Ontological Coach

In metaphysics, ontology is the philosophical study of being, as well as related concepts such as existence, becoming, and reality.

Ontology addresses questions like how entities are grouped into categories and which of these entities exist on the most fundamental level. Ontologists often try to determine what the categories or highest kinds are and how they form a system of categories that encompasses classification of all entities. Commonly proposed categories include substances, properties, relations, states of affairs and events. These categories are characterized by fundamental ontological concepts, including particularity and universality, abstractness and concreteness, or possibility and necessity. Of special interest is the concept of ontological dependence, which determines whether the entities of a category exist on the most fundamental level. Disagreements within ontology are often about whether entities belonging to a certain category exist and, if so, how they are related to other entities.

When used as a countable noun, the words ''ontology'' and ''ontologies'' refer not to the science of being but to theories within the science of being. Ontological theories can be divided into various types according to their theoretical commitments. Monocategorical ontologies hold that there is only one basic category, but polycategorical ontologies rejected this view. Hierarchical ontologies assert that some entities exist on a more fundamental level and that other entities depend on them. Flat ontologies, on the other hand, deny such a privileged status to any entity.

Etymology

The compound word ''ontology'' ('study of being') combines
: ''onto-'' ( Greek: ; ) and
: ''-logia'' ().

While the etymology is Greek, the oldest extant records of the word itself is a New Latin form , which appeared
: in 1606 in the ''Ogdoas Scholastica'' by Jacob Lorhard (''Lorhardus''), and
: in 1613 in the ''Lexicon philosophicum'' by Rudolf Göckel (''Goclenius'').
The first occurrence in English of ''ontology'', as recorded by the ''Oxford English Dictionary'', came in 1664 through ''Archelogia philosophica nova...'' by Gideon Harvey. The word was first used, in its Latin form, by philosophers, and based on the Latin roots (and in turn on the Greek ones).

Overview

Ontology is closely associated with Aristotle's question of 'being ''qua'' being': the question of what all entities in the widest sense have in common. The ''Eleatic principle'' is one answer to this question: it states that being is inextricably tied to causation, that "Power is the mark of Being". One problem with this answer is that it excludes abstract objects. Another explicit but little accepted answer can be found in Berkeley's slogan that "to be is to be perceived". Intimately related but not identical to the question of 'being ''qua'' being' is the problem of categories. Categories are usually seen as the highest kinds or genera. A system of categories provides a classification of entities that is exclusive and exhaustive: every entity belongs to exactly one category. Various such classifications have been proposed, they often include categories for substances, properties, relations, states of affairs or events. At the core of the differentiation between categories are various fundamental ontological concepts and distinctions, for example, the concepts of ''particularity and universality'', of ''abstractness and concreteness'', of ''ontological dependence'', of ''identity'' and of ''modality''. These concepts are sometimes treated as categories themselves, are used to explain the difference between categories or play other central roles for characterizing different ontological theories. Within ontology, there is a lack of general consensus concerning how the different categories are to be defined. Different ontologists often disagree on whether a certain category has any members at all or whether a given category is fundamental.

Particulars and universals

Particulars or individuals are usually contrasted with universals. Universals concern features that can be exemplified by various different particulars. For example, a tomato and a strawberry are two particulars that exemplify the universal redness. Universals can be present at various distinct locations in space at the same time while particulars are restricted to one location at a time. Furthermore, universals can be fully present at different times, which is why they are sometimes referred to as ''repeatables'' in contrast to non-repeatable particulars. The so-called problem of universals is the problem to explain how different things can agree in their features, e.g. how a tomato and a strawberry can both be red. Realists about universals believe that there are universals. They can solve the ''problem of universals'' by explaining the commonality through a universal shared by both entities. Realists are divided among themselves as to whether universals can exist independently of being exemplified by something ("ante res") or not ("in rebus"). Nominalists, on the other hand, deny that there are universals. They have to resort to other notions to explain how a feature can be common to several entities, for example, by positing either fundamental resemblance-relations between the entities (resemblance nominalism) or a shared membership to a common natural class (class nominalism).

