A system of categories is a complete list of highest kinds or genera.Traditionally, following Aristotle, these have been thought of ashighest genera of entities (in the widest sense of the term), so thata system of categories undertaken in this realist spirit would ideallyprovide an inventory of everything there is, thus answering the mostbasic of metaphysical questions: “What is there?”Skepticism about our ability to discern a unique system of basiccategories of ‘reality itself’ has led others to approachcategory systems not with the aim of cataloging the highest kinds inthe world itself, but rather with the aim of elucidating thecategories of our conceptual system or language. Thus Kant makes theshift to a conceptualist approach by drawing out the categories thatarea priori necessary for any possible cognition of objects.Since such categories are guaranteed to apply to any possible objectof cognition, they retain a certain sort of ontological import,although this application is limited to phenomena, not the thing initself. After Kant, it has been common to approach the project ofcategories in a neutral spirit that Brian Carr (1987, 7) calls“categorial descriptivism”, as describing the categorialstructure that the world would haveaccording to our thought,experience, or language, while refraining from making commitmentsabout whether or not these categories are occupied, or are onticallyfundamental. Edmund Husserl approaches categories in something likethis way, since he begins by laying out categories ofmeanings, which may then be used to draw outontological categories (categories of possible objects meant)as the correlates of the meaning categories, without concern for anyempirical matter about whether or not there really are objects of thevarious ontological categories discerned.

A system of ontological categories drawn out in any of these modes hasthe potential for a great many uses in philosophy, but those who wouldoffer such systems of categories also face a variety of difficulties.They must address the issue of what the proper methods are by means ofwhich categories are to be distinguished, how many categories thereare and what they are, whether or not there is a singlesummumgenus subsuming all other categories, and whether we shoulddistinguish a single system of categories or multiple dimensions ofcategories – issues on which there has been littleagreement.

Over the past hundred years, skepticism about the possibility ofoffering a uniquely true and complete system of ontological categorieshas led discussion of categories to shift from attempts to offercomplete systems of categories to attempts merely to draw particulardistinctions, especially among our conceptual or linguisticcategories. Work on category differences, unlike that on categorysystems, does not generally purport to answer deep metaphysicalquestions about what things or kinds of things exist; instead,category differences are typically articulated as a way of diagnosingand avoiding various philosophical problems and confusions.Nonetheless, even those who merely argue for category differences owean account of the conditions under which two concepts, terms, orobjects belong to different categories.

• 1. Category Systems
  • 1.1 Aristotelian Realism
  • 1.2 Kantian Conceptualism
  • 1.3 Husserlian Descriptivism
  • 1.4 Contemporary Category Systems
  • 1.5 Skepticism about Category Systems
  • 1.6 Categories in Other Disciplines
• 2. Category Differences
  • 2.1 The Uses of Category Distinctions
  • 2.2 The Ryle/Husserl Method of Distinguishing Categories
  • 2.3 Fregean Approaches to Distinguishing Categories
• Bibliography
• Academic Tools
• Other Internet Resources
• Related Entries

1. Category Systems

1.1 Aristotelian Realism

Philosophical interest in categories may be traced back to Aristotlewho, in his treatiseCategories, attempts to enumerate themost general kinds into which entities in the world divide. He doesnot begin from a single highest kind, but rather lists the followingas the ten highest categories of things “said without anycombination” (Categories 1b25):

• Substance (e.g., man, horse)
• Quantity (e.g., four-foot, five-foot)
• Quality (e.g., white, grammatical)
• Relation (e.g., double, half)
• Place (e.g., in the Lyceum, in the market-place)
• Date (e.g., yesterday, last year)
• Posture (e.g., is lying, is sitting)
• State (e.g., has shoes on, has armor on)
• Action (e.g., cutting, burning)
• Passion (e.g., being cut, being burned)

There are two sorts of substance: a primary substance is, e.g., anindividual man or horse; the species (and genera) of theseindividuals (e.g.,man,animal) are secondarysubstances. While the ten categories are all equally highest kinds,primary substances nonetheless have a certain sort of priority, since“all the other things are either said of the primary substancesas subjects or in them as subjects. So if the primary substances didnot exist it would be impossible for any of the other things toexist” (Categories 2b4).

Elsewhere, inMetaphysics (998b22), Aristotle arguesexplicitly that there cannot be a highest genus (e.g., ofbeing orunity) shared by entities of differentcategories (cf. Ackrill 1963, 81). For a species is defined in termsof its subsuming genus and differentia (e.g., man is definable as ananimal that is rational), and while the genus (animal) may bepredicated of the species (man), it may not be predicated of thedifferentia (rational). As a result, ifbeing (or unity) werea genus, no differentiae could be said to have being (or to be one);but “the differentiae of any genus must each of them both havebeing and be one” (Metaphysics 998b22–3).

The ancient Greek term ‘kategoria’ described what could besaid against someone in a court of law, and indeed Aristotle uses whatcan be saidof orin a subject as a route todistinguishing categories. There is controversy in the literature,however, about precisely how he arrived at his categories (Studtmann2007). On one prominent interpretation, put forward by J. L. Ackrill,Aristotle arrived at his list of categories by way of distinguishing“different questions which may be asked about something”and noting “that only a limited range of answers can beappropriately given to any particular question” (Ackrill 1963,78–9), e.g., the question ‘what is it’ can only beasked of a substance, and only answers describing substances areappropriate. The question ‘how much’, by contrast,requires a quantity for an answer, and so on.

But although on this interpretation Aristotle seems to have arrived athis categories by considering different sorts of question and answer,the categories he was offering were supposed to be categories ofentities, not of language; language was just a clue to truths aboutthe world. As J. L. Ackrill writes, Aristotle’sCategories “is not primarily or explicitly about names,but about the things that names signify…Aristotle reliesgreatly on linguistic facts and tests, but his aim is to discovertruths about non-linguistic items” (1963, 71).

Other interpretations have also been suggested about howAristotle’s categories were derived. Some hold thatAristotle’s list was arrived at by reflecting on grammaticalcategories, and assuming a parallelism between structures of languageand structures of the world (Baumer 1993). But others have developedinterpretations that do not consider Aristotle to have arrived at hiscategories by considering linguistic matters such as grammaticalstructure or the questions we may ask. Instead, they take them toarise from more worldly considerations such as which types of entityany sensible particular must be related to (Moravcsik 1967). For anoverview of the interpretive options, see Studtmann (2007).

In any case, regardless of how the categories were derived,Aristotle’s approach to categories is generally taken to be inthe spirit of what Brian Carr calls “categorial realism”– an approach conceiving of a system of categories as a list ofthe highest genera of beings (not merely of language or thought– even if those may be used in deriving the metaphysicalcategories). As Studtmann (2007) puts it, Aristotle “ assumesrather than defends a posture of realism with respect to themetaphysical structures of the world”. Given this approach, acomplete system of categories would offer a systematic inventory ofwhat there is, considered at the most abstract level (although it isnot clear whether Aristotle intended his categories to be exhaustive).Thus on a categorial realist approach, providing a system ofcategories can be seen as one, or eventhe central task ofmetaphysics (cf. Grossman 1983, 3). Such a system of categories mayalso play a central role in answering individual questions of nature,providing the most general sort of answer to questions of the form“What is this?”, and providing the basis for definitionsof narrower sorts of things by specifying the most general category(genus) under which things of this sort fall, and the differentia thatdistinguishes them from other things of the same category. This hasendured as the paradigmatic approach to categories, and several recentauthors have offered new theories of categories in this spirit ofAristotelian realism (see §1.4 below).

