamental categories they belong. In the modern period, the scope of metaphysics was extended to cover topics such as the distinction between mind and body and free will. Some philosophers follow Aristotle in describing metaphysics as "first philosophy", implying that it is the most basic inquiry while all other branches of philosophy depend on it in some way.



Metaphysics is traditionally understood as a study of mind-independent features of reality. Starting with Immanuel Kant's critical philosophy, an alternative conception gained prominence that focuses on conceptual schemes rather than external reality. Kant distinguishes transcendent metaphysics, which aims to describe the objective features of reality beyond sense experience, from critical metaphysics, which outlines the aspects and principles underlying all human thought and experience. Regarding the analysis of conceptual schemes, philosopher P. F. Strawson contrasts descriptive metaphysics, which articulates conceptual schemes commonly used to understand the world, with revisionary metaphysics, which aims to produce better conceptual schemes.

Metaphysics differs from the individual sciences by studying very general and abstract aspects of reality. The individual sciences, by contrast, examine more specific and concrete features and restrict themselves to certain classes of entities, such as the focus on physical things in physics, living entities in biology, and cultures in anthropology. It is disputed to what extent this contrast is a strict dichotomy rather than a gradual continuum.

Philosophers engaged in metaphysics are called metaphysicians or metaphysicists. Outside the academic discourse, the term metaphysics is sometimes used in a different sense for the study of occult and paranormal phenomena, like metaphysical healing, auras, and the power of pyramids.

Etymology
The word metaphysics has its origin in the ancient Greek words metá (μετά, meaning after, above, and beyond) and phusiká (φυσικά) as a short form of ta metá ta phusiká: that is, what comes after the physics. This is frequently interpreted in the sense that metaphysics discusses topics that, due to their generality and comprehensiveness, lie beyond the realm of physics and its focus on empirical observation. Metaphysics got its name by a historical accident when Aristotle's book on this subject was published. Aristotle did not use the term metaphysics but his editor (likely Andronicus of Rhodes) may have coined it for its title to indicate that this book should be studied after Aristotle's book published on physics: literally after physics. The term entered the English language through the Latin word metaphysica.

Branches
The nature of metaphysics can also be characterized in relation to its main branches. An influential division from early modern philosophy distinguishes between general and special or specific metaphysics. General metaphysics, also called ontology, takes the widest perspective and studies the most fundamental aspects of being. It investigates the features that all entities have in common and how entities can be divided into different categories. Categories are the most general kinds, such as substance, property, relation, and fact. Ontologists research which categories there are, how they depend on one another, and how they form a system of categories that provides an encompassing classification of all entities.

Special metaphysics considers being from more narrow perspectives and is divided into subdisciplines based on the perspective they take. Metaphysical cosmology examines changeable things and investigates how they are connected to form a world as a totality of entities extending through space and time. Rational psychology restricts itself to exploring metaphysical foundations and problems concerning the mind, such as its relation to matter and the freedom of the will. Natural theology studies the divine and its role as the first cause. The scope of special metaphysics overlaps with other philosophical disciplines and it is often not clear whether a topic belongs to it rather than to disciplines like philosophy of mind and theology.

Applied metaphysics is a young subdiscipline. It belongs to applied philosophy and studies the applications of metaphysics, both within philosophy and other fields of inquiry. In ethics and philosophy of religion, it concerns topics like the ontological foundation of moral claims and religious doctrines. Applications outside philosophy include the use of ontologies in artificial intelligence, economics, and sociology to classify entities as well as questions in psychiatry and medicine about the metaphysical status of diseases.

Meta-metaphysics is the metatheory of metaphysics and investigates the nature and methods of metaphysics. It also examines how metaphysics differs from other philosophical and scientific disciplines and how it is relevant to them. While the discussions of its topics have a long history in metaphysics, it has only recently developed into a systematic field of inquiry.