Abstract and concrete

Many philosophers agree that there is an exclusive and exhaustive distinction between ''concrete objects'' and ''abstract objects''. Some philosophers consider this to be the most general division of being. Examples of concrete objects include plants, human beings and planets while things like numbers, sets and propositions are abstract objects. But despite the general agreement concerning the paradigm cases, there is less consensus as to what the characteristic marks of concreteness and abstractness are. Popular suggestions include defining the distinction in terms of the difference between (1) existence inside or outside space-time, (2) having causes and effects or not and (3) having contingent or necessary existence.

Ontological dependence

An entity ''ontologically depends'' on another entity if the first entity cannot exist without the second entity. Ontologically independent entities, on the other hand, can exist all by themselves. For example, the surface of an apple cannot exist without the apple and so depends on it ontologically. Entities often characterized as ontologically dependent include properties, which depend on their bearers, and boundaries, which depend on the entity they demarcate from its surroundings. As these examples suggest, ontological dependence is to be distinguished from causal dependence, in which an effect depends for its existence on a cause. It is often important to draw a distinction between two types of ontological dependence: rigid and generic. Rigid dependence concerns the dependence on one specific entity, as the surface of an apple depends on its specific apple. Generic dependence, on the other hand, involves a weaker form of dependence, on merely a certain type of entity. For example, electricity generically depends on there being charged particles, but it does not depend on any specific charged particle. Dependence-relations are relevant to ontology since it is often held that ontologically dependent entities have a less robust form of being. This way a ''hierarchy'' is introduced into the world that brings with it the distinction between more and less fundamental entities.

Identity

Identity is a basic ontological concept that is often expressed by the word "same". It is important to distinguish between ''qualitative identity'' and ''numerical identity''. For example, consider two children with identical bicycles engaged in a race while their mother is watching. The two children have the ''same'' bicycle in one sense (''qualitative identity'') and the ''same'' mother in another sense (''numerical identity''). Two qualitatively identical things are often said to be indiscernible. The two senses of ''sameness'' are linked by two principles: the principle of ''indiscernibility of identicals'' and the principle of '' identity of indiscernibles''. The principle of ''indiscernibility of identicals'' is uncontroversial and states that if two entities are numerically identical with each other then they exactly resemble each other. The principle of ''identity of indiscernibles'', on the other hand, is more controversial in making the converse claim that if two entities exactly resemble each other then they must be numerically identical. This entails that "no two distinct things exactly resemble each other". A well-known counterexample comes from Max Black, who describes a symmetrical universe consisting of only two spheres with the same features. Black argues that the two spheres are indiscernible but not identical, thereby constituting a violation of the principle of ''identity of indiscernibles''.

The problem of ''identity over time'' concerns the question of ''persistence'': whether or in what sense two objects at different times can be ''numerically identical''. This is usually referred to as ''diachronic identity'' in contrast to ''synchronic identity''. The statement that " e table in the next room is identical with the one you purchased last year" asserts diachronic identity between the table now and the table then. A famous example of a denial of diachronic identity comes from Heraclitus, who argues that it is impossible to step into the same river twice because of the changes that occurred in-between. The traditional position on the problem of ''persistence'' is endurantism, the thesis that diachronic identity in a strict sense is possible. One problem with this position is that it seems to violate the principle of ''indiscernibility of identicals'': the object may have undergone changes in the meantime resulting in it being discernible from itself. Perdurantism or four-dimensionalism is an alternative approach holding that ''diachronic identity'' is possible only in a loose sense: while the two objects differ from each other strictly speaking, they are both temporal parts that belong to the same temporally extended whole. ''Perdurantism'' avoids many philosophical problems plaguing ''endurantism'', but ''endurantism'' seems to be more in touch with how we ordinarily conceive ''diachronic identity''.