1.2 Kantian Conceptualism

Others, however, have shied away from this robustly realist approachto categories, generally on grounds of skepticism about our ability todiscern intrinsic divisions in ‘reality itself’, and haveinstead treated the project of categories as a matter of laying outthe highest categories governing our conceptual scheme. This shift inapproach to what Carr (1987, 6) calls “categorialconceptualism” was made famous by Immanuel Kant. While Kantfamously denied that we have access to intrinsic divisions (if any) ofthe thing in itself that lies behind appearances or phenomena, he heldthat wecan discover the essential categories that governhuman understanding, which are the basis for any possible cognition ofphenomena. Thus, as H. J. Paton puts it, for Kant “We can havea priori knowledge by means of the categories, only if thecategories are due to the nature of the mind and are imposed by themind on the objects which it knows” (1936, 258).

In hisCritique of Pure Reason, Kant arrives at his list ofcategories by first enumerating the forms of possiblejudgment (A70/B95–A93/B109). On this view, objectiveempirical judgments (i.e., empirical judgments which purport to referto objects rather than merely subjective seemings or connections ofsense impressions, and which purport to be universally valid for alljudging subjects) are endowed with their objectivity and generality invirtue of the a priori concepts embodied in the relevant forms ofjudgment. If we can identify all of the possible forms of objectiveempirical judgment, we can then hope to use them as the basis todiscover all of the most general concepts or categories that areemployed in making such judgments, and thus that are employed in anycognition of objects (Körner 1955, 48–49).

Thus in distinguishing his categories, Kant begins from Aristotelianlogic in outlining four respects in which one can classify anyjudgment: according to its quantity, quality, relation, or modality.In each of these respects or ‘moments’ of judgment, thereare three alternative classifications; thus, e.g., in respect ofquantity, a judgment may be universal, particular, or singular; inrespect of its relation, a judgment may be categorical, hypothetical,or disjunctive, and so on. These Aristotelian ways of classifyingjudgments are the clue to discerning the twelve correlated concepts ofthe understanding. So, e.g., from noting that all judgments are eitheruniversal (e.g., All swans are white), particular (e.g., Some swansare white) or singular (e.g., Cygmund is white), we can arrive at thethree corresponding categories of quantity: unity, plurality, andtotality. Via this route, Kant ultimately distinguishes twelve pureconcepts of the understanding (A80/B106), divided into four classes ofthree:

• Quantity
  • Unity
  • Plurality
  • Totality
• Quality
  • Reality
  • Negation
  • Limitation
• Relation
  • Inherence and Subsistence (substance and accident)
  • Causality and Dependence (cause and effect)
  • Community (reciprocity)
• Modality
  • Possibility
  • Existence
  • Necessity

The categories are presented as forming a single exhaustive list, withthe four classes of categories imposing four different forms of unityon the object known (Paton 1936, 295–9). Thus, one mayseparately inquire after an object’s quantity, quality,relation, and modality, receiving one of the three sub-answers in eachcase on the way to a more complete characterization of the object.

Although these are categories of the understanding, they nonethelessretain a certain sort of ontological import, as it isapriori that they apply universally to all objects of possiblecognition (A79/B105). In this way, by delineating the concepts thatarea priori necessary for thecognition of objects,we can acquire knowledge of categories governing any possibleobject of cognition, and so acquire a sort of descriptive setof ontological categories, though these must be understood explicitlyas categoriesof objects of possible cognition, not of thething in itself. Thus Kant was able to treat his system of concepts asa system of categories in something like the Aristotelian sense,“for our primary purpose is the same as his [Aristotle’s],although widely diverging from it in manner of execution”(A80/B105). Nonetheless, it is clear that for Kant the categories findtheir original source in principles of human understanding, not inintrinsic divisions in mind-independent reality, and are discoverableby paying attention to possible forms of human judgment, not by studyof the world itself, nor by study of our contingent manners ofspeaking.

1.3 Husserlian Descriptivism

Edmund Husserl introduced two sorts of innovation to the study ofcategories. First, while Aristotle used language as a clue toontological categories, and Kant treated concepts as the route tocategories of objects of possible cognition, Husserl explicitlydistinguished categories ofmeanings from categories ofobjects, and attempted to draw out the law-like correlationsbetween categories of each sort (Smith 2007, 139ff.). Secondly,whereas Aristotle and Kant each lay out a single system of categories,Husserl distinguishes two ways of arriving at top-level ontologicalclassifications: byformalization and bygeneralization, yielding two separate, orthogonal, systems ofcategories, in two different dimensions (cf. Smith 2004, chapter8).

Husserl is careful to distinguish categories ofmeanings (byway of which we can thinkabout the highest kinds or‘essences’ of objects) from the categoriesmeant– the latter are the categoriesof objects, orontological categories, considered as the highest essencesthat entities might have: “by ‘categories’ we canunderstand, on the one hand, concepts in the sense of meanings, but onthe other also, and to better effect, the formal essences themselveswhich find their expression in these meanings” (1913 [1962],61–2). But although the two sorts of categories must bedistinguished, according to Husserl categories of the two sorts areessentially correlated (see below), so we can learn about one by wayof the other.

Regardless of whether we are studying categories of meanings or ofobjects, Husserl is quite clear that the study of categories, for him,is an entirelya priori matter; the categories of meaningsand objects alike “arise … solely in relation to ourvarying thought-functions: their concrete basis is solely to be foundin possible acts of thought, as such, or in the correlates which canbe grasped in these” (1913 [2000], 237). As he puts it later, intheIdeas, the study of categories is a study of essences,based inessential insights about the types of meanings andcorrelative types of things. Such studies of essence may be conductedby way of imaginative variation of cases, independently of any matterof fact, including whether or not there actually is anythingof the ontological kinds distinguished (1913 [1962], 51).Thus Husserl’s ontological categories, in this sense, aredescriptive categories of highest essences of possible things(that might fall under those essences), and do not purport to providean inventory of what thingsactually exist (as a matter ofempirical fact).

Husserl provides an extensive discussion of categories of meaning intheLogical Investigations, arguing that differences incategories of meaning (which seem to be more like syntactic thansemantic categories) can be distinguished by noting where nonsenseresults from substituting one term for another. E.g., in the sentence“This tree is green” we may substitute “chair”– but not “careless” – for “tree”without turning sense into nonsense, marking the difference betweenthe meaning categories ofnominative material andadjectival material (1913 [2000], 511–512).Husserl’s understanding of ‘nonsense’ is ratherstrict: he counts only those strings of words that aresyntactically incorrect (so that they form a mere ‘heapof words’ and cannot be combined into any unified meaning(Husserl 1913 [2000], 522)) as strictly nonsensical, and thus as signsof differences in categories of meaning. (Husserl repeatedlydistinguishes the nonsense of verbal formations like “a roundor” (in which no unified meaning emerges) from cases of mereabsurdity such as “a round square”, in which theexpression has a unified meaning, although it isa priorithat no object can correspond to the expression (1913 [2000],516–17)).

