Metaphysics

== Topics ==
=== Existence and categories of being ===
{{main|Existence|Theory of categories}}

Metaphysicians often see existence or being as one of the most basic and general concepts.<ref>{{multiref | {{harvnb|Lowe|2005|p=[https://www.oxfordreference.com/display/10.1093/acref/9780199264797.001.0001/acref-9780199264797-e-828 277]}} | {{harvnb|White|2019|pp=135, 200}} | {{harvnb|Gibson|1998|pp=1–2}} | {{harvnb|Jubien|2004|pp=[https://books.google.com/books?id=uoCh8mpbZO4C&pg=PA47 47–48]}} | {{harvnb|Vallicella|2010|p=16}} }}</ref> To exist means to form part of [[reality]] and existence marks the difference between real entities and imaginary ones.<ref>{{multiref | {{harvnb|Lowe|2005|p=[https://www.oxfordreference.com/display/10.1093/acref/9780199264797.001.0001/acref-9780199264797-e-828 277]}} | {{harvnb|AHD staff|2022}} }}</ref> 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.<ref>{{multiref |1={{harvnb|Casati|Fujikawa|loc=Lead Section, §1. Existence as a Second-Order Property and Its Relation to Quantification}} |2={{harvnb|Blackburn|2008|loc=[https://www.oxfordreference.com/display/10.1093/oi/authority.20110803095804522;jsessionid=AEB41DBFF0B3EBCDCCB8A1E0DAFD4EB0 existence]}} }}</ref> 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.<ref>{{multiref |1={{harvnb|Casati|Fujikawa|loc=Lead Section, §2. Existence as a First-Order Property and Its Relation to Quantification}} |2={{harvnb|Blackburn|2008|loc=[https://www.oxfordreference.com/display/10.1093/oi/authority.20110803095804522;jsessionid=AEB41DBFF0B3EBCDCCB8A1E0DAFD4EB0 existence]}} }}</ref> It is controversial whether all entities have this property. According to [[Alexius Meinong]], there are some objects that do not exist, including merely possible objects like [[Santa Claus]] and [[Pegasus]].<ref name="auto">{{multiref |1={{harvnb|Van Inwagen|2023}} |2={{harvnb|Nelson|2022|loc=Lead Section, §2. Meinongianism}} |3={{harvnb|Jubien|2004|p=[https://books.google.com/books?id=uoCh8mpbZO4C&pg=PA49 49]}} }}</ref>{{efn|According to Meinong, existence is not a synonym of being: all entities have being but not all entities have existence.<ref name="auto"/>}} A related question is whether existence is the same for all entities or whether there are different modes or degrees of existence.<ref>{{multiref |1={{harvnb|Casati|Fujikawa|loc=Lead Section, §3. How Many Ways of Being Existent?}} |2={{harvnb|McDaniel|2017|p=[https://books.google.com/books?id=8F0vDwAAQBAJ&pg=PT89 77]}} }}</ref> 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.<ref>{{multiref |1={{harvnb|Poidevin|Peter|Andrew|Cameron|2009|pp=227–228}} |2={{harvnb|Van Inwagen|2023}} }}</ref>