Existence and categories of being
Metaphysicians often see existence or being as one of the most basic and general concepts. To exist means to form part of reality and existence marks the difference between real entities and imaginary ones. According to the orthodox view, existence is a second-order property or a property of properties: if an entity exists then its properties are instantiated. A different position states that existence is a first-order property, meaning that it is similar to other properties of entities, such as shape or size. It is controversial whether all entities have this property. According to Alexius Meinong, there are nonexistent objects, including merely possible objects like Santa Claus and Pegasus. A related question is whether existence is the same for all entities or whether there are different modes or degrees of existence. For instance, Plato held that Platonic forms, which are perfect and immutable ideas, have a higher degree of existence than matter, which is only able to imperfectly mirror Platonic forms.

Another key concern in metaphysics is the division of entities into different groups based on underlying features they have in common. Theories of categories provide a system of the most fundamental kinds or the highest genera of being by establishing a comprehensive inventory of everything. One of the earliest theories of categories was provided by Aristotle, who proposed a system of 10 categories. Substances (e.g. man and horse), are the most important category since all other categories like quantity (e.g. four), quality (e.g. white), and place (e.g. in Athens) are said of substances and depend on them. Kant understood categories as fundamental principles underlying human understanding and developed a system of 12 categories, which are divided into the four classes quantity, quality, relation, and modality. More recent theories of categories were proposed by C. S. Peirce, Edmund Husserl, Samuel Alexander, Roderick Chisholm, and E. J. Lowe. Many philosophers rely on the contrast between concrete and abstract objects. According to a common view, concrete objects, like rocks, trees, and human beings, exist in space and time, undergo changes, and impact each other as cause and effect, while abstract objects, like numbers and sets, exist outside space and time, are immutable, and do not enter into causal relations.

Particulars
Particulars are individual entities and include both concrete objects, like Aristotle, the Eiffel Tower, or a specific apple, and abstract objects, like the number 2 or a specific set in mathematics. Also called individuals, they are unique, non-repeatable entities and contrast with universals, like the color red, which can at the same time exist in several places and characterize several particulars. A widely held view is that particulars instantiate universals but are not themselves instantiated by something else, meaning that they exist in themselves while universals exist in something else. Substratum theory analyzes particulars as a substratum, also called bare particular, together with various properties. The substratum confers individuality to the particular while the properties express its qualitative features or what it is like. This approach is rejected by bundle theorists, who state that particulars are only bundles of properties without an underlying substratum. Some bundle theorists include in the bundle an individual essence, called haecceity, to ensure that each bundle is unique. Another proposal for concrete particulars is that they are individuated by their space-time location.

Concrete particulars encountered in everyday life, like rocks, tables, and organisms, are complex entities composed of various parts. For example, a table is made up of a tabletop and legs, each of which is itself made up of countless particles. The relation between parts and wholes is studied by mereology. The problem of the many is about which groups of entities form mereological wholes, for instance, whether a dust particle on the tabletop forms part of the table. According to mereological universalists, every collection of entities forms a whole, meaning that the parts of the table without the dust particle form one whole while they together with it form a second whole. Mereological moderatists hold that certain conditions have to be fulfilled for a group of entities to compose a whole, for example, that the entities touch one another. Mereological nihilists reject the idea that there are any wholes. According to them, there are no tables and chairs but only particles that are arranged table-wise and chair-wise. A related mereological problem is whether there are simple entities that have no parts, as atomists claim, or not, as continuum theorists contend.

Universals
Universals are general entities, encompassing both properties and relations, that express what particulars are like and how they resemble one another. They are repeatable, meaning that they are not limited to a unique existent but can be instantiated by different particulars at the same time. For example, the particulars Nelson Mandela and Mahatma Gandhi instantiate the universal humanity, similar to how a strawberry and a ruby instantiate the universal red.