Correlated with the categories of meanings areontologicalcategories; e.g., object, state of affairs, unit, plurality, number,and relation are (formal) categories that categorize objects, notmeanings (Husserl 1913 [2000], 237). Categories of the two sorts are,according to Husserl, connected by ‘ideal laws’. Thus, forexample, presumably objects are the ontological correlates of themeaning category of nominative expressions, properties are theontological correlates of adjectival expressions, and states ofaffairs are the ontological correlates of propositions. So whileHusserl does not (to my knowledge) explicitly lay out a method ofdiscerningontological categories, it may be that we canderive them by beginning from the above nonsense test fordistinguishing meaning categories, and then shifting attention to thecorrelative ontological categories, since “pure truthsconcerning meaning can be transformed into pure truths concerning theobject” (1913 [1962], 61).

As well as explicitly distinguishing categories of meanings fromcategories of the correlated objects that could possibly be‘meant’, Husserl introduced a second innovation to thestudy of categories by distinguishing highestformal essences(which Husserl calls ‘categories’) from highestmaterial essences (which he calls ‘regions’)(1913 [1962], §10; cf. Smith 1995, 329–330 and Smith 2007,142–148). Thus far I have been describing theformalontological categories, the correlates of the different meaningcategories distinguishable by the nonsense test. In fact, Husserlreserves the term ‘category’ for the highest formalgenera, which are distinguished by a process offormalization– a removal of content. These ‘categorial essences’begin with ‘object in general’ at the top of the tree,which is then divided at the next level into categories including (asexamples) object, state of affairs, property, relation, number, etc.(compare lists 1913 [2000], 237 and 1913 [1962], 61). Much asAristotle distinguished (independent) primary substances from(dependent) things of other kinds, within his formal categoriesHusserl distinguishes the ‘substrative’ category ofindividuals (or, more properly, the merethis-there) from thedependent ‘syntactic objectivities’ – the correlatesof nominative terms that are derived from ways in which we speak aboutthe primary substances (1913 [1962], 62–3 and 67) (as, e.g., thenominative term ‘this relation of brightness’ may bederived from claims that ‘A is brighter than B’ (1913[2000], 797–8)).

Husserl’s material categories, by contrast, classify entitiesaccording to their nature or essence, with the highest material generato be arrived at by a process ofgeneralization to the mostgeneral kind of content involved, rather than by theformalization that involves an emptying of all content (1913[1962], 65). The highest material categories are the three regions:nature (including physical objects and events), culture (includingartifacts, social entities, and values), and consciousness (cf. Smith2004). While formal and material category systems each form ahierarchy (1913 [1962], 64), considered jointly their categories arenot mutually exclusive, since one and the same entity may becategorized either in terms of its material nature or its form. Forfurther discussion of Husserl’s categories, see Smith (2007,135–161).

Husserl is nowhere explicit about the proper method for distinguishingmaterial ontological categories, but he does distinguish materialabsurdity from formal absurdity, and from the formal nonsense thatmarks the difference in meaning categories (1913 [2000], 523).Expressions areformally absurd if it isa priorithat no object could correspond to them, based purely on formal,logical laws, without regard to which particular material concepts areemployed, e.g., “a round not-round thing” is formallyabsurd; its absurdity would remain regardless of which adjective wesubstituted for ‘round’ or which noun for‘thing’. On the other hand, expressions are materiallyabsurd if the impossibility of there being any corresponding object isbased in the particular material concepts employed, e.g., ‘around square’ is a materially absurd expression based in theparticular meanings of ‘round’ and ‘square’.Thus presumably one could attempt to distinguish material ontologicalcategories by the material absurdity that results from substitutingexpressions for objects of differentmaterial kinds; ‘around table’, for example, makes perfect sense, but if wesubstitute for ‘table’ a term for a geometric figure suchas ‘square’ or for a day of the week such as‘Thursday’, we get a materially absurd statement (to whichit isa priori that nothing corresponds). As we will see in§2.2 below, Gilbert Ryle developed Husserl’s nonsense testfor category differences in something like this way.

Roman Ingarden (1960 [1964], 22ff) took Husserl’smulti-dimensional ontology one step further. Like Husserl, hedistinguished formal categories from material categories, but he alsodistinguished categories in a third dimension: existential categories(those describing an entity’s mode of being). The highestexistential categories on Ingarden’s list are the real(spatio-temporal being), the ideal (abstract), the absolute(completely independent, atemporal), and purely intentional(consciousness-dependent). While any conceivable entity should beuniquely locatable in a single category of each dimension, the threesorts of ontology are mutually orthogonal, providing different mostabstract ways of considering the putative entity in question. Thus,e.g., a sculpture might be categorized formally as an object,materially as a work of art, and existentially as purelyintentional.

1.4 Recent Category Systems

In the twentieth century, systems of ontological categories fellsomewhat out of fashion (for reasons I will discuss in §1.5below), with most discussion of categories shifting to merelyarticulating categorydifferences rather than aiming tooutline a comprehensive system of categories.

One important exception to this comes in the work of Samuel Alexander,who, in his 1920 workSpace, Time and Deity develops a theoryof categories in the realist spirit. Alexander defends a monistontology in which he posits Space-Time as “the one monisticentity that encompasses every entity and every feature inreality” (Fisher 2015, 246). He sees the categories as groundedon the intrinsic nature of Space-Time, and posits as categorialfeatures only those which are ‘pervasive’, that is,instantiated by every entity. The categories he identifies come inthree ‘grades’ (or ranks of increasing complexity, inwhich the latter grades presuppose the former), giving us thefollowing system:

• Grade 1
  • Existence
  • Universality
  • Relation
  • Order
• Grade 2
  • Substance
  • Causality
  • Quantity
  • Number
• Grade 3
  • Motion

In recent years there have also been several notable attempts to offernew systems of categories in either the realist or descriptivistspirit, although little agreement exists about what the categories areor how one could decide among competing systems.

Ingvar Johansson (1989) and Roderick Chisholm (1996) both take aneo-Aristotelian realist approach to categories, attempting to lay outa complete system of the categories, where this is understood asproviding a list of categories of real entities in the world. IngvarJohansson explicitly insists that his interest is in the world:“This book is a book about the world. I am concerned withontology, not merely with language” (1989, 1), and attempts tooffer “a realist theory of categories regarded as real aspectsof being” (1989, 2). His list (1989, 20) includes nine maincategories (some of which subdivide further):

• Space-time
• State of affairs
• Quality
  • Substance
  • Property
• External Relation
• Grounded Relation
• Inertia
• Spontaneity
• Tendency
• Intentionality
  • Real
    • Presentational
    • Representational
  • Fictional

Unlike Aristotle, Johansson makes no explicit use of language indiscerning ontological categories, instead appealing to the method ofsuccessive abstraction (Johansson 1989, 1–2). Thus, e.g., wearrive at the category ‘quality’ by moving up inabstraction from a particular shade of dark red, to red, color, andfinally quality. Similarly (to use an example of Sellars’) onemight try to arrive at the category of ‘substance’ byconsidering an individual entity, say, Fido, and moving by successiveabstraction from “Fido is a dachshund” through “Fidois a dog” and “Fido is an animal”, ultimatelyreaching “Fido is a substance” (1970 [1974], 321). LikeAristotle’s categories, Johansson’s categories top outwith a number of distinctions without a highest single categorysubsuming them all.