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.<ref>{{multiref | {{harvnb|Thomasson|2022|loc=Lead Section}} | {{harvnb|Loux|Crisp|2017|pp=11–12}} | {{harvnb|Wardy|1998|loc=Lead Section}} }}</ref> One of the earliest theories of categories was provided by Aristotle, who proposed a [[Categories_(Aristotle)#The_praedicamenta|system of 10 categories]]. [[Substance theory|Substances]] (e.g. man and horse), are the most important category since all other categories like [[quantity]] (e.g. four), [[Quality (philosophy)|quality]] (e.g. white), and place (e.g. in Athens) are said of substances and depend on them.<ref>{{multiref | {{harvnb|Thomasson|2022|loc=§ 1.1 Aristotelian Realism}} | {{harvnb|Studtmann|2024|loc=§ 2. The Ten-Fold Division}} | {{harvnb|Wardy|1998|loc=§ 1. Categories in Aristotle}} }}</ref> Kant understood categories as fundamental principles underlying human understanding and developed a [[Category (Kant)|system of 12 categories]], which are divided into the four classes quantity, quality, relation, and modality.<ref>{{multiref | {{harvnb|Thomasson|2022|loc=§ 1.2 Kantian Conceptualism}} | {{harvnb|Wardy|1998|loc=§ 1. Categories in Kant}} }}</ref> More recent theories of categories were proposed by [[Edmund Husserl]], [[Samuel Alexander]], [[Roderick Chisholm]], and [[E. J. Lowe (philosopher)|E. J. Lowe]].<ref>{{multiref | {{harvnb|Thomasson|2022|loc=§ 1.3 Husserlian Descriptivism, § 1.4 Contemporary Category Systems}} | {{harvnb|Grim|Rescher|2023|p=[https://books.google.com/books?id=KgPPEAAAQBAJ&pg=PT39 39]}} }}</ref> Many philosophers rely on the contrast between [[Abstract and concrete|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 [[number]]s and [[Set (mathematics)|sets]], exist outside space and time, are immutable, and do not enter into causal relations.<ref>{{multiref | {{harvnb|Falguera|Martínez-Vidal|Rosen|2022|loc=Lead Section, § 1. Introduction, § 3.5 The Ways of Negation}} | {{harvnb|Erasmus|2018|p=[https://books.google.com/books?id=ze5FDwAAQBAJ&pg=PA93 93]}} | {{harvnb|Macnamara|2009|p=[https://books.google.com/books?id=pvKD5fvyLXIC&pg=PA94 94]}} }}</ref>

=== Particulars ===
[[Particular]]s 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'',{{efn|Some philosophers use the two terms in slightly different ways.<ref>{{harvnb|Bigelow|1998|loc=Lead Section}}</ref>}} they are unique, non-repeatable entities and contrast with [[Universal (metaphysics)|universals]], like the color red, which can at the same time exist in several places and characterize several particulars.<ref>{{multiref | {{harvnb|Lowe|2005|p=683}} | {{harvnb|MacLeod|Rubenstein|loc=Lead Section, § 1a. The Nature of Universals}} | {{harvnb|Bigelow|1998|loc=Lead Section}} | {{harvnb|Campbell|2006|loc=§ Particularity and Individuality}} | {{harvnb|Maurin|2019|loc=Lead Section}} }}</ref> 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 theory|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.<ref>{{multiref | {{harvnb|Maurin|2019|loc=Lead Section}} | {{harvnb|Campbell|2006|loc=§ Particularity and Individuality}} | {{harvnb|Bigelow|1998|loc=Lead Section, § 3. Bundles of Properties}} | {{harvnb|Loux|Crisp|2017|pp=82–83}} }}</ref>

Concrete particulars encountered in everyday life, like rocks, tables, and organisms, are complex entities [[Composition (objects)|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]].<ref>{{multiref | {{harvnb|Loux|Crisp|2017|pp=250–251}} | {{harvnb|Varzi|2019|loc=Lead Section, § 1. 'Part' and Parthood}} | {{harvnb|Cornell|loc=Lead Section, § 2. The Special Composition Question}} | {{harvnb|Tallant|2017|pp=19–21}} }}</ref> 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 nihilism|Mereological nihilists]] reject the idea that there are any wholes. They deny that, strictly speaking, there is a table and talk instead of particles that are arranged table-wise.<ref>{{multiref | {{harvnb|Loux|Crisp|2017|pp=82–83}} | {{harvnb|Cornell|loc=Lead Section, § 2. The Special Composition Question}} | {{harvnb|Brenner|2015|p=1295}} | {{harvnb|Tallant|2017|pp=19–21, 23–24, 32–33}} }}</ref> A related mereological problem is whether there are simple entities that have no parts, as [[Atomism|atomists]] claim, or not, as continuum theorists contend.<ref>{{multiref | {{harvnb|Berryman|2022|loc=§ 2.6 Atomism and Particle Theories in Ancient Greek Sciences}} | {{harvnb|Varzi|2019|loc=§ 3.4 Atomism, Gunk, and Other Options}} }}</ref>

