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A system of categories is a complete list of highest kinds or genera. Traditionally, following Aristotle, these have been thought of as highest genera of entities (in the widest sense of the term), so that a system of categories undertaken in this realist spirit would ideally provide an inventory of everything there is, thus answering the most basic of metaphysical questions: “What is there?” Skepticism about the possibilities for discerning the different categories of ‘reality itself’ has led others to approach category systems not with the aim of cataloging the highest kinds in the world itself, but rather with the aim of elucidating the categories of our conceptual system.
Roman Ingarden (1960/1964, 22ff) took Husserl's multi-dimensional ontology one step further. Like Husserl, he distinguished formal categories from material categories, but he also distinguished categories in a third dimension: existential categories (those describing an entity’s mode of being). The highest existential 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 be uniquely locatable in a single category of each dimension, the three sorts of ontology are mutually orthogonal, providing different most abstract 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 purely intentional.
1.4 Contemporary Category Systems
By the mid twentieth century, systems of ontological categories had fallen somewhat out of fashion (for reasons I will discuss in §1.5 below), with most discussion of categories shifting to merely articulating category differences rather than outlining a comprehensive system of categories. Nonetheless, in recent years there have been several notable attempts to offer new systems of categories in either the realist or descriptivist spirit, although little agreement exists about what the categories are or how one could decide among competing systems.
Ingvar Johansson (1989) and Roderick Chisholm (1996) both take a neo-Aristotelian realist approach to categories, attempting to lay out a complete system of the categories, where this is understood as providing a list of categories of real entities in the world. Ingvar Johansson explicitly insists that his interest is in the world: “This book is a book about the world. I am concerned with ontology, not merely with language” (1989, 1), and attempts to offer “a realist theory of categories regarded as real aspects of being” (1989, 2). His list (1989, 20) includes nine main categories (some of which subdivide further):
Unlike Aristotle, Johansson makes no explicit use of language in discerning ontological categories, instead appealing to the method of successive abstraction (Johansson 1989, 1–2). Thus, e.g., we arrive at the category ‘quality’ by moving up in abstraction from a particular shade of dark red, to red, color, and finally quality. Similarly (to use an example of Sellars') one might try to arrive at the category of ‘substance’ by considering an individual entity, say, Fido, and moving by successive abstraction from “Fido is a dachshund” through “Fido is a dog” and “Fido is an animal”, ultimately reaching “Fido is a substance” (1970/1974, 321). Like Aristotle's categories, Johansson's categories top out with a number of distinctions without a highest single category subsuming them all.
Like Aristotle and Johansson, Roderick Chisholm presents his work on categories as being “about the ultimate categories of reality” (1996, 3). Unlike them, however, Chisholm (1996, 3) lays out categories in the form of a porphyrian tree starting from a single most general category comprising everything, but divided into successively narrower genera at lower levels of branching. (For an interesting discussion of whether such general terms as ‘entity’ or ‘thing’ could be seen as naming a highest category, see Thompson 1957, cf. §2.3 below). Chisholm's system of categories thus reads:
Other contemporary authors have approached the issue of categories in a purely descriptive spirit. Reinhardt Grossman, for example, distinguishes eight highest categories (1983, xvi):
But although Grossman characterizes his book as an attempt to “bring Aristotle's Categories up-to-date” (1983, xv), he is explicit in denying that he is making any claims about whether or not there are things belonging to any of the eight categories he distinguishes, taking this as beyond the scope of ontology (1983, 10–12).
Joshua Hoffman and Gary Rosenkrantz (1994) lay out a tree-form system of categories, with entity the summum genus, subdivided into abstract and concrete (rather than Chisholm's contingent and necessary), each of which is further subdivided:
They, too, explicitly offer their system of categories in the spirit of categorial description, as offering an analysis of the various possible categories of being, rather than making any claims about which of these categories is non-empty (1994, 7–8).
E. J. Lowe takes categories to be categories of “what kinds of things can exist and coexist” (2006, 5). Such categories, he argues, are to be individuated according to the existence and/or identity conditions of their members; fundamental categories are those regarding which the existence and identity conditions for their members cannot be exhaustively specified in terms of ontological dependence relations they bear to entities of other categories (2006, 8). Accordingly, he argues that there are four fundamental ontological categories: objects (individual substances such as Fido), modes (property or relation instances such as Fido's four-leggedness), kinds (substantial universals such as the kind dog), and attributes (property or relation universals, such as being four-legged). But although he argues that there are exactly four fundamental categories, Lowe nonetheless takes a hierarchical approach to arranging categories. The four fundamental categories appear at the third level of his hierarchical chart; the categories that appear at the higher levels (particulars and universals at the second level; entity at the top) are “mere abstractions and do no serious ontological work on their own account” (2006, 39). His fuller chart of categories appears as follows:
1.5 Skepticism about Category Systems
Both realist and descriptivist category systems, at least as traditionally presented, seem to presuppose that there is a unique true answer to the question of what categories of entity there are—indeed the discovery of this answer is the goal of most such inquiries into ontological categories. Grossman, for example, argues that a list of categories must be complete, contain everything, with everything in its proper place (1983, 4). Johansson similarly takes his project as to “develop a coherent system of all the most abstract categories needed to give a true description of the world” (1989, 1). Arguments about which of the many systems of categories offered is correct likewise seem to presuppose that there is a uniquely correct list of categories.