Like Aristotle and Johansson, Roderick Chisholm presents his work oncategories as being “about the ultimate categories ofreality” (1996, 3). Unlike them, however, Chisholm (1996, 3)lays out categories in the form of a Porphyrian tree starting from asingle most general category comprising everything, but divided intosuccessively narrower genera at lower levels of branching. (For aninteresting discussion of whether such general terms as‘entity’ or ‘thing’ could be seen as naming ahighest category, see Thompson 1957, cf. §2.3 below).Chisholm’s system of categories thus reads:

• Entia
  • Contingent
    • States
      • Events
    • Individuals
      • Boundaries
      • Substances
  • Necessary
    • States
    • Nonstates
      • Attributes
      • Substance

Other contemporary authors have approached the issue of categories ina purely descriptive spirit. Reinhardt Grossmann, for example,distinguishes eight highest categories (1983, xvi):

• Individuals
• Properties
• Relations
• Classes
• Structures
• Quantifiers
• Facts
• Negation

But although Grossmann characterizes his book as an attempt to“bring Aristotle’sCategories up-to-date”(1983, xv), he is explicit in denying that he is making any claimsabout whether or not there are things belonging to any of the eightcategories he distinguishes, taking this as beyond the scope ofontology (1983, 10–12).

Joshua Hoffman and Gary Rosenkrantz (1994) lay out a tree-form systemof categories, withentity thesummum genus,subdivided intoabstract andconcrete (rather thanChisholm’scontingent andnecessary), each ofwhich is further subdivided:

• Entity
  • Abstract
    • Property
    • Relation
    • Proposition
  • Concrete
    • Event
    • Time
    • Place
    • Substance
      • Material Object
      • Spirit
    • Limit
    • Collection
    • Privation
    • Trope

They, too, explicitly offer their system of categories in the spiritof categorial description, as offering an analysis of the variouspossible categories of being, rather than making any claims aboutwhich of these categories is non-empty (1994, 7–8).

E. J. Lowe takes categories to be categories of “what kinds ofthings can exist and coexist” (2006, 5). Such categories, heargues, are to be individuated according to the existence and/oridentity conditions of their members; fundamental categories are thoseregarding which the existence and identity conditions for theirmembers cannot be exhaustively specified in terms of ontologicaldependence relations they bear to entities of other categories (2006,8). Accordingly, he argues that there are four fundamental ontologicalcategories: objects (individual substances such as Fido), modes(property or relation instances such as Fido’s four-leggedness),kinds (substantial universals such as the kinddog), andattributes (property or relation universals, such asbeingfour-legged). But although he argues that there are exactly fourfundamental categories, Lowe nonetheless takes a hierarchicalapproach to arranging categories. The four fundamental categoriesappear at the third level of his hierarchical chart; the categoriesthat appear at the higher levels (particulars and universals at thesecond level; entity at the top) are “mere abstractions and dono serious ontological work on their own account” (2006, 39).His fuller chart of categories appears as follows:

• Entities
  • Particulars
    • Objects
      • Substances
      • Non-substances
    • Modes (monadic and relational)
  • Universals
    • Kinds
    • Attributes (properties and relations)

Others, taking the project of developing categories in an explicitlyrealist spirit and driven by the goal of offering a parsimoniousontology, have aimed to offer a more minimal system of fundamentalontological categories. For example, Laurie Paul (2016) has recentlydefended a ‘one-category’ ontology that accepts (at thefundamental level) only the existence of intrinsic characters orqualities. Against those who accept more categories, she argues thatwe have no need (for example) for a fundamental division betweenindividuals and their properties, and that a one-category ontology isboth more parsimonious and has a better claim to carve the world atits ‘ontological joints’.

1.5 Skepticism about Category Systems

Both realist and descriptivist category systems, at least astraditionally presented, seem to presuppose that there is a uniquetrue answer to the question of what categories of entity there are– indeed the discovery of this answer is the goal of most suchinquiries into ontological categories. Grossman, for example, arguesthat a list of categories must be complete, contain everything, witheverything in itsproper place (1983, 4). Johansson similarlytakes his project as to “develop a coherent system ofall the most abstract categories needed to give a truedescription of the world” (1989, 1). Arguments about which ofthe many systems of categories offered is correct likewise seem topresuppose that there is a uniquely correct list of categories.

But actual category systems offered vary so much that even a shortsurvey of past category systems like that above can undermine thebelief that such a unique, true and complete system of categories maybe found. Given such a diversity of answers to the question of whatthe ontological categories are, by what criteria could we possiblychoose among them to determine which is uniquely correct?

Some minimal standards of adequacy immediately suggest themselves(Butchvarov 1995, 75). Whether one takes a realist or descriptivistapproach to providing a system of categories, if that system issupposed to be comprehensive, it clearly must meet at least thestandard of being exhaustive – providing a category foreverything there is (on the realist approach) or might be (on thedescriptive approach). Nonetheless, one may, as Hoffman andRosenkrantz (1994, 140) do, present a system ofsomefundamental categories without taking it to be exhaustive.

Another minimal criterion of adequacy is generally taken to be thatthe highest categories (or, for tree systems, the categories at eachlevel of branching) be mutually exclusive, ensuring that whateverthere is (or might be) finds its place in exactly one highestcategory, or one category at each level (Chisholm 1989, 162). (Thisstill allows for nested categories, so that something may belong toboth a more specific category like substance and a more generalcategory like individual.)

But these criteria are not enough to provide the needed reassurance.First, we lack assurance that most proposed category systems meet eventhese minimal conditions. As mentioned above, Aristotle drew out hiscategories largely by considering the types of question that could beasked (and the types of answer appropriate to them). It is difficultto know, however, how one can be assured that all kinds of questionshave been surveyed, and so difficult to know that an exhaustive listof categories has been offered – a point Aristotle does notattempt to demonstrate (Ackrill 1963, 80–81). Indeed, the factthat Aristotle provides different lists of categories in differentplaces suggests that he did not consider his list final andexhaustive. Similarly, Kant’s system of categories can bethought to be exhaustive only as long as the list of forms of judgmentfrom which he derives them exhausts the possible forms of judgment– but we have reason to think this is not so (Körner 1955,50). Johansson, as we have seen, instead uses the method of successiveabstraction. But it is not clear how following such a method couldensure either that the categories thereby distinguished are exhaustive(how do we know we have considered something of each highest kind ifwe do not yet know what the highest kinds are?) or even mutuallyexclusive.