Modality

Modality concerns the concepts of possibility, actuality and necessity. In contemporary discourse, these concepts are often defined in terms of possible worlds. A possible world is a complete way how things could have been. The actual world is one possible world among others: things could have been different from what they actually are. A proposition is possibly true if there is at least one possible world in which it is true; it is necessarily true if it is true in all possible worlds. '' Actualists'' and ''possibilists'' disagree on the ''ontological status'' of possible worlds. Actualists hold that reality is at its core actual and that possible worlds should be understood in terms of actual entities, for example, as fictions or as sets of sentences. Possibilists, on the other hand, assign to possible worlds the same ''fundamental ontological status'' as to the actual world. This is a form of modal realism, holding that reality has ''irreducibly modal features''. Another important issue in this field concerns the distinction between ''contingent'' and ''necessary beings''. Contingent beings are beings whose existence is possible but not necessary. Necessary beings, on the other hand, could not have failed to exist. It has been suggested that this distinction is the highest division of being.

Substances

The category of ''substances'' has played a central role in many ontological theories throughout the history of philosophy. "Substance" is a technical term within philosophy not to be confused with the more common usage in the sense of chemical substances like gold or sulfur. Various definitions have been given but among the most common features ascribed to substances in the philosophical sense is that they are ''particulars'' that are ''ontologically independent'': they are able to exist all by themselves. Being ontologically independent, substances can play the role of ''fundamental entities'' in the ''ontological hierarchy''. If 'ontological independence' is defined as including ''causal independence'' then only self-caused entities, like Spinoza's God, can be substances. With a specifically ontological definition of 'independence', many everyday objects like books or cats may qualify as substances. Another defining feature often attributed to substances is their ability to ''undergo changes''. Changes involve something existing ''before'', ''during'' and ''after'' the change. They can be described in terms of a persisting substance gaining or losing properties, or of ''matter'' changing its ''form''. From this perspective, the ripening of a tomato may be described as a change in which the tomato loses its greenness and gains its redness. It is sometimes held that a substance can have a property in two ways: '' essentially'' and ''accidentally''. A substance can survive a change of ''accidental properties'' but it cannot lose its ''essential properties'', which constitute its nature.

Properties and relations

The category of ''properties'' consists of entities that can be exemplified by other entities, e.g. by substances. Properties characterize their bearers, they express what their bearer is like. For example, the red color and the round shape of an apple are properties of this apple. Various ways have been suggested concerning how to conceive properties themselves and their relation to substances. The traditionally dominant view is that properties are universals that inhere in their bearers. As universals, they can be shared by different substances. Nominalists, on the other hand, deny that universals exist. Some nominalists try to account for properties in terms of resemblance relations or class membership. Another alternative for nominalists is to conceptualize properties as simple particulars, so-called tropes. This position entails that both the apple and its redness are particulars. Different apples may still exactly resemble each other concerning their color, but they do not share the same particular property on this view: the two color-tropes are ''numerically distinct''. Another important question for any theory of properties is how to conceive the relation between a bearer and its properties. Substratum theorists hold that there is some kind of substance, ''substratum'' or ''bare particular'' that acts as bearer. Bundle theory is an alternative view that does away with a substratum altogether: objects are taken to be just a bundle of properties. They are held together not by a substratum but by the so-called ''compresence-relation'' responsible for the bundling. Both substratum theory and bundle theory can be combined with conceptualizing properties as universals or as particulars.

An important distinction among properties is between ''categorical'' and ''dispositional'' properties. Categorical properties concern what something is like, e.g. what qualities it has. Dispositional properties, on the other hand, involve what powers something has, what it is able to do, even if it is not actually doing it. For example, the shape of a sugar cube is a categorical property while its tendency to dissolve in water is a dispositional property. For many properties there is a lack of consensus as to how they should be classified, for example, whether colors are categorical or dispositional properties. Categoricalism is the thesis that on a fundamental level there are only categorical properties, that dispositional properties are either non-existent or dependent on categorical properties. Dispositionalism is the opposite theory, giving ontological primacy to dispositional properties. Between these two extremes, there are dualists who allow both categorical and dispositional properties in their ontology.