A topic discussed since ancient philosophy, the problem of universals consists in the challenge of characterizing the ontological status of universals. Realists argue that universals are real, mind-independent entities that exist in addition to particulars. According to Platonic realists, universals exist also independently of particulars, which implies that the universal red would continue to exist even if there were no red things. A more moderate form of realism, inspired by Aristotle, states that universals depend on particulars, meaning that they are only real if they are instantiated. Nominalists reject the idea that universals exist in either form. For them, the world is composed exclusively of particulars. The position of conceptualists constitutes a middle ground: they state that universals exist, but only as concepts in the mind used to order experience by classifying entities.

Natural and social kinds are often understood as special types of universals. Entities belonging to the same natural kind share certain fundamental features characteristic of the structure of the natural world. In this regard, natural kinds are not an artificially made-up classification but are discovered, usually by the natural sciences, and include kinds like electrons, H2O, and tigers. Scientific realists and anti-realists are in disagreement about whether natural kinds exist. Social kinds are studied by social metaphysics and characterized as useful social constructions that, while not purely fictional, fail to reflect the fundamental structure of mind-independent reality. Uncontroversial examples include money or baseball.

Possibility and necessity
The concepts of possibility and necessity convey what can or must be the case, expressed in statements like "it is possible to find a cure for cancer" and "it is necessary that two plus two equals four". They belong to modal metaphysics, which investigates the metaphysical principles underlying them, in particular, why it is the case that some modal statements are true while others are false. Some metaphysicians hold that modality is a fundamental aspect of reality, meaning that besides facts about what is the case, there are additional facts about what could or must be the case. A different view argues that modal truths are not about an independent aspect of reality but can be reduced to non-modal characteristics, for example, to facts about what properties or linguistic descriptions are compatible with each other or to fictional statements.

Borrowing a term from German philosopher Gottfried Wilhelm Leibniz's theodicy, many metaphysicians use the concept of possible worlds to analyze the meaning and ontological ramifications of modal statements. A possible world is a complete and consistent way of how things could have been. For example, the dinosaurs were wiped out in the actual world but there are possible worlds in which they are still alive. According to possible world semantics, a statement is possibly true if it is true in at least one possible world while it is necessarily true if it is true in all possible worlds. Modal realists argue that possible worlds exist as concrete entities in the same sense as the actual world, with the main difference being that the actual world is the world we live in while other possible worlds are inhabited by counterparts. This view is controversial and various alternatives have been suggested, for example, that possible worlds only exist as abstract objects or that they are similar to stories told in works of fiction.

Space, time, and change
Space and time are dimensions that entities occupy. Spacetime realists state that space and time are fundamental aspects of reality and exist independently of the human mind. This view is rejected by spacetime idealists, who hold that space and time are constructions of the human mind in its attempt to organize and make sense of reality. Spacetime absolutism or substantivalism understands spacetime as a distinct object, with some metaphysicians conceptualizing it as a box that contains all other entities within it. Spacetime relationism, by contrast, sees spacetime not as an object but as relations between objects, such as the spatial relation of being next to and the temporal relation of coming before.

In the metaphysics of time, an important contrast is between the A-series and the B-series. According to the A-series theory, the flow of time is real, meaning that events are categorized into the past, present, and future. The present keeps moving forward in time and events that are in the present now will change their status and lie in the past later. From the perspective of the B-series theory, time is static and events are ordered by the temporal relations earlier-than and later-than without any essential difference between past, present, and future. Eternalism holds that past, present, and future are equally real while according to presentists, only entities in the present exist.

Material objects persist through time and change in the process, like a tree that grows or loses leaves. The main ways of conceptualizing persistence through time are endurantism and perdurantism. According to endurantism, material objects are three-dimensional entities that are wholly present at each moment. As they change, they gain or lose properties but remain the same otherwise. Perdurantists see material objects as four-dimensional entities that extend through time and are made up of different temporal parts. At each moment, only one part of the object is present but not the object as a whole. Change means that an earlier part is qualitatively different from a later part. For example, if a banana ripens then there is an unripe part followed by a ripe part.