Secondly, even if we can verify that the standards of mutualexclusivity and exhaustiveness are met, these conditions alone are fartoo weak to uniquely pick out a system of categories. Provided oneaccepts the law of the excluded middle, an endless supply of mutuallyexclusive and exhaustive classifications can be generated at will: wecan divide things into the spatio-temporally located and thenot-spatio-temporally-located, the intentional and thenon-intentional, the extended and the non-extended, to name but a fewof the more relevant ways in which things could be divided. Indeed oneof the sources of puzzlement about categories comes from the fact thatphilosophers have selected so many different sorts of divide asthe fundamental category difference – for Descartes,the extended and the thinking (unextended), for Chisholm thecontingent and the necessary, for Hoffman and Rosenkrantz the concreteand the abstract, and so on. Thus another reason for skepticism aboutthe existence of aunique set of categories comes from thefact that categories are supposed to be the most abstract genera underwhich things (may) fall. But from any given entity, abstraction mayapparently be done in a variety of ways – even if we are carefulto do so in ways that ensure mutual exclusivity andexhaustiveness.

Doubts about possibilities for discovering the one true categorysystem have led many to eschew talk of category systems altogether,and others to adopt some kind of relativism about category systemsthat ceases to take systems of categories seriously as candidate listsofthe single set of highest genera under which anythingfalls (or could fall). Jan Westerhoff (2005), for example, argues thatthere is no unique, absolute set of ontological categories. On hisview categories in metaphysics turn out to be analogous to axioms inmathematical theories; in each case, there may be more than one way tosystematize our knowledge from a relatively simple basis. The resultis a kind of relativity about systems of categories: “which setof ontological categories we choose is primarily a matter ofconvenience, in the same way as specific axiomatizations ofpropositional logic or Newtonian mechanics are more convenient to usethan others” (2005, 218). As a result, Westerhoff argues, wemust reassess the importance of ontological categories in metaphysics– these should not be considered “the most fundamentalparts of the world, but... the most fundamental parts ofoursystematization of the world” (2005, 135).

Others have taken the variety of category systems explicitly offeredor presupposed by philosophers as mere evidence of the particularpresuppositions of their thought, or prejudices of their age –not as evidence about anything to do with the world and its divisions.Thus, e.g., Stephan Körner’s discussion of categorialframeworks is designed to make explicit how a thinker’sframework categorizes objects, making use of certain individuativeprinciples, and making clear his reasons for holding that framework(1970, 10). R. G. Collingwood, in similar vein, treats the task ofmetaphysics generally as merely uncovering the “presuppositionsunderlying ordinary science” (1940 [1998]).

The specific worries about (1) guaranteeing the mutual exclusivenessand joint exhaustiveness of the categories, and (2) whether or not anysingle system of categories could purport to be uniquely correct, can,however, be met by certain ways of formulating ontological categories.The first sort of worry can be met by ensuring that categories (of thesame level) are defined in ways that guarantee mutual exclusivenessand exhaustiveness. Thus, e.g., Thomasson (1999, chapter 8)distinguishes categories in terms of what relations of dependence apurported entity hasor lacks on mental states (and a seconddimension is distinguished in terms of what relations of dependence apurported entity has or lacks on spatio-temporally located objects),so that the law of the excluded middle alone ensures mutualexclusiveness and exhaustiveness of the categories distinguished.(Dummett’s method of distinguishing categories provides anotherroute for guaranteeing mutual exclusivity – see §2.3below).

Multi-dimensional systems (Husserl 1913 [1962], §10; Ingarden1960 [1964], Chapter 2; Thomasson 1999, Chapter 8; Smith 2007, Chapter8) address the second worry to some extent by acknowledging that thedifferent dimensions of categorization are possible, and that noone-dimensional list can purport to completeness. In principle,multi-dimensionalists may even accept that there is no fixed number orlimit on how many one-dimensional lists of categories there may be,though each such list may purport to provide a unique, correct,exhaustive categorization of entities considered in the chosenrespect.

In any case, given the great potential uses of a system of categories(many of which do not depend on claims that that category system isuniquely ‘correct’), we should not prematurely abandonattempts at developing and evaluating systems of categorization. Evenif we do not think of a category system as providing a realistinventory of all that exists or a description of the fundamentalintrinsic ‘divisions of reality’, a system of categorieslaid out in the descriptivist spirit provides a framework within whichexistence questions can be answered in a systematic and wholesale way,by enumerating categories so that we can then undertake furtherinvestigations into whether or not there really is anything of eachkind. Working from within a categorial framework can help ensure thatwhatever ontology we provide is principled and unified, avoidingad hoc and piecemeal decisions. The descriptivist’scategories also provide a tool that may be used elsewhere in ontology,e.g., in helping to ensure that comparisons of parsimony arelegitimately made (by examining which categories of entity areaccepted and which denied by various theories), and in checking thatpotential solutions to metaphysical problems are not overlooked bytacit use of a category system that is not exhaustive (Thomasson 1999,Chapters 8 and 9). Another important use of systems of categories isthat, with a proposed set of categories laid out, we can, as DanielNolan (2011) suggests, go on to investigate questions about therelationships among entities that are placed in different categories:for example, questions about whether events depend on or are groundedin things, or (as Nolan suggests) whether things and events mayultimately be identified as belonging to a single category.Assumptions about categorizations play such a strong role inphilosophical discussions (e.g., discussions of the Cartesian theoryof mind, Platonist theories of mathematics, etc.), that doing the workon categories necessary to make these categorial assumptions explicitand open them for examination must remain a highly useful exerciseregardless of doubts about the prospects for discovering a uniquelycorrect system of categories.

1.6 Categories in Other Disciplines

For those who approach categories in a descriptive spirit, as a matterof determining the categories of our language or thought, it isnatural to turn to linguistics or cognitive science forassistance.

A prominent approach to determining the ontological categories thatare implicit in the use of natural language is via Natural LanguageOntology, which provides one way of undertaking a descriptivistapproach to categories. As Friederike Moltmann (2017) makes clear,however, the methodology for doing natural language ontology isimportantly different from attempts to determine a common senseontology by simply asking what ontological claims or categories peopleexplicitly accept or would accept on reflection. So, for example,Natural Language Ontology doesn’t determine the ontologicalcategories of a natural language by looking to explicit assertionsspeakers make (or would assent to) about categories, such as“objects are not events”. Instead, natural languagecategory distinctions are revealed by uncovering thepresuppositions of sentences used by ordinary speakers. Forexample, the fact that one can acceptably say “The buildingexisted last year” but not “The building took place lastyear”, and “John’s arrival took place lastweek” but not “John’s arrival existed lastweek”, it has been argued, presupposes a difference of categorybetween material objects and events, since the conditions ofapplicability for these predicates apparently presuppose that theyapply to entities of different categories (Moltmann 2017, Section3.1). Since Natural Language Ontology “concerns itself with theontological categories and structures a speaker accepts when using alanguage, not those a speaker accepts when engaging in some form ofphilosophical reflection”, its results may differ widely fromthe ontological categories many philosophers would reflectivelyaccept, and even from those commonly attributed to natural language(Moltmann 2017, Section 1). We have reason to engage in naturallanguage ontology, Moltmann argues, since it may give us “thebest indication of how we, implicitly, conceive of things”(2017, Section 7). A question that remains is whether there will be auniform ontology found across all natural languages, perhaps one fixedby our cognitive structure.