=== Universals ===
{{main|Universal (metaphysics)}}

Universals are general entities, encompassing both [[Property (philosophy)|properties]] and [[Relations (philosophy)|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''.<ref>{{multiref | {{harvnb|MacLeod|Rubenstein|loc=Lead Section}} | {{harvnb|Bigelow|1998a|loc=Lead Section}} | {{harvnb|Cowling|2019|loc=Lead Section}} | {{harvnb|Loux|Crisp|2017|pp=17–19}} }}</ref>

A topic discussed since ancient philosophy, the [[problem of universals]] consists in the challenge of characterizing the ontological status of universals.<ref>{{multiref | {{harvnb|MacLeod|Rubenstein|loc=Lead Section, § 1c. The Problem of Universals}} | {{harvnb|Rodriguez-Pereyra|2000|pp=255–256}} | {{harvnb|Loux|Crisp|2017|pp=17–19}} }}</ref> Realists argue that universals are real, mind-independent entities that exist in addition to particulars. According to [[Theory of forms|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 [[Aristotelianism#Problem of universals|moderate form of realism]], inspired by Aristotle, states that universals depend on particulars, meaning that they are only real if they are instantiated. [[Nominalism|Nominalists]] reject the idea that universals exist in either form. For them, the world is composed exclusively of particulars. The position of [[Conceptualism|conceptualists]] constitutes a middle ground: they state that universals exist, but only as [[concept]]s in the mind used to order experience by classifying entities.<ref>{{multiref | {{harvnb|MacLeod|Rubenstein|loc=Lead Section, § 2. Versions of Realism, § 3. Versions of Anti-Realism}} | {{harvnb|Bigelow|1998a|loc=§ 4. Nominalism and Realism}} | {{harvnb|Loux|Crisp|2017|pp=17–19, 45}} }}</ref>

[[Natural kind|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,{{efn|The classified entities do not have to occur naturally and can encompass man-made products, such as synthetic chemical substances.<ref>{{multiref | {{harvnb|Brzović|loc=Lead Section}} | {{harvnb|Bird|Tobin|2024|loc=Lead Section}} }}</ref>}} usually by the natural sciences, and include kinds like [[electrons]], [[H2O|{{chem2|H2O}}]], and tigers. [[Scientific realism|Scientific realists]] and anti-realists are in disagreement about whether natural kinds exist.<ref>{{multiref | {{harvnb|Brzović|loc=Lead Section, § 3. Metaphysics of Natural Kinds}} | {{harvnb|Bird|Tobin|2024|loc=Lead Section, § 1.2 Natural Kind Realism}} | {{harvnb|Liston|loc=Lead Section}} }}</ref> Social kinds are basic concepts used in the social sciences, such as [[Race (human categorization)|race]], [[gender]], [[money]], and [[disability]].<ref>{{multiref | {{harvnb|Ásta|2017|pp=[https://www.taylorfrancis.com/chapters/edit/10.4324/9781315768571-27/social-kinds-%C3%A1sta 290–291]}} | {{harvnb|Bird|Tobin|2024|loc=§ 2.4 Natural Kinds and Social Science}} }}</ref> They are studied by [[Social metaphysics|social metaphysics]] and group entities based on similarities they share from the perspective of certain practices, conventions, and institutions. They are often characterized as useful social constructions that, while not purely fictional, fail to reflect the fundamental structure of mind-independent reality.<ref>{{multiref | {{harvnb|Ney|2014|pp=259–263}} | {{harvnb|Rea|2021|pp=185–186}} }}</ref>

=== 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.<ref>{{multiref | {{harvnb|Parent|loc=Lead Section}} | {{harvnb|Loux|Crisp|2017|pp=149–150}} | {{harvnb|Koons|Pickavance|2015|pp=154–155}} | {{harvnb|Mumford|2012|loc=§ 8. What Is Possible?}} }}</ref>{{efn|A further topic concerns different types of modality, such as the contrast between physical, metaphysical, and logical necessity based on whether the necessity has its source in the [[Scientific law|laws of nature]], the essences of things, or the laws of logic.<ref>{{multiref | {{harvnb|Hanna|2009|p=[https://books.google.com/books?id=ka9BhOL1ev8C&pg=PA196 196]}} | {{harvnb|Hale|2020|p=[https://books.google.com/books?id=OSvsDwAAQBAJ&pg=PA142 142]}} }}</ref>}} 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.<ref>{{multiref | {{harvnb|Goswick|2018|pp=97–98}} | {{harvnb|Wilsch|2017|pp=428–429, 446}} }}</ref> 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 [[Fictionalism|fictional statements]].<ref>{{multiref | {{harvnb|Goswick|2018|pp=97–98}} | {{harvnb|Parent|loc=§ 3. Ersatzism, § 4. Fictionalism}} | {{harvnb|Wilsch|2017|pp=428–429}} }}</ref>