''Relations'' are ways in which things, the relata, stand to each other. Relations are in many ways similar to properties in that both characterize the things they apply to. Properties are sometimes treated as a special case of relations involving only one relatum. Central for ontology is the distinction between ''internal'' and ''external'' relations. A relation is ''internal'' if it is fully determined by the features of its relata. For example, an apple and a tomato stand in the ''internal relation'' of similarity to each other because they are both red. Some philosophers have inferred from this that internal relations do not have a proper ontological status since they can be reduced to intrinsic properties. ''External'' relations, on the other hand, are not fixed by the features of their relata. For example, a book stands in an ''external'' relation to a table by lying on top of it. But this is not determined by the book's or the table's features like their color, their shape, etc.

States of affairs and events

States of affairs are complex entities, in contrast to substances and properties, which are usually conceived as simple. Complex entities are built up from or constituted by other entities. Atomic states of affairs are constituted by one particular and one property exemplified by this particular. For example, the state of affairs that Socrates is wise is constituted by the particular "Socrates" and the property "wise". Relational states of affairs involve several particulars and a relation connecting them. States of affairs that ''obtain'' are also referred to as facts. It is controversial which ontological status should be ascribed to states of affairs that do not obtain. States of affairs have been prominent in 20th-century ontology as various theories were proposed to describe the world as composed of states of affairs. It is often held that states of affairs play the role of truthmakers: judgments or assertions are true because the corresponding state of affairs obtains.

Events take place in time, they are sometimes thought of as involving a change in the form of acquiring or losing a property, like the lawn's becoming dry. But on a liberal view, the retaining of a property without any change may also count as an event, e.g. the lawn's staying wet. Some philosophers see events as universals that can repeat at different times, but the more dominant view is that events are particulars and therefore non-repeatable. Some events are complex in that they are composed of a sequence of events, often referred to as a process. But even simple events can be conceived as complex entities involving an object, a time and the property exemplified by the object at this time. So-called process philosophy or process ontology ascribes ontological primacy to changes and processes as opposed to the emphasis on static being in the traditionally dominant substance metaphysics.

Reality of things

The word 'real' is derived from the Latin word ''res'', which is often translated as 'thing'. The word 'thing' is often used in ontological discourse as if it had a presupposed meaning, not needing an explicit philosophical definition because it belongs to ordinary language. Nevertheless, what is a thing and what is real or substantial are concerns of ontology. Different views are held about this. Plato proposed that underlying, and constituting the real basis of, the concretely experienced world are ' forms' or 'ideas', which today are generally regarded as high abstractions. In earlier days, philosophers used the term 'realism' to refer to Plato's belief that his 'forms' are 'real'; nowadays, the term 'realism' often has an almost opposite meaning, so that Plato's belief is sometimes called 'idealism'. Philosophers debate whether entities such as tables and chairs, lions and tigers, philosophical doctrines, numbers, truth, and beauty, are to be regarded as 'things', or as something or nothing 'real'.

Types of ontologies

Ontological theories can be divided into various types according to their theoretical commitments. Particular ontological theories or types of theories are often referred to as "ontologies" ( singular or plural). This usage contrasts with the meaning of "ontology" ( only singular) as a branch of philosophy: the ''science of being'' in general.

Flat vs polycategorical vs hierarchical

One way to divide ontologies is by the number of basic categories they use. ''Monocategorical'' or ''one category ontologies'' hold that there is only one basic category while ''polycategorical ontologies'' imply that there are several distinct basic categories. Another way to divide ontologies is through the notion of ontological hierarchy. Hierarchical ontologies assert that some entities exist on a more fundamental level and that other entities depend on them. Flat ontologies, on the other hand, deny such a privileged status to any entities. Jonathan Schaffer provides an overview of these positions by distinguishing between ''flat'' ontologies (non-hierarchical), ''sorted'' ontologies (polycategorical non-hierarchical) and ''ordered'' ontologies (polycategorical hierarchical).