One might, of course, turn to cognitive science to attempt to addressthe question of whether there is a fixed system of categoriesdetermined by our cognitive structure. And indeed, discussions ofcategories also play an important role in cognitive science, where thegoal is not to discover the fundamental categories of being, butrather the means by which experiencers come to categorize their world.Here, debates have centered on how humans in fact come to group thingsinto categories – whether this involves lists of definitional(observable or hidden) features, resemblance to prototypes, prominentfeatures weighted probabilistically, etc. Debates also concern therelation between conceptual and linguistic categories, which levels ofcategory are more basic, whether there is a most basic set ofcategories, whether or to what extent categorizations are consistentacross cultural groups, and whether or not some fundamental categoriesare innate. The psychologist Susan Carey (2011) has engaged in anumber of studies on infants and primates which, she argues, suggestthat there are a number of concepts of ‘core cognition’that are innate, designed to represent certain classes of entity inthe world, and that are shared across pre-linguistic human infants,adults, and other primates. These include the concept of object (takenas a sortal concept that makes use of boundedness and spatio-temporalcontinuity in individuation), quantity, intentional agency, andcausation. For further discussion of the debates about categorizationin cognitive science see Lakoff (1987) and Rakison and Oakes(2003).

Recently, work on ontological categories has attracted interest notonly among philosophers, but also in information science and thebiomedical sciences, where ontologies are used to organize theknowledge represented in information systems (Smith 2003). In somecases, the ontologies developed are domain-specific (e.g. specific tomedical information, geographic information, etc.), but there has alsobeen a great deal of interest in developing a ‘top-level’ontology of maximally general categories applicable to all specificdomains and enabling data sharing across systems. It is such top-levelontologies that draw upon philosophical work on ontological categoriesmost directly, although categorial distinctions also play a crucialrole in domain-specific ontologies. Both sorts of philosophical workon categorization promise to have a wide variety of practicalapplications to information management that are just beginning to beexplored (see Sowa 1995, Munn & Smith (eds.) 2008)

2. Category Differences

Much recent work on categories has been influenced by skepticism aboutthe possibility of offering a system of ontological categories.Difficulties like those mentioned above have undermined the idea thata uniquely true and comprehensive system of ontological categories canbe found. The skepticism that comes from noting the proliferation ofcategory systems is compounded by general skepticism aboutmetaphysics. In some cases this has come from imputations of logicalpositivists that all metaphysical talk is nonsense. More recently, theskepticism has arisen from general doubts about the epistemology ofmetaphysics (Bennett 2009, Kriegel 2013, Thomasson 2015), as well asmore specific doubts that we can make sense of the idea that the worldhas a distinctly ‘ontological structure’, or that we coulddiscover what that structure is.

As a result, while categories have continued to play a central role inanalytic philosophy in the past century, and while some have continuedto pursue work on categories in the realist spirit, others haveshifted their focus to identifyingdifferences insemantic categories rather than drawing outsystemsofontological categories. Thus when Gilbert Ryle (1949, 1938[1971]) talks of categories, he does not speak directly of categoriesofentities, but rather of differing logical types ofconcepts, where such type differences are detectable by theabsurdities that result from substituting in terms of one sort forterms of another in sentences of certain kinds (see §2.2 below).Wilfrid Sellars, developing a strategy of Ockham’s, arguesexplicitly that we may construe category statements as disguisedmetalinguistic statements about the conceptual role of certainexpressions (and their functional counterparts in other languages).According to Sellars, “Socrates is a substance”, forexample, has the sense of “The ·Socrates· is abasic mental singular term”, and “Yellow is aquality” has the sense of “The ·yellow· is a(one-place) predicate (in mentalese)” (1970 [1974], 328) (wherethe “·___·” notation has the function ofenabling us to speak about linguistic roles without being tied to aparticular natural language). As a result, we can replicate the workdone by traditional category distinctions between, e.g., substance andquality, without committing ourselves ontologically to the existenceof qualities or other abstracta (1970 [1974], 329). On Sellars’view, the categories are “metaconceptual, second-orderfunctional classifications of the most fundamental types offirst-order conceptual roles within a norm-governed linguisticpractice or conceptual framework” (O’Shea forthcoming,section 1). An interesting upshot of this Sellarsian way of looking atthe categories is that are not fixed, but may change over time as theconceptual roles in our norm-governed practices change (O’Sheaforthcoming, section 2).

2.1 The Uses of Category Distinctions

Those who focus on articulating category distinctions rather than onlaying out complete systems of categories generally invoke categoriesnot in hopes of providing answers to such basic metaphysical questionsas ‘what exists’, but rather as a way of exposing,avoiding, or dissolving various presumed philosophical mistakes,confusions, and paradoxes.

Thus, e.g., Russell and Whitehead introduced type theory (which mightin some sense be considered a theory of categories) to avoid a certainform of paradox found in Fregean set theory (where we must considerthe putative set of all non-self membered sets, which is a member ofitself if and only if it is not a member of itself), liar’sparadoxes (“This sentence is false”, which is true if andonly if it is false), etc. On their analysis, paradoxes like thesearise from the attempt to form an illegitimate totality by trying tocollect into a single totality a collection that has members thatpresuppose the existence of the totality. To avoid such paradoxes, wemust accept that “Whatever involvesall of a collectionmust not be one of the collection” (1913 [1962], 37) and thusthat such totalities (involving all of a collection) must be of ahigher type, making, e.g., classes of sets of a higher type than aresets of individuals, and so on, leading to an infinite hierarchy oftypes. The type-mixing paradox-generating claims are rejected asill-formed and meaningless (1913 [1962]).

Most famously, Ryle (1949) introduced the idea of the category mistakeas a way of dispelling the confusions he thought to be rampant in theCartesian theory of the mind, and thus of dissolving many apparentproblems in philosophy of mind. According to Ryle, one makes acategory mistake when one mistakes the logical type or category of acertain expression (1949, 16–17). Thus, e.g., a foreigner wouldmake a category mistake if he observed the various colleges,libraries, and administrative offices of Oxford, and then asked to beshown the university. The foreigner mistakes the university foranother institution like those he has seen, when in fact it issomething of another category altogether: “the way in which allthat he has already seen is organized” (1949, 16). The categorymistake behind the Cartesian theory of mind, on Ryle’s view, isbased in representing mental concepts such as believing, knowing,aspiring, or detesting as acts or processes (and concluding they mustbe covert, unobservable acts or processes), when the concepts ofbelieving, knowing, and the like are actually dispositional (1949,33). Properly noting category distinctions may help alleviate avariety of philosophical problems and perplexities, and the idea ofthe category mistake was widely wielded (by Ryle and others) with thisaim. Ofra Magidor suggests that it is “far from clear what Ryletook the central mistake in the dualistic position to be” (2013,10). Jonah Goldwater (forthcoming), however, argues that, inTheConcept of Mind, the category mistakes Ryle identifies all havethe form of mistakenly conjoining entities that belong in twodifferent categories – implicitly assigning their conjuncts to ashared category. But on Ryle’s view (Goldwater argues) there isoften no single highest category (‘existent’) under whichwe can subsume the conjoined entities, and so we cannot sensiblyconjoin, count, or quantify over them together. This, Goldwaterargues, not only clarifies the basis for Ryle’s critique of bothCartesian and physicalist theories of mind, but also has the potentialto dissolve various current debates in metaphysics, such as argumentsagainst co-location that are based on denying (for example) that thereis a statueand a lumpboth on the pedestal. HuwPrice (2009, 330–335) argues that the category differences Ryleidentifies can be seen as underlain byfunctional differencesin the sorts of language used. The thought that category mistakes aresymptoms of underlying functional differences, as Price puts it,suggests that to evaluate claims of category mistakes, and theirrelevance to traditional ‘metaphysical’ problems, we need“first-order scientific inquiries into the underlying functionsof language in human life” (Price 2009, 335). Thomasson (2022,23–29) picks up this idea, and begins to suggest how work insystemic functional linguistics can provide a way of understandingdifferences in linguistic function that may underlie a range ofphilosophical problems.