Following [[Gottfried Wilhelm Leibniz]], many metaphysicians use the concept of [[possible world]]s 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.<ref>{{multiref | {{harvnb|Menzel|2023|loc=Lead Section, § 1. Possible Worlds and Modal Logic}} | {{harvnb|Berto|Jago|2023|loc=Lead Section}} | {{harvnb|Pavel|1986|p=[https://books.google.com/books?id=OF7ZA54UpYYC&pg=PA50 50]}} | {{harvnb|Campbell|2006|loc=§ Possible Worlds}} }}</ref> For example, the [[dinosaurs]] were [[Cretaceous–Paleogene extinction event|wiped out]] in the actual world but there are possible worlds in which they are still alive.<ref>{{harvnb|Nuttall|2013|p=[https://books.google.com/books?id=luc-Hf_bEOIC&pg=PT135 135]}}</ref> 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.<ref>{{multiref | {{harvnb|Menzel|2023|loc=Lead Section, § 1. Possible Worlds and Modal Logic}} | {{harvnb|Kuhn|2010|p=[https://books.google.com/books?id=c_eYe6NVaDMC&pg=PA13 13]}} }}</ref> [[Modal realism|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 [[Counterpart theory|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]].<ref>{{multiref | {{harvnb|Parent|loc=Lead Section, § 2. Lewis' Realism, § 3. Ersatzism, § 4. Fictionalism}} | {{harvnb|Menzel|2023|loc=Lead Section, § 2. Three Philosophical Conceptions of Possible Worlds}} | {{harvnb|Campbell|2006|loc=§ Modal Realism}} }}</ref>

=== Space, time, and change ===
{{main|Philosophy of space and time}}

[[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.<ref>{{multiref | {{harvnb|Dainton|2010|pp=[https://www.cambridge.org/core/books/abs/time-and-space/spatial-antirealism/CE284C8D3088AF598998951618D4867D 245–246]}} | {{harvnb|Janiak|2022|loc=§ 4.2 Absolute/Relational Vs. Real/Ideal}} | {{harvnb|Pelczar|2015|p=[https://books.google.com/books?id=tkjTBgAAQBAJ&pg=PA115 115]}} }}</ref> 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. [[Relationalism#Relationalism (theory of space and time)|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''.<ref>{{multiref | {{harvnb|Hoefer|Huggett|Read|2023|loc=Lead Section}} | {{harvnb|Benovsky|2016|pp=[https://books.google.com/books?id=4KHWCwAAQBAJ&pg=PA19 19–20]}} | {{harvnb|Romero|2018|p=[https://books.google.com/books?id=5wJ2DwAAQBAJ&pg=PA135 135]}} }}</ref>

In the metaphysics of time, an important contrast is between the [[A series and B series|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.<ref>{{multiref | {{harvnb|Dyke|2002|p=[https://books.google.com/books?id=IxIlqbSWg4gC&pg=PA138 138]}} | {{harvnb|Koons|Pickavance|2015|pp=182–185}} | {{harvnb|Carroll|Markosian|2010|pp=160–161}} }}</ref> [[Eternalism (philosophy of time)|Eternalism]] holds that past, present, and future are equally real while according to [[Presentism (philosophy of time)|presentists]], only entities in the present exist.<ref>{{multiref | {{harvnb|Carroll|Markosian|2010|pp=179–181}} | {{harvnb|Loux|Crisp|2017|pp=206, 214–215}} | {{harvnb|Romero|2018|p=[https://books.google.com/books?id=5wJ2DwAAQBAJ&pg=PA135 135]}} }}</ref>