But actual category systems offered vary so much that even a short survey of past category systems like that above can undermine the belief that such a unique, true and complete system of categories may be found. Given such a diversity of answers to the question of what the ontological categories are, by what criteria could we possibly choose 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 descriptivist approach to providing a system of categories, if that system is supposed to be comprehensive, it clearly must meet at least the standard of being exhaustive—providing a category for everything there is (on the realist approach) or might be (on the descriptive approach). Nonetheless, one may, as Hoffman and Rosenkrantz (1994, 140) do, present a system of some fundamental categories without taking it to be exhaustive.
Another minimal criterion of adequacy is generally taken to be that the highest categories (or, for tree systems, the categories at each level of branching) be mutually exclusive, ensuring that whatever there is (or might be) finds its place in exactly one highest category, or one category at each level (Chisholm 1989, 162). (This still allows for nested categories, so that something may belong to both a more specific category like substance and a more general category like individual.)
But these criteria are not enough to provide the needed reassurance. First, we lack assurance that most proposed category systems meet even these minimal conditions. As mentioned above, Aristotle drew out his categories largely by considering the types of question that could be asked (and the types of answer appropriate to them). It is difficult to know, however, how one can be assured that all kinds of questions have been surveyed, and so difficult to know that an exhaustive list of categories has been offered—a point Aristotle does not attempt to demonstrate (Ackrill 1963, 80–81). Indeed, the fact that Aristotle provides different lists of categories in different places suggests that he did not consider his list final and exhaustive. Similarly, Kant’s system of categories can be thought to be exhaustive only as long as the list of forms of judgment from 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 successive abstraction. But it is not clear how following such a method could ensure either that the categories thereby distinguished are exhaustive (how do we know we have considered something of each highest kind if we do not yet know what the highest kinds are?) or even mutually exclusive.
Secondly, even if we can verify that the standards of mutual exclusivity and exhaustiveness are met, these conditions alone are far too weak to uniquely pick out a system of categories. Provided one accepts the law of the excluded middle, an endless supply of mutually exclusive and exhaustive classifications can be generated at will: we can divide things into the spatio-temporally located and the not-spatio-temporally-located, the intentional and the non-intentional, the extended and the non-extended, to name but a few of the more relevant ways in which things could be divided. Indeed one of the sources of puzzlement about categories comes from the fact that philosophers have selected so many different sorts of divide as the fundamental category difference—for Descartes, the extended and the thinking (unextended), for Chisholm the contingent and the necessary, for Hoffman and Rosenkrantz the concrete and the abstract, and so on. Thus another reason for skepticism about the existence of a unique set of categories comes from the fact that categories are supposed to be the most abstract genera under which things (may) fall. But from any given entity, abstraction may apparently be done in a variety of ways—even if we are careful to do so in ways that ensure mutual exclusivity and exhaustiveness.
Doubts about possibilities for discovering the one true category system have led many to eschew talk of category systems altogether, and others to adopt some kind of relativism about category systems that ceases to take systems of categories seriously as candidate lists of the single set of highest genera under which anything falls (or could fall). Jan Westerhoff (2005), for example, argues that there is no unique, absolute set of ontological categories. On his view categories in metaphysics turn out to be analogous to axioms in mathematical theories; in each case, there may be more than one way to systematize our knowledge from a relatively simple basis. The result is a kind of relativity about systems of categories: “which set of ontological categories we choose is primarily a matter of convenience, in the same way as specific axiomatizations of propositional logic or Newtonian mechanics are more convenient to use than others” (2005, 218). As a result, Westerhoff argues, we must reassess the importance of ontological categories in metaphysics--these should not be considered “the most fundamental parts of the world, but... the most fundamental parts of our systematization of the world” (2005, 135).
Others have taken the variety of category systems explicitly offered or presupposed by philosophers as mere evidence of the particular presuppositions 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 categorial frameworks is designed to make explicit how a thinker’s framework categorizes objects, making use of certain individuative principles, and making clear his reasons for holding that framework (1970, 10). R. G. Collingwood, in similar vein, treats the task of metaphysics generally as merely uncovering the “presuppositions underlying ordinary science” (1940/1998).
The specific worries about (1) guaranteeing the mutual exclusiveness and joint exhaustiveness of the categories, and (2) whether or not any single 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 the same level) are defined in ways that guarantee mutual exclusiveness and exhaustiveness. Thus, e.g., Thomasson (1999, chapter 8) distinguishes categories in terms of what relations of dependence a purported entity has or lacks on mental states (and a second dimension distinguished in terms of what relations of dependence a purported entity has or lacks on spatio-temporally located objects), so that the law of the excluded middle alone ensures mutual exclusiveness and exhaustiveness of the categories distinguished. (Dummett’s method of distinguishing categories provides another route for guaranteeing mutual exclusivity—see §2.3 below).