Work on category distinctions also has other applications in assessingtraditional debates in metaphysics. Thomasson (2007) argues thatvarious mistakes and puzzlements in ontology can be traced to themistaken use of category-neutral existential and quantificationalclaims. A great many arguments in ontology rely on claims aboutwhether, in various situations, there is someobject present(or how many objects there are), where the term ‘object’must be used in a category-neutral way for the argument to go through(Thomasson 2007, 112–118). But if existential andquantificational claims must tacitly presuppose some category orcategories of entity over which we are quantifying, then sucharguments go astray. Thomasson (2007) gives independent grounds forthinking that all quantification must at least tacitly presuppose acategory or categories of entity over which we are quantifying, andargues that adopting that view provides the uniform basis fordissolving a number of problems supposed to arise with accepting anontology of ordinary objects. Jonah Goldwater (2021) argues that thearguments standardly given for being eliminativist about a certainkind of entity all rely on mistaken principles or judgments aboutontological categories, to which the right response is typically torectify these mistakes about categories, not to eliminate theentities. This analysis, Goldwater argues, provides a kind of indirectsupport for ‘permissivist’ ontologies, that would acceptthe existence of numbers, properties, holes, or other entities towhich metaphysicians have often sought to avoid commitment.

Work on category distinctions is also relevant to debates inlinguistics and philosophy of language about what, exactly, a categorymistake is, and why category mistakes are infelicitous. Analyzingcategory mistakes and why they are infelicitous (as Magidor 2019 makesclear) has further relevance for linguistic theories of syntax,semantics, pragmatics, and of metaphorical and fictional discourse.Magidor (2013, 2019) surveys past answers to the question of whatmakes a category mistake infelicitous, including: that they aresyntactically ill-formed, that they are meaningless, that they aremeaningful but lacking in truth-value, and that they are (despitebeing well formed, meaningful and having truth-value) pragmaticallyinappropriate. Magidor argues against the first three options, anddefends instead a presuppositional account of why sentences that seemto contain category mistakes are infelicitous. Roughly, on her view, asentence like ‘Two is green’ triggers the presuppositionthat two is colored – a presupposition that is difficult toaccommodate (2013, 132). Thus, on her analysis, the sentence isinfelicitous, but still has a truth-value (it is false).

2.2 The Ryle/Husserl Method of Distinguishing Categories

While those who only make use of the idea of category differences(rather than purporting to offer a category system) have no need toworry about how to provide an exhaustive list of categories, theynonetheless owe an account of the conditions under which we canlegitimately claim that two entities, concepts, or terms are ofdifferent categories, so that we know when a category mistake is (andis not) being made. Otherwise, they would face the charge ofarbitrariness orad hocery in views about which categoriesthere are or where category differences lie. Yet there is little moreagreement about the proper criteria for distinguishing categories thanthere is about what categories there are.

Ryle famously considered absurdities to be the key to detectingcategory differences. But although Ryle made the method famous, heapparently derived the idea from Husserl’s method ofdistinguishing categories of meaning (cf. Ryle 1970, 8; Simons 1995,120; Thomasson 2002, 124–8, and §1.3 above). But whileHusserl usedsyntactic nonsense as a way of detectingdifferences in categories ofmeaning (yielding differentgrammatical categories), Ryle broadened the idea, takingabsurdities more widely conceived to be symptoms ofdifferences inlogical orconceptual categories(1938 [1971], 180). Thus, e.g., the statement “She came home ina flood of tears and a sedan-chair” (Ryle 1949, 22) is perfectlywell-formed syntactically, but nonetheless Ryle classifies it as asentence that is absurd, where the absurdity is a symptom of the factthat the sentence conjoins terms of different categories.

In his earlier paper “Categories”, Ryle describes the testfor category differences as follows: “Two proposition-factorsare of different categories or types, if there are sentence-framessuch that when the expressions for those factors are imported asalternative complements to the same gap-signs, the resultant sentencesare significant in the one case and absurd in the other” (1938[1971], 181) – in other words, two expressions (or rather: whatthey signify) differ in category if there are contexts in whichsubstituting one expression for the other results in absurdity. Thistest, of course, provides no way of establishing that two expressionsare of the same category (but only that they are not), sincethere is an infinite number of sentence-frames, and one may always yetbe found that does not permit the substitution to be made withoutabsurdity. It also leaves open and merely intuitive the notion of‘absurdity’ itself; in fact, Ryle concludes his paper“Categories” with the question “But what are thetests of absurdity?” (1938 [1971], 184). Ryle’s approachwas further developed, in a more formal fashion, by Fred Sommers(1959, 1971).

J. J. C. Smart (1953) criticized Ryle’s criterion for drawingcategory distinctions on grounds that it could apparently be used toestablish a category difference between any two expressionswhatsoever. “Thus ‘the seat of the – is hard’works if ‘chair’ or ‘bench’ is put into theblank, but not if ‘table’ or ‘bed’ is. And iffurniture words do not form a category, we may well ask what do”(1953, 227). Without a test for absurdity apart from a certain kind ofintuitive unacceptability to native speakers, we seem to be leftwithout a means of declaring ‘Saturday is in bed’ to be acategory violation but ‘The seat of the bed is hard’ notto be. Bernard Harrison attempts to meet this challenge bydistinguishing the sorts of inappropriateness that result fromviolations of category facts (such as the former) from those thatresult from mere violations of facts of usage (the latter) (1965,315–16). The use of the term ‘bed’ could conceivablybe extended in ways that would make ‘The seat of the bed ishard’ acceptable (e.g., if beds came to be made with seats),whereas ‘Saturday’ could not conceivably be extended in away that would make ‘Saturday is in bed’ acceptable– any such attempted ‘extension’ would just involveusing ‘Saturday’ homonymously (e.g., as the name for a dayof the week and for a person) (1965, 316–18). For furtherdiscussion of intersubstitutability approaches to drawing categorydistinctions, see Westerhoff (2005, 40–59 and 2002,338–339). Westerhoff (2004) develops a method of distinguishingcategories based on substitutability in worldly states of affairsrather than language.

2.3 Fregean Approaches to Distinguishing Categories

Frege treats distinctions in categories as correlates of distinctionsin types of linguistic expression. The category ofobject,for example, is distinguished by reference to the linguistic categoryofproper name (Dummett 1973 [1981], 55–56; cf. Wright1983, 13 and Hale 1987, 3–4) – i.e., an object just is thecorrelate of a proper name, where proper names are held to include allsingular terms (including singular substantival phrases preceded bythe definite article). Broadly Fregean approaches have been morerecently developed and defended by Michael Dummett (1973 [1981]) andBob Hale (2010).