Causality
Causality is the relation between cause and effect whereby one entity produces or affects another entity. For instance, if a person bumps a glass and spills its contents then the bump is the cause and the spill is the effect. Besides the single-case causation between particulars in this example, there is also general-case causation expressed in general statements such as "smoking causes cancer". The term agent causation is used if people and their actions cause something. Causation is usually interpreted deterministically, meaning that a cause always brings about its effect. This view is rejected by probabilistic theories, which claim that the cause merely increases the probability that the effect occurs. This view can be used to explain that smoking causes cancer even though this is not true in every single case.

The regularity theory of causation, inspired by David Hume's philosophy, states that causation is nothing but a constant conjunction in which the mind apprehends that one phenomenon, like putting one's hand in a fire, is always followed by another phenomenon, like a feeling of pain. According to nomic regularity theories, the regularities take the forms of laws of nature studied by science. Counterfactual theories focus not on regularities but on how effects depend on their causes. They state that effects owe their existence to the cause and would not be present without them. According to primitivism, causation is a basic concept that cannot be analyzed in terms of non-causal concepts, such as regularities or dependence relations. One form of primitivism identifies causal powers inherent in entities as the underlying mechanism. Eliminativists reject the above theories by holding that there is no causation.

Mind and free will


Mind encompasses phenomena like thinking, perceiving, feeling, and desiring as well as the underlying faculties responsible for these phenomena. The mind–body problem is the challenge of clarifying the relation between physical and mental phenomena. According to Cartesian dualism, minds and bodies are distinct substances. They causally interact with each other in various ways but can, at least in principle, exist on their own. This view is rejected by monists, who argue that reality is made up of only one kind. According to idealism, everything is mental, including physical objects, which may be understood as ideas or perceptions of conscious minds. Materialists, by contrast, state that all reality is at its core material. Some deny that mind exists but the more common approach is to explain mind in terms of certain aspects of matter, such as brain states, behavioral dispositions, or functional roles. Neutral monists argue that reality is fundamentally neither material nor mental and suggest that matter and mind are both derivative phenomena. A key aspect of the mind–body problem is the hard problem of consciousness, which concerns the question of how physical systems like brains can produce phenomenal consciousness.

The status of free will as the ability of a person to choose their actions is a central aspect of the mind–body problem. Metaphysicians are interested in the relation between free will and causal determinism, the view that everything in the universe, including human behavior, is determined by preceding events and laws of nature. It is controversial whether causal determinism is true, and, if so, whether this would imply that there is no free will. According to incompatibilism, free will cannot exist in a deterministic world since there is no true choice or control if everything is determined. Hard determinists infer from this observation that there is no free will while libertarians conclude that determinism must be false. Compatibilists take a third approach by arguing that determinism and free will do not exclude each other, for instance, because a person can still act in tune with their motivation and choices even if they are determined by other forces. Free will plays a key role in ethics regarding the moral responsibility people have for what they do.

Others
Identity is a relation that every entity has to itself as a form of sameness. It refers to numerical identity when the very same entity is involved, as in the statement "the morning star is the evening star" (both are the planet Venus). In a slightly different sense, it encompasses qualitative identity, also called exact similarity and indiscernibility, which is the case when two distinct entities are exactly alike, such as perfect identical twins. The principle of the indiscernibility of identicals is widely accepted and holds that numerically identical entities exactly resemble one another. The converse principle, known as identity of indiscernibles or Leibniz's Law, is more controversial and states that two entities are numerically identical if they exactly resemble one another. Another distinction is between synchronic and diachronic identity. Synchronic identity relates an entity to itself at the same time while diachronic identity is about the same entity at different times, as in statements like "the table I bought last year is the same as the table in my dining room now". Personal identity is a related topic in metaphysics that uses the term identity in a slightly different sense and concerns questions like what personhood is or what makes someone a person.