Multi-dimensional systems (Husserl 1913/1962, §10; Ingarden 1960/1964, Chapter 2; Thomasson 1999, Chapter 8; Smith forthcoming, Chapter 8) address the second worry to some extent by acknowledging that the different dimensions of categorization are possible, and that no one-dimensional list can purport to completeness. In principle, multi-dimensionalists may even accept that there is no fixed number or limit 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 chosen respect.
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 is uniquely acceptable), we should not abandon attempts at systems of categorization prematurely. Even if we do not think of a category system as providing a realist inventory of all that exists, a system of categories laid out in the descriptivist spirit provides a framework within which existence questions can be answered in a systematic and wholesale way, by enumerating categories so that we can then undertake further investigations into whether or not there really is anything of each kind. Working from within a categorial framework can help ensure that whatever ontology we provide is principled and unified, avoiding ad hoc and piecemeal decisions. The descriptivist's categories also provide a tool that may be used elsewhere in ontology, e.g., in helping to ensure that comparisons of parsimony are legitimately made (by examining which categories of entity are accepted and which denied by various theories), and in checking that potential solutions to metaphysical problems are not overlooked by tacit use of a category system that is not exhaustive (Thomasson 1999, Chapters 8 and 9). Indeed assumptions about categorizations play such a strong role in philosophical discussions (e.g., discussions of the Cartesian theory of mind, Platonist theories of mathematics, etc.), that doing the work on categories necessary to make these categorial assumptions explicit and open them for examination must remain a highly useful exercise regardless of doubts about the prospects for discovering a uniquely correct system of categories.
1.6 Categories in Other Disciplines
Recently, work on ontological categories has attracted interest not only among philosophers, but also in information science and the biomedical sciences, where ontologies are used to organize the knowledge represented in information systems (Smith 2003). In some cases, the ontologies developed are domain-specific (e.g. specific to medical information, geographic information, etc.), but there has also been a great deal of interest in developing a ‘top-level’ ontology of maximally general categories applicable to all specific domains; it is such top-level ontologies that draw upon philosophical work on ontological categories most directly, although categorial distinctions also play a crucial role in domain-specific ontologies.
Discussion of categories also plays an important role in cognitive science, where the goal is not to discover the fundamental categories of being, but rather the means by which experiencers come to categorize their world. Here, debates have centered on how humans in fact come to group things into categories--whether this involves lists of definitional (observable or hidden) features, resemblance to prototypes, prominent features weighted probabalistically, etc. Debates also concern the relation between conceptual and linguistic categories, which levels of category are more basic, and whether or to what extent categorizations are consistent across cultural groups. For further discussion of the debates about categorization in cognitive science see Lakoff (1987) and Rakison and Oakes (2003). Whether information from cognitive science about our formation of conceptual and linguistic categories has any import for our understanding of what ontological categories there are is itself, of course, a contentious issue (to which I will return briefly in section 2.3 below).
2. Category Differences
Recent work on categories has been strongly directed by reaction to skepticism about the possibility of offering a system of ontological categories. On the one hand, difficulties like those mentioned above have undermined the idea that a uniquely true and comprehensive system of categories could be developed. As a result, while categories have continued to play a central role in analytic philosophy in the past century, the focus has shifted to articulating particular category differences, without attempting to provide an exhaustive inventory of metaphysical categories or even presupposing that such a list is possible.
On the other hand, an even more influential source of skepticism about the possibility of laying out a system of ontological categories in the realist spirit derives from the general positivist rejection of metaphysics: if all metaphysical talk is nonsense, then talk about what sorts of thing there really are in the world is just part of the general rubbish constituting metaphysics, and debates about, e.g., whether or not substance is a category of being, whether there are ten or twelve or twenty-seven top level categories, whether or not there is a single highest category, etc. are all meaningless. Moreover, even for philosophers who regained comfort with certain forms of metaphysical talk, naturalistic concerns about ontological commitment to such abstract entities as qualities, relations, etc., have made many shy away from providing a realist list of categories that would include any such things (Sellars 1970/1974, 322–3).
In response to such positivist and naturalistic qualms, talk about categories over the past century has also tended to shift to identifying differences in semantic categories rather than drawing out systems of ontological categories. Thus when Gilbert Ryle (1949, 1938/1971) talks of categories, he does not speak directly of categories of entities, but rather of differing logical types of concepts, where such type differences are detectable by the absurdities that result from substituting in terms of one sort for terms of another in sentences of certain kinds (see §2.2 below). Wilfrid Sellars, developing a strategy of Ockham’s, argues explicitly that we may construe category statements as disguised metalinguistic statements about the role of certain expressions (and their functional counterparts in other languages). According to Sellars, “Socrates is a substance”, for example, has the sense of “The ·Socrates· is a basic mental singular term”, and “Yellow is a quality” has the sense of “The ·yellow· is a (one-place) predicate (in mentalese)” (1970/1974, 328) (where the “·___·” notation has the function of enabling us to speak about linguistic roles without being tied to a particular natural language). As a result we can replicate the work done by traditional category distinctions between, e.g., substance and quality, without committing ourselves ontologically to the existence of qualities or other abstracta (1970/1974, 329).