''Flat'' ontologies are only interested in the difference between existence and non-existence. They are ''flat'' because each flat ontology can be represented by a simple set containing all the entities to which this ontology is committed. An influential exposition of this approach comes from Willard Van Orman Quine which is why it has been referred to as the Quinean approach to meta-ontology. This outlook does not deny that the existing entities can be further subdivided and may stand in various relations to each other. These issues are questions for the more specific sciences, but they do not belong to ontology in the Quinean sense.

''Polycategorical'' ontologies are concerned with the categories of being. Each polycategorical ontology posits a number of categories. These categories are exclusive and exhaustive: every existing entity belongs to exactly one category. A recent example of a polycategorical ontology is E.J. Lowe's four-category-ontology. The four categories are object, kind, mode and attribute. The fourfold structure is based on two distinctions. The first distinction is between substantial entities (objects and kinds) and non-substantial entities (modes and attributes). The second distinction is between particular entities (objects and modes) and universal entities (kinds and attributes). Reality is built up through the interplay of entities belonging to different categories: particular entities instantiate universal entities, and non-substantial entities characterize substantial entities.

''Hierarchical'' ontologies are interested in the degree of fundamentality of the entities they posit. Their main goal is to figure out which entities are fundamental and how the non-fundamental entities depend on them. The concept of fundamentality is usually defined in terms of metaphysical grounding. Fundamental entities are different from non-fundamental entities because they are not grounded in other entities. For example, it is sometimes held that elementary particles are more fundamental than the macroscopic objects (like chairs and tables) they compose. This is a claim about the grounding-relation between microscopic and macroscopic objects. Schaffer's priority monism is a recent form of a hierarchical ontology. He holds that on the most fundamental level there exists only one thing: the world as a whole. This thesis does not deny our common-sense intuition that the distinct objects we encounter in our everyday affairs like cars or other people exist. It only denies that these objects have the most fundamental form of existence. An example of a ''hierarchical'' ontology in continental philosophy comes from Nicolai Hartmann. He asserts that reality is made up of four levels: the inanimate, the biological, the psychological and the spiritual. These levels form a hierarchy in the sense that the higher levels depend on the lower levels while the lower levels are indifferent to the higher levels.

Thing ontologies vs fact ontologies

''Thing ontologies'' and ''fact ontologies'' are one-category-ontologies: they both hold that all fundamental entities belong to the same category. They disagree on whether this category is the category of things or of facts. A slogan for fact ontologies comes from Ludwig Wittgenstein: "The world is the totality of facts, not of things".

One difficulty in characterizing this dispute is to elucidate what things and facts are, and how they differ from each other. Things are commonly contrasted with the properties and relations they instantiate. Facts, on the other hand, are often characterized as having these things and the properties/relations as their constituents. This is reflected in a rough linguistic characterization of this difference where the subjects and objects of an assertion refer to things while the assertion as a whole refers to a fact.

Reism is one form of thing ontology. Franz Brentano developed a version of reism in his later philosophy. He held that only concrete particular things exist. Things can exist in two forms: either as spatio-temporal bodies or as temporal souls. Brentano was aware of the fact that many common-sense expressions seem to refer to entities that do not have a place in his ontology, like properties or intentional objects. This is why he developed a method to paraphrase these expressions in order to avoid these ontological commitments.

D. M. Armstrong is a well-known defender of fact ontology. He and his followers refer to facts as states of affairs. States of affairs are the basic building blocks of his ontology: they have particulars and universals as their constituents but they are primary in relation to particulars and universals. States of affairs have ontologically independent existence while " propertied particulars and uninstantiated universals are false abstractions".

Constituent ontologies vs blob theories

''Constituent ontologies'' and ''blob theories'', sometimes referred to as ''relational ontologies'', are concerned with the internal structure of objects. Constituent ontologies hold that objects have an internal structure made up of constituents. This is denied by blob theories: they contend that objects are structureless "blobs".