Various contemporary metaphysicians rely on the concepts of truth, truth-bearer, and truthmaker to conduct their inquiry. Truth is a property of being in accord with reality. Truth-bearers are entities that can be true or false, such as linguistic statements and mental representations. A truthmaker of a statement is the entity whose existence makes the statement true. For example, the statement "a tomato is red" is true because there exists a red tomato as its truthmaker. Based on this observation, it is possible to pursue metaphysical research by asking what the truthmakers of statements are, with different areas of metaphysics being dedicated to different types of statements. According to this view, modal metaphysics asks what makes statements about what is possible and necessary true while the metaphysics of time is interested in the truthmakers of temporal statements about the past, present, and future.

Methodology
Metaphysicians employ a variety of methods to arrive at metaphysical theories and formulate arguments for and against them. Traditionally, a priori methods are the dominant approach. They rely on rational intuition and abstract reasoning from general principles rather than sensory experience. A posteriori approaches, by contrast, ground metaphysical theories in empirical observations and scientific theories. Some metaphysicians use perspectives from fields such as physics, psychology, linguistics, and history to conduct their inquiry. The two approaches are not exclusive and it is possible to combine elements from both. Which method a metaphysician employs often depends on their conception of the nature of metaphysics, for example, whether they see it as an inquiry into the mind-independent structure of reality, as metaphysical realists claim, or the principles underlying thought and experience, as some metaphysical anti-realists contend.

A priori approaches often rely on intuitions, that is, non-inferential impressions about the correctness of specific claims or general principles. For example, arguments for the A-theory of time, which states that time flows from the past through the present and into the future, often rely on pre-theoretical intuitions associated with the sense of the passage of time. Some approaches use intuitions to establish a small set of self-evident fundamental principles, known as axioms, and employ deductive reasoning to build complex metaphysical systems by drawing conclusions from these axioms. Intuition-based approaches can be combined with thought experiments, which help evoke and clarify intuitions by linking them to imagined situations while using counterfactual thinking to assess the possible consequences of these situations. To explore the relation between matter and consciousness, some theorists compare humans to philosophical zombies, that is, hypothetical creatures identical to humans but without conscious experience. A related method relies on commonly accepted beliefs instead of intuitions to formulate arguments and theories. The common-sense approach is often used to criticize metaphysical theories that deviate a lot from how the average person thinks about an issue. For example, common-sense philosophers have argued that mereological nihilism is false since it implies that commonly accepted things, like tables, do not exist.

Conceptual analysis, a method particularly prominent in analytic philosophy, aims to decompose metaphysical concepts into component parts to clarify their meaning and identify essential relations. In phenomenology, the method of eidetic variation is used to investigate essential structures underlying phenomena. To study the essential features of any kind of object, it proceeds by imagining this object and varying its features to identify which ones are essential and cannot be changed. The transcendental method is a further approach and examines the metaphysical structure of reality by observing what entities there are and studying the conditions of possibility without which these entities could not exist.

Some approaches give less importance to a priori reasoning and see metaphysics instead as a practice continuous with the empirical sciences that generalizes their insights while making their underlying assumptions explicit. This approach is known as naturalized metaphysics and is closely associated with the work of Willard Van Orman Quine. He relies on the idea that true sentences from the sciences and other fields have ontological commitments, that is, they imply that certain entities exist. For example, if the sentence "some electrons are bonded to protons" is true then it can be used to justify that electrons and protons exist. Quine used this insight to argue that one can learn about metaphysics by closely analyzing scientific claims to understand what kind of metaphysical picture of the world they presuppose.

In addition to methods of conducting metaphysical inquiry, there are various methodological principles used to decide between competing theories by comparing their theoretical virtues. Ockham's Razor is a well-known principle that gives preference to simple theories, in particular, to theories that assume that few entities exist. Other principles consider explanatory power, theoretical usefulness, and proximity to established beliefs.