Material objects persist through time and undergo changes in the process, like a tree that grows or loses leaves.<ref>{{multiref | {{harvnb|Miller|2018|loc=Lead Section}} | {{harvnb|Costa|loc=Lead Section}} | {{harvnb|Simons|2013|p=[https://books.google.com/books?id=ql27m77IveQC&pg=PA166 166]}} }}</ref> 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 undergo changes, 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|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.<ref>{{multiref | {{harvnb|Miller|2018|loc=Lead Section}} | {{harvnb|Costa|loc=Lead Section, § 1. Theories of Persistence}} | {{harvnb|Simons|2013|p=[https://books.google.com/books?id=ql27m77IveQC&pg=PA166 166]}} | {{harvnb|Hawley|2023|loc=3. Change and Temporal Parts}} }}</ref>

=== Causality ===
{{main|Causality}}

Causality is the relation between cause and effect whereby one entity produces or affects another entity.<ref>{{multiref | {{harvnb|Carroll|Markosian|2010|pp=20–22}} | {{harvnb|Tallant|2017|pp=218–219}} }}</ref> For instance, if a person bumps a glass and spills its contents then the bump is the cause and the spill is the effect.<ref>{{harvnb|Carroll|Markosian|2010|p=20}}</ref> 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".<ref>{{multiref | {{harvnb|Carroll|Markosian|2010|pp=21–22}} | {{harvnb|Williamson|2012|p=[https://books.google.com/books?id=yWWEIvNgUQ4C&pg=PA186 186]}} }}</ref> The term [[agent causation]] is used if people and their actions cause something.<ref>{{multiref | {{harvnb|Ney|2014|pp=219, 252–253}} | {{harvnb|Tallant|2017|pp=233–234}} }}</ref> Causation is usually interpreted deterministically, meaning that a cause always brings about its effect. This view is rejected by [[Probabilistic causation|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.<ref>{{multiref | {{harvnb|Ney|2014|pp=228–231}} | {{harvnb|Williamson|2012|pp=[https://books.google.com/books?id=yWWEIvNgUQ4C&pg=PA185 185–186]}} }}</ref>

The [[Humeanism#Causality and necessity|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.<ref>{{multiref | {{harvnb|Lorkowski|loc=Lead Section, § 2. Necessary Connections and Hume’s Two Definitions, § 4. Causal Reductionism}} | {{harvnb|Carroll|Markosian|2010|pp=24–25}} | {{harvnb|Tallant|2017|pp=220–221}} }}</ref> According to nomic regularity theories, the regularities take the forms of [[Scientific law|laws of nature]] studied by science.<ref>{{harvnb|Ney|2014|pp=223–224}}</ref> [[Counterfactual conditional|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.<ref>{{multiref | {{harvnb|Carroll|Markosian|2010|p=26}} | {{harvnb|Tallant|2017|pp=221–222}} | {{harvnb|Ney|2014|pp=224–225}} }}</ref> 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.<ref>{{multiref | {{harvnb|Ney|2014|pp=231–232}} | {{harvnb|Mumford|2009|pp=94–95}} | {{harvnb|Mumford|Anjum|2013}} | {{harvnb|Koons|Pickavance|2015|pp=63–64}} }}</ref> Eliminativists reject the above theories by holding that there is no causation.<ref>{{harvnb|Tallant|2017|pp=231–232}}</ref>

=== Mind and free will ===
{{main|Mind|Free will}}

[[File:Dualism-vs-Monism.png|thumb|right|upright=1.3|alt=Diagram of approaches to the mind–body problem|Different approaches toward resolving the mind–body problem]]