Bundle theories are examples of constituent ontologies. Bundle theorists assert that an object is nothing but the properties it "has". On this account, a regular apple could be characterized as a bundle of redness, roundness, sweetness, etc. Defenders of bundle theory disagree on the nature of the bundled properties. Some affirm that these properties are universals while others contend that they are particulars, so-called "tropes".

Class nominalism, on the other hand, is a form of blob theory. Class nominalists hold that properties are classes of things. To instantiate a property is merely to be a member of the corresponding class. So properties are not constituents of the objects that have them.

Information science and natural sciences

In information science ontologies are classified in various ways, using criteria such as the degree of abstraction and field of application:

# '' Upper ontology'': concepts supporting development of an ontology, meta-ontology.
# '' Domain ontology'': concepts relevant to a particular topic, domain of discourse, or area of interest, for example, to information technology or to computer languages, or to particular branches of science.
# ''Interface ontology'': concepts relevant to the juncture of two disciplines.
# ''Process ontology'': inputs, outputs, constraints, sequencing information, involved in business or engineering processes.
In the biomedical sciences, ontologies have been used to create terminologies for various aspects of living organism or medical applications. A prominent example is the gene ontology, but many other ontologies exist, e.g. for anatomical terms or physiology. Standards have been established to maintain and organize biological ontologies under the OBO (Open Biological Ontologies) project.

History

Ancient Greek

In the Greek philosophical tradition, Parmenides was among the first to propose an ontological characterization of the fundamental nature of existence. In the prologue (or ''proem'') to ''On Nature'', he describes two views of existence. Initially, nothing comes from nothing, thus existence is eternal. This posits that existence is what may be conceived of by thought, created, or possessed. Hence, there may be neither void nor vacuum; and true reality neither may come into being nor vanish from existence. Rather, the entirety of creation is eternal, uniform, and immutable, though not infinite (Parmenides characterized its shape as that of a perfect sphere). Parmenides thus posits that change, as perceived in everyday experience, is illusory.

Opposite to the Eleatic monism of Parmenides is the pluralistic conception of being. In the 5th century BC, Anaxagoras and Leucippus replaced the reality of ''being'' (unique and unchanging) with that of ''becoming'', therefore by a more fundamental and elementary ''ontic'' plurality. This thesis originated in the Hellenic world, stated in two different ways by Anaxagoras and by Leucippus. The first theory dealt with "seeds" (which Aristotle referred to as "''homeomeries''") of the various substances. The second was the atomistic theory, which dealt with reality as based on the vacuum, the atoms and their intrinsic movement in it.

The materialist ''atomism'' proposed by Leucippus was indeterminist, but Democritus ( 460 – 370 BC) subsequently developed it in a deterministic way. Later (4th century BC), Epicurus took the original ''atomism'' again as indeterministic. He saw reality as composed of an infinity of indivisible, unchangeable corpuscles or atoms (from the Greek ''atomon'', lit. 'uncuttable'), but he gives weight to characterize atoms whereas for Leucippus they are characterized by a "figure", an "order" and a "position" in the cosmos. Atoms are, besides, creating the whole with the intrinsic movement in the ''vacuum'', producing the diverse flux of being. Their movement is influenced by the ''parenklisis'' (Lucretius names it ''clinamen'') and that is determined by chance. These ideas foreshadowed the understanding of traditional physics until the advent of 20th-century theories on the nature of atoms.

Plato

Plato developed the distinction between true reality and illusion, in arguing that what is real are eternal and unchanging forms or ideas (a precursor to ''universals''), of which things experienced in sensation are at best merely copies, and real only in so far as they copy ("partake of") such forms. In general, Plato presumes that all nouns (e.g., "beauty") refer to real entities, whether sensible bodies or insensible forms. Hence, in ''The Sophist'', Plato argues that ''being'' is a ''form'' in which all existent things participate and which they have in common (though it is unclear whether "Being" is intended in the sense of existence, copula, or identity); and argues, against Parmenides, that forms must exist not only of ''being'', but also of Negation and of non-being (or ''Difference'').