Criticism


Despite its status as one of the main branches of philosophy, metaphysics has received numerous criticisms putting into question its status as a legitimate field of inquiry. One type of criticism states that metaphysical inquiry is impossible because humans do not have the cognitive capacities needed to access the ultimate nature of reality. This line of thought leads to a form of skepticism about the possibility of metaphysical knowledge. It is often followed by empiricists like Hume, who argue that there is no good source of metaphysical knowledge since metaphysics lies outside the field of empirical knowledge and relies on dubious intuitions about the realm beyond sensory experience. A closely related concern about the unreliability of metaphysical theorizing is that there are deep and lasting disagreements about metaphysical issues, indicating a lack of overall progress.

Another criticism holds that the problem lies not with human cognitive abilities but with metaphysical statements themselves, which are claimed to be neither true nor false but meaningless. According to logical positivists, for instance, the meaning of a statement is given by the procedure used to verify it, usually in terms of the observations that would confirm it. Based on this controversial assumption, they argue that metaphysical statements are meaningless since they do not make predictions about experience.

A slightly weaker position allows metaphysical statements to have meaning while holding that metaphysical disagreements are merely verbal disputes about different ways to describe the world. According to this view, the disagreement in the metaphysics of composition about whether there are tables or only particles arranged table-wise is a trivial debate about linguistic preferences without any substantive consequences for the nature of reality. The position that metaphysical disputes have no meaning or no significant point is called metaphysical or ontological deflationism. This view is opposed by serious metaphysicians, who contend that metaphysical disputes are about substantial features of the underlying structure of reality. A closely related debate between ontological realists and anti-realists concerns the question of whether there are any objective facts that determine which metaphysical theories are true. A different criticism, formulated by pragmatists, sees the fault of metaphysics not in its cognitive ambitions or the meaninglessness of its statements, but in its practical irrelevance and lack of usefulness.

It is questionable to what extent the criticisms of metaphysics affect the discipline as a whole or only certain issues or approaches in it. For example, it could be the case that certain metaphysical disputes are merely verbal while others are substantive.

Relation to other disciplines
Metaphysics is related to many fields of inquiry by investigating their basic concepts and relation to the fundamental structure of reality. For example, scientists often rely on concepts such as law of nature, causation, necessity, and spacetime to formulate their theories and predict or explain the outcomes of experiments. While the main focus of scientists is on the application of these concepts to specific situations, metaphysics examines their general nature and how they depend on each other. Physicists formulate specific laws of nature, like laws of gravitation and thermodynamics, to describe how physical systems behave under various conditions. Metaphysicians, by contrast, ask what all laws of nature have in common, for example, whether they merely describe contingent regularities or express necessary relations. At the same time, new scientific findings have also influenced existing and inspired new metaphysical theories. Einstein's theory of relativity, for instance, prompted various metaphysicians to conceive space and time as a unified dimension rather than as independent dimensions. Empirically focused metaphysicians often rely on scientific theories to ground their theories about the nature of reality in empirical observations.

Similar issues pertain to the social sciences where metaphysicians investigate their basic concepts and analyze their metaphysical implications. This includes questions like whether social facts arise from non-social facts, whether social groups and institutions have mind-independent existence, and how they persist through time. Metaphysical assumptions and topics in psychology and psychiatry include the questions about the relation between body and mind, whether the nature of the human mind is historically fixed, and what the metaphysical status of diseases is.

Metaphysics is similar to both physical cosmology and theology in its interest in the first causes and the universe as a whole. Key differences are that metaphysics relies on rational inquiry while physical cosmology gives more weight to empirical observations and theology is additionally based on divine revelation and faith-based doctrines. Historically, cosmology and theology were considered subfields of metaphysics.