Hale develops and defends the Fregean idea that “the division ofnon-linguistic entities into different types or categories [is]dependent upon a prior categorization of the types of expressions bymeans of which we refer to them” (2010, 403). As he develops theidea, to be an object is “to be the referent of apossible singular term, to be a property is to be thereferent of apossible (first-level) predicate, and so on forother cases” (2010, 411). He also argues that this encourages adeflationary approach to existence questions according to which we mayargue for the existence of entities of a certain kind by simplyarguing “that there are true statements involving expressions ofthe relevant kind” (2010, 406).

Dummett (1973 [1981]) also aims to develop and precisify a broadlyFregean approach to category distinctions. Frege leaves thedistinction between so-called ‘proper names’ and otherparts of speech merely intuitively understood, but Dummett arguesthat, e.g., one could make a start at criteria for distinguishingproper names by requiring substitutability of terms while preservingthe well-formedness of a sentence (which, as we have seen in§1.3, also plays a role in Husserl’s distinction of meaningcategories), and while preserving the validity of various patterns ofinference (where the latter requirement is needed to distinguishproper names from other substantival terms such as‘someone’ and ‘nobody’) (1973 [1981], 58 ff.).(For further refinements of these criteria, see Dummett (1973 [1981],61–73) and Hale (1987, Chapter 2).)

In line with Frege’s requirement (1884 [1968], §62) thatnames must be associated with a criterion of identity, Dummett arguesthat an additional test (beyond these formal tests) is needed todistinguish genuine proper names (to which objects correspond) fromother sorts of expression: “Even though an expression passes themore formal tests we devised, it is not to be classified as a propername, or thought of as standing for an object, unless we can speak ofa criterion of identity, determined by the sense of the expression,which applies to the object for which it stands” (1973 [1981],79).

Thus once grammatical categories are distinguished, enabling us tothereby distinguish the logical categoryobject by referenceto the linguistic category ofproper name, we can go on todraw out category distinctions among objects. To avoid confusion,Dummett calls the first range of distinctions (among logicalcategories of objects, properties, relations, etc.) distinctions among‘types’ and the second range of distinctions (within thetypeobject) distinctions among ‘categories’(1973 [1981], 76).

Since, as Dummett argues (in a point further developed in Lowe 1989and Wiggins 2001), proper names and sortal terms must be associatedwith a criterion of identity that determines the conditions underwhich the term may be correctly applied again to one and the samething (1973 [1981], 73–75), we may use the associated criteriaof identity in order to distinguish categories of objects referred to.All of those names and general sortal terms (usable in forming complexnames) that share a criterion of identity are said to be terms of thesamecategory, even if the criteria of application for theassociated sortals vary (1973 [1981], 546). Thus, e.g., the sortalterms ‘horse’ and ‘cow’ (similarly, names ofhorses and cows) are terms of the same category, since they share theidentity criteria suitable for animals.

As Lowe (1989, 108–118) notes, this approach to categoriesblocks certain reductivist moves in metaphysics. For, e.g., if sortalterms such as ‘person’ and ‘organism’ areassociated with different identity conditions, then those who seek toreductively identify persons with biological organisms are involved ina category mistake.

The idea that category distinctions among objects may be drawn out interms of the identity and/or existence conditions associated withterms of each category has recently gained popularity. Though theydiffer in details, versions of the approach have been utilized notonly by Frege, Dummett and Hale but also by Lowe (2006, 6) andThomasson (2007).

This approach to drawing category distinctions among objects can avoidvarious potential problems and sources of skepticism. It is notsubject to problems like those Smart raised for Ryle’scriterion, for days of the week clearly have different identityconditions than do persons, whereas beds and chairs seem to shareidentity conditions (those suitable for artifacts). Such a method ofdrawing out categories also is not subject to the sorts of skepticismraised above for category systems. Here there is no claim to providean exhaustive list of categories, and for a principled reason:different categories may come into discussion as long as nominativeterms or concepts associated with distinct identity conditions may beinvented.

Following this method also guarantees that the categoriesdistinguished are mutually exclusive, for it is a corollary of thisposition that entities may be identified only if they are governed bythe same identity conditions (and meet those), so that it is ruled outa priori that one and the same entity could belong to two ormore distinct categories, in violation of the mutual exclusivityrequirement.

This method of distinguishing categories also provides a principledway of answering some of the central questions for theories ofcategories, including whether or not there is a singlesummumgenus, and what the relationship is between linguistic/conceptualand ontological categories. Such completely general terms as“thing” “entity” or “object”, onDummett’s view, are not genuine sortal terms, since they fail toprovide any criteria of identity. Thus clearly on this view (as onAristotle’s) there is nosummum genus under whichcategories such asartifact,animal, etc. could bearranged as species, since (lacking criteria of identity) suchcandidate catch-all terms as ‘object’,‘being’, ‘entity’ and the like are not evensortal terms and so cannot be categorial terms.

Views that, like the Rylean and Fregean approaches, distinguishcategories by way of language, are sometimes criticized as capableonly of noting differences in category of certain linguisticexpressions. For why, it might be asked, should that have anything totell us about differences in the categories ofrealthings?

Hale argues that there is no serious alternative to using types ofexpression that aim at referring to entities of different types if wehope to characterize such basic logical categories (or types) asobject and property (2010, 408). For what it is to be an object orproperty evidently cannot be conveyed merely by ostension, nor by moresubstantive criteria, without being restrictive in ways that beg thequestion against various views of what objects or properties thereare. Moreover, he argues that we can avoid making our (logical)categories unduly dependent on what language we actually have bytreating objects and properties as correlates ofpossible,not merely actual, expressions of the relevant sorts (2010, 411).

Dummett’s way of understanding categories of objects also opensthe way for a reply to this objection. For Dummett argues that,without some associated categorial concept, we cannot single outobjects (even using names or demonstratives) (1973 [1981], 571).Categorial concepts are necessary for us to single out‘things’ at all, and cannot be derived from considering‘things’ preidentified without regard to categories. (Itwould thus follow from this that Johansson’s idea that we couldarrive at categories by abstraction from considering individualthings would be wrong-headed.) On this view, then, categoriesnot onlymay butmust be distinguished primarily byway of distinguishing the identity conditions criterially associatedwith the proper use of different sortal terms and names. If we cannotrefer to, discover, or single out objects at all except by way of acertain categorial conception (providing application and identityconditions), then the categorial differences in our sortal terms ornames (marked by their differences in identity conditions) areipso facto, and automatically, category differences in thethings singled out by these terms – the possibility of a‘mistake’ here just does not arise, and the connectionbetween the category of an expression used to refer to a given entityand the category of the entity referred to is ensured.

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Aristotle, General Topics: categories |category mistakes |Husserl, Edmund |Ingarden, Roman |Kant, Immanuel |metaphysics |ontology, natural language |Russell, Bertrand |sortals

Acknowledgments

Many thanks go to Willem de Vries, Simon Evnine, Jonathan Lowe,Friederike Moltmann, Linda Palmer, David Woodruff Smith, JenniferUleman, and Achille Varzi for very helpful comments on earlier draftsof this entry. Thanks also to Amanda McMullen and an anonymous refereefor help in identifying new literature relevant to the (2013) revisedversion of this entry.