Aristotle

In his ''Categories'', Aristotle (384–322 BCE) identifies ten possible kinds of things that may be the subject or the predicate of a proposition. For Aristotle there are four different ontological dimensions:

# according to the various categories or ways of addressing a being as such
# according to its truth or falsity (e.g. fake gold, counterfeit money)
# whether it exists in and of itself or simply 'comes along' by accident
# according to its potency, movement (energy) or finished presence ( ''Metaphysics'' Book Theta).

Hindu philosophy

Ontology features in the Samkhya school of Hindu philosophy from the first millennium BCE. Samkhya philosophy regards the universe as consisting of two independent realities: puruṣa (pure, contentless consciousness) and prakṛti (matter). The substance dualism between puruṣa and prakṛti is similar but not identical to the substance dualism between mind and body that, following the works of Descartes, has been central to many disputes in the Western philosophical tradition. Samkhya sees the mind as being the subtle part of prakṛti. It is made up of three faculties: the sense mind (manas), the intellect ( buddhi), and the ego ( ahaṁkāra). These faculties perform various functions but are by themselves unable to produce consciousness, which belongs to a distinct ontological category and for which puruṣa alone is responsible. The Yoga school agrees with Samkhya philosophy on the fundamental dualism between puruṣa and prakṛti but it differs from Samkhya's atheistic position by incorporating the concept of a "personal, yet essentially inactive, deity" or "personal god" ( Ishvara).Lloyd Pflueger, Person Purity and Power in Yogasutra, in Theory and Practice of Yoga (Editor: Knut Jacobsen), Motilal Banarsidass, , pages 38-39 These two schools stand in contrast to Advaita Vedanta, which adheres to non-duality by revealing that the apparent plurality of things is an illusion ( Maya) hiding the true oneness of reality at its most fundamental level (Brahman).

Medieval

''Medieval ontology'' was strongly influenced by Aristotle's teachings. The thinkers of this period often relied on Aristotelian categories like ''substance'', ''act and potency'' or ''matter and form'' to formulate their own theories. Important ontologists in this epoch include Avicenna, Thomas Aquinas, Duns Scotus and William of Ockham.

Avicenna

According to Avicenna's interpretation of Greek Aristotelian and Platonist ontological doctrines in medieval metaphysics, being is either necessary, contingent ''qua'' possible, or impossible. Necessary being is that which cannot but be, since its non-being would entail a contradiction. Contingent ''qua'' possible being is neither necessary nor impossible for it to be or not to be. It is ontologically neutral, and is brought from potential existing into actual existence by way of a cause that is external to its essence. Its being is borrowed – unlike the necessary existent, which is self-subsisting and impossible not to be. As for the impossible, it necessarily does not exist, and the affirmation of its being would involve a contradiction.

Aquinas

Fundamental to Thomas Aquinas's ontology is his distinction between essence and existence: all entities are conceived as composites of essence and existence. The essence of a thing is what this thing is like, it signifies the definition of this thing. God has a special status since He is the only entity whose essence is identical to its existence. But for all other, finite entities there is a ''real distinction'' between essence and existence. This distinction shows itself, for example, in our ability to understand the essence of something without knowing about its existence. Aquinas conceives of existence as an ''act of being'' that actualizes the potency given by the essence. Different things have different essences, which impose different limits on the corresponding ''act of being''. The paradigm examples of essence-existence-composites are material substances like cats or trees. Aquinas incorporates Aristotle's distinction between matter and form by holding that the essence of ''material'' things, as opposed to the essence of ''immaterial'' things like angels, is the composition of their matter and form. So, for example, the essence of a marble statue would be the composition of the marble (its matter) and the shape it has (its form). Form is universal since substances made of different matter can have the same form. The forms of a substance may be divided into substantial and accidental forms. A substance can survive a change of an accidental form but ceases to exist upon a change of a substantial form.