Metaphysics in the form of ontology plays a central role in computer science to classify objects and formally represent information about them. Unlike metaphysicians, computer scientists are usually not interested in providing a single all-encompassing characterization of reality as a whole but instead employ many different ontologies, each one concerned only with a limited domain of entities. For example, a college database may use an ontology with categories such as person, teacher, student, and exam to represent information about academic activities. Ontologies provide standards or conceptualizations for encoding and storing information in a structured way, which makes it possible to use and transform the information by computational processes for a variety of purposes. Some knowledge bases integrate information belonging to various domains, which brings with it the problem of handling data that was formulated using different ontologies. They do so by providing an upper ontology that defines concepts on a higher level of abstraction to apply to all domains. Influential upper ontologies include Suggested Upper Merged Ontology and Basic Formal Ontology.

Logic as the study of correct reasoning is often used by metaphysicians as a tool to engage in their inquiry and express insights using precise logical formulas. Another relation between the two fields concerns the metaphysical assumptions associated with logical systems. Many logical systems like first-order logic rely on existential quantifiers to express existential statements. For instance, in the logical formula $$\exists x \text{Horse}(x)$$ the existential quantifier $$\exists$$ is applied to the predicate $$\text{Horse}$$ to express that there are horses. Following Quine, various metaphysicians assume that existential quantifiers carry ontological commitments, meaning that existential statements imply that the entities over which one quantifies form part of reality.

History


The history of metaphysics examines how the inquiry into the basic structure of reality has evolved in the course of history. Metaphysics has its origin in speculations about the nature and origin of the cosmos that go back to ancient civilizations. In ancient India starting in the 7th century BCE, the Upanishads were written as religious and philosophical texts that examine how ultimate reality constitutes the ground of all being. They further explore the nature of the self and how it can reach liberation by understanding ultimate reality. This period also saw the emergence of Buddhism in the 6th century BCE, which denies the existence of an independent self and understands the world as a cyclic process. At about the same time in ancient China, the school of Daoism was formed and explored the natural order of the universe, known as Dao, and how it is characterized by the interplay of yin and yang as two correlated forces.

In ancient Greece, metaphysics emerged in the 6th century BCE with the pre-Socratic philosophers, who gave rational explanations of the whole cosmos by examining the first principles from which everything arises. Following them, Plato (427–347 BCE) formulated his theory of forms, which states that eternal forms or ideas possess the highest kind of reality while the material world is only an imperfect reflection of them. Aristotle (384–322 BCE) accepted Plato's idea that there are universal forms but held that they cannot exist on their own but depend on matter. He also proposed a system of categories and developed a comprehensive framework of the natural world through his theory of the four causes. Starting in the 4th century BCE, Hellenistic philosophy explored the rational order underlying the cosmos and the idea that it is made up of indivisible atoms. Neoplatonism emerged towards the end of the ancient period in the 3rd century CE and introduced the idea of "the One" as a transcendent and ineffable entity that is the source of all of creation.

Meanwhile, in Indian Buddhism, the Madhyamaka school developed the idea that all phenomena are inherently empty without a permanent essence while the consciousness-only doctrine of the Yogācāra school stated that experienced objects are mere transformations of consciousness that do not reflect external reality. The Hindu school of Samkhya philosophy introduced a metaphysical dualism with pure consciousness and matter as its fundamental categories. In China, the school of Xuanxue explored metaphysical problems such as the contrast between being and non-being.



Medieval Western philosophy was strongly influenced by ancient Greek philosophy. Boethius (477–524 CE) attempted to harmonize Plato's and Aristotle's theories of universals by stating that universals can exist both in matter and mind. His theory inspired the philosophies of nominalism and conceptualism, as in the thought of Peter Abelard (1079–1142 CE). Thomas Aquinas (1224–1274 CE) understood metaphysics as the discipline that investigates the different meanings of being, such as the contrast between substance and accident, and principles applying to all beings, such as the principle of identity. William of Ockham (1285–1347 CE) proposed the methodological principles of Ockham's razor as a tool to decide between competing metaphysical theories. Arabic–Persian philosophy, which had its prime period from the early 9th century CE to the late 12th century CE, employed many ideas of the ancient Greek philosophers to interpret and clarify the teachings of the Quran. Avicenna (980–1037 CE) developed a comprehensive philosophical system that examined the contrast between existence and essence and distinguished between contingent and necessary existence. Medieval India saw the emergence of the monist school of Advaita Vedanta in the 8th century CE, which holds that everything is one and that the idea of many entities existing independently is an illusion. In China, Neo-Confucianism arose in the 9th century CE and explored the concept of li as the rational principle that is the ground of being and reflects the order of the universe.