Mind encompasses phenomena like [[thinking]], [[perceiving]], [[feeling]], and [[desiring]] as well as the underlying faculties responsible for these phenomena.<ref>{{harvnb|Morton|2005|p=[https://www.oxfordreference.com/display/10.1093/acref/9780199264797.001.0001/acref-9780199264797-e-1608?rskey=jqUtkA&result=1601 603]}}</ref> 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.<ref>{{multiref | {{harvnb|McLaughlin|1999|pp=684–685}} | {{harvnb|Kim|2005|p=608}} }}</ref> This view is rejected by [[Monism|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 [[disposition]]s, or functional roles.<ref>{{multiref | {{harvnb|McLaughlin|1999|pp=685–691}} | {{harvnb|Kim|2005|p=608}} | {{harvnb|Ramsey|2022|loc=Lead Section}} }}</ref> [[Neutral monism|Neutral monists]] argue that reality is fundamentally neither material nor mental and suggest that matter and mind are both derivative phenomena.<ref>{{multiref | {{harvnb|Stubenberg|Wishon|2023|loc=Lead Section; § 1.3 Mind and Matter Revisited}} | {{harvnb|Griffin|1998}} }}</ref> 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.<ref>{{harvnb|Weisberg|loc=Lead Section, § 1. Stating the Problem}}</ref>

The status of free will as the ability of a person to choose their [[Action (philosophy)|actions]] is a central aspect of the mind–body problem.<ref>{{multiref | {{harvnb|Timpe|loc=Lead Section}} | {{harvnb|Olson|2001|loc=[https://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/mind-body-problem Mind–Body Problem]}} | {{harvnb|Armstrong|2018|p=[https://books.google.com/books?id=uf7EDwAAQBAJ&pg=PA94 94]}} }}</ref> 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 [[Libertarianism (metaphysics)|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 in regard to the [[moral responsibility]] people have for what they do.<ref>{{multiref | {{harvnb|O’Connor|Franklin|2022|loc=Lead Section, § 2. The Nature of Free Will}} | {{harvnb|Timpe|loc=Lead Section, § 1. Free Will, Free Action and Moral Responsibility, § 3. Free Will and Determinism}} | {{harvnb|Armstrong|2018|p=[https://books.google.com/books?id=uf7EDwAAQBAJ&pg=PA94 94]}} }}</ref>

=== Others ===
[[Identity (philosophy)|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". 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.<ref>{{multiref | {{harvnb|Kirwan|2005|pp=417–418}} | {{harvnb|Noonan|Curtis|2022|loc=Lead Section}} }}</ref> 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]], is more controversial and states that two entities are numerically identical if they exactly resemble one another.<ref>{{multiref | {{harvnb|Sleigh|2005|p=418}} | {{harvnb|Kirwan|2005|pp=417–418}} | {{harvnb|Noonan|Curtis|2022|loc=§ 2. The Logic of Identity}} }}</ref> 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".<ref>{{multiref | {{harvnb|Gallois|2016|loc=§ 2.1 Diachronic and Synchronic Identity}} | {{harvnb|Noonan|Curtis|2022|loc=Lead Section, § 5. Identity Over Time}} }}</ref> [[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.<ref>{{multiref | {{harvnb|Noonan|Curtis|2022|loc=Lead Section}} | {{harvnb|Olson|2023|loc=Lead Section, § 1. The Problems of Personal Identity}} | {{harvnb|Korfmacher}} }}</ref>

Various contemporary metaphysicians rely on the concepts of [[truth]] and [[Truthmaker theory|truthmakers]] to conduct their inquiry.<ref>{{multiref | {{harvnb|Tallant|2017|pp=1–4}} | {{harvnb|Koons|Pickavance|2015|pp=15–17}} }}</ref> Truth is a property of linguistic statements or mental representations that are in accord with reality. A truthmaker of a statement is the entity whose existence makes the statement true.<ref>{{multiref | {{harvnb|Lowe|2005a|p=926}} | {{harvnb|Imaguire|2018|p=[https://books.google.com/books?id=X4dnDwAAQBAJ&pg=PA34 34]}} | {{harvnb|Tallant|2017|pp=1–4}} | {{harvnb|Koons|Pickavance|2015|pp=15–17}} | {{harvnb|Asay|2020|p=[https://books.google.com/books?id=nRXeDwAAQBAJ&pg=PA11 11]}} }}</ref> For example, the statement "a tomato is red" is true because there exists a red tomato as its truthmaker.<ref>{{harvnb|Tallant|2017|p=1}}</ref> 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.<ref>{{multiref | {{harvnb|Tallant|2017|pp=1–4, 163–165}} | {{harvnb|Koons|Pickavance|2015|pp=15–17, 154}} }}</ref>
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