Modern

Ontology is increasingly seen as a separate domain of philosophy in the ''modern period''. Many ontological theories of this period were rationalistic in the sense that they saw ontology largely as a deductive discipline that starts from a small set of first principles or axioms, a position best exemplified by Baruch Spinoza and Christian Wolff. This rationalism in metaphysics and ontology was strongly opposed by Immanuel Kant, who insisted that many claims arrived at this way are to be dismissed since they go beyond any possible experience that could justify them.

Descartes

René Descartes' ontological distinction between mind and body has been one of the most influential parts of his philosophy. On his view, minds are thinking things while bodies are extended things. ''Thought'' and ''extension'' are two ''attributes'' that each come in various ''modes'' of being. Modes of ''thinking'' include judgments, doubts, volitions, sensations and emotions while the shapes of material things are modes of ''extension''. Modes come with a lower degree of reality since they depend for their existence on a substance. Substances, on the other hand, can exist on their own. Descartes' substance dualism asserts that every finite substance is either a thinking substance or an extended substance. This position does not entail that minds and bodies ''actually are'' separated from each other, which would defy the intuition that we both have a body and a mind. Instead, it implies that minds and bodies ''can'', at least in principle, be separated, since they are distinct substances and therefore are capable of independent existence. A longstanding problem for substance dualism since its inception has been to explain how minds and bodies can causally interact with each other, as they apparently do, when a volition causes an arm to move or when light falling on the retina causes a visual impression.

Spinoza

Baruch Spinoza is well-known for his ''substance monism:'' the thesis that only one substance exists. He refers to this substance as "God or Nature", emphasizing both his pantheism and his naturalism. This substance has an infinite amount of attributes, which he defines as "what the intellect perceives of substance as constituting its essence". Of these attributes, only two are accessible to the human mind: thought and extension. ''Modes'' are properties of a substance that follow from its attributes and therefore have only a dependent form of existence. Spinoza sees everyday-things like rocks, cats or ourselves as mere modes and thereby opposes the traditional Aristotelian and Cartesian conception of categorizing them as substances. Modes compose deterministic systems in which the different modes are linked to each other as cause and effect. Each deterministic system corresponds to one attribute: one for extended things, one for thinking things, etc. Causal relations only happen within a system while the different systems run in parallel without causally interacting with each other. Spinoza calls the system of modes ''Natura naturata'' ("nature natured") and opposes it to ''Natura naturans'' ("nature naturing"), the attributes responsible for the modes. Everything in Spinoza's system is necessary: there are no contingent entities. This is so since the attributes are themselves necessary and since the system of modes follows from them.

Wolff

Christian Wolff defines ontology as the science of being in general. He sees it as a part of metaphysics besides cosmology, psychology and natural theology. According to Wolff, it is a deductive science, knowable a priori and based on two fundamental principles: the principle of non-contradiction ("it cannot happen that the same thing is and is not") and the principle of sufficient reason ("nothing exists without a sufficient reason for why it exists rather than does not exist"). ''Beings'' are defined by their ''determinations'' or ''predicates'', which cannot involve a contradiction. Determinates come in 3 types: ''essentialia'', ''attributes'', and ''modes''. ''Essentialia'' define the nature of a being and are therefore necessary properties of this being. ''Attributes'' are determinations that follow from essentialia and are equally necessary, in contrast to ''modes'', which are merely contingent. Wolff conceives ''existence'' as just one determination among others, which a being may lack. Ontology is interested in being at large, not just in actual being. But all beings, whether actually existing or not, have a sufficient reason. The sufficient reason for things without actual existence consists in all the determinations that make up the essential nature of this thing. Wolff refers to this as a "reason of being" and contrasts it with a "reason of becoming", which explains why some things have actual existence.

Schopenhauer

Arthur Schopenhauer was a proponent of metaphysical voluntarism

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