In the early modern period, René Descartes (1596–1650) developed a substance dualism according to which body and mind exist as independent entities that causally interact. This idea was rejected by Baruch Spinoza (1632–1677), who formulated a monist philosophy according to which there is only one substance that has both physical and mental attributes developing side-by-side without interacting. Gottfried Wilhelm Leibniz (1646–1716) introduced the concept of possible worlds and articulated a metaphysical system, known as monadology, that understands the universe as a collection of simple substances that are synchronized without causally interacting with one another. Christian Wolff (1679–1754), conceptualized the scope of metaphysics by introducing the distinction between general and special metaphysics. According to the idealism of George Berkeley (1685–1753), everything is mental, including material objects, which are ideas perceived by the mind. David Hume (1711–1776) made various contributions to metaphysics, including the regularity theory of causation and the idea that there are no necessary connections between distinct entities. At the same time, his empiricist outlook led him to formulate a stark criticism of metaphysical theories that aim to arrive at ultimate principles inaccessible to sensory experience. This skeptical outlook was embraced by Immanuel Kant (1724–1804). He tried to reconceptualize metaphysics as a critical inquiry into the basic principles and categories of thought and understanding rather than seeing it as an attempt to comprehend mind-independent reality.

Many developments in the later modern period were shaped by Kant's philosophy. German idealists employed his idealistic outlook in their attempt to find a unifying principle as the foundation of all reality. Georg Wilhelm Friedrich Hegel (1770–1831) developed a comprehensive system of philosophy that examines how absolute spirit manifests itself. He inspired the British idealism of Francis Herbert Bradley (1846–1924), who interpreted absolute spirit as the all-inclusive totality of being. Arthur Schopenhauer (1788–1860) was a strong critic of German idealism and articulated a different metaphysical vision that takes a blind and irrational will as the underlying principle of reality. Pragmatists like C. S. Peirce (1839–1914) and John Dewey (1859–1952) conceived metaphysics as an observational science of the most general features of reality and experience.



At the turn of the 20th century in analytic philosophy, philosophers like Bertrand Russell (1872–1970) and George Edward Moore's (1873–1958) led a "revolt against idealism". Logical atomists, like Russell and the early Ludwig Wittgenstein (1889–1951), conceived the world as a multitude of atomic facts, which inspired later metaphysicians such as D. M. Armstrong (1926–2014). Alfred North Whitehead (1861–1947) developed process metaphysics as an attempt to provide a holistic description of both the objective and the subjective worlds.

Rudolf Carnap (1891–1970) and other logical positivists formulated a wide-ranging criticism of metaphysical statements by holding that they are meaningless since there is no way to verify them. Other criticisms of traditional metaphysics identified misunderstandings of ordinary language as the source of many traditional metaphysical problems or challenged complex metaphysical deductions by relying on common sense.

The decline of logical positivism saw a revival of metaphysical theorizing. Willard Van Orman Quine (1908–2000) tried to naturalize metaphysics by connecting it to the empirical sciences. His student David Lewis (1941–2001) employed the concept of possible worlds to formulate his modal realism. Saul Kripke (1940–2022) helped revive discussions of identity and essentialism, and distinguished necessity as a metaphysical notion from the epistemic notion of a priori.

In continental philosophy, Edmund Husserl (1859–1938) engaged in ontology through a phenomenological description of experience while his student Martin Heidegger (1889–1976) developed fundamental ontology as an attempt to clarify the meaning of being. Heidegger's philosophy inspired general criticisms of metaphysics by postmodern thinkers like Jacques Derrida (1930–2004).