The meaning of facets in nondisciplinary classifications 

Abstract: Disciplines are felt by many to be a constraint in classification, though they are a structuring principle of most bibliographic classification schemes. A nondisciplinary approach has been explored by the Classification Research Group, and research in this direction has been resumed recently by the Integrative Level Classification project. This paper focuses on the role and the definition of facets in nondisciplinary schemes. A generalized definition of facets is suggested with reference to predicate logic, allowing for having facets of phenomena as well as facets of disciplines. The general categories under which facets are often subsumed can be related ontologically to the evolutionary sequence of integrative levels. As a facet can be semantically connected with phenomena from any other part of a general scheme, its values can belong to three types, here called extra-defined foci (either special or general), and context-defined foci. Nondisciplinary freely faceted classification is being tested by applying it to little bibliographic samples stored in a MySQL database, and developing Web search interfaces to demonstrate possible uses of the described techniques.

In memory of Douglas Foskett, whose writings 
are far greater than the sum of their words.

1: Introduction

Several authors in classification research remarked at various times that disciplines are an arbitrary constraint on classification schemes, and produce obstacles to cross-disciplinary indexing and searching. Among them are Brown (1906, 8-11), Kyle (1959),  Foskett (1961, 138-139; 1970), Austin (1969), Beghtol (1998), Williamson (1998), Szostak (2004, 221), and López-Huertas (pers. comm.). 
Defining classes as disciplines and their subdivisions is a top-down approach: first the universe of knowledge is cut into a certain number of fields, then each of them is further subdivided, and so on. An alternative bottom-up approach has been advocated by members of the Classification Research Group (CRG). In particular, Farradane (1961) claimed that classification should follow the inductive method of the sciences, starting from simple concepts (isolates) and combining them by relational operators. Foskett (1970), Austin (1969), and others explored the possibility of a new general classification scheme based on phenomena of the real world instead of disciplines: phenomena can be arranged in a sound order by the integrative level to which they belong, and classmarks can be obtained by combining the constant notations of each compounding phenomenon (Gnoli & Poli, 2004).
On the other hand, the main advancement in 20th century classification theory, facet analysis (Vickery, 1960), has usually been applied to disciplinary schemes. In them, each facet is defined as belonging to one general category, such as Objects, Parts, Properties, Materials, Actions, Operations, Agents, Space, and Time. Within each discipline, a given category, e.g. Operations, gets a more specific meaning to give a facet, e.g. Therapies in medicine or Working in technology. Therefore, the meaning of a facet depends on its disciplinary context. 
What is, then, the meaning of facets when they refer no more to discipline classes, but to phenomenon classes? The question is even more relevant, as faceted classification is now becoming popular in information architecture as an effective model to structure search interfaces and website menus. Several websites and software applications, including file management systems and email management systems, claim to adopt faceted classification, though often their interpretation of facet theory is partial or use inappropriate terminology (La Barre, 2004). In many cases, what is indexed in these sites and applications is not disciplinary knowledge but concrete objects, like wines or cars to be sold, or factual information, like that on working processes in an enterprise. In these situations, the meaning of facets cannot depend on a limited list of disciplines, but has to be understood in a more flexible sense.

2: A predicate logic model of facets

As an example, let’s suppose that a disciplinary classification uses botany as a main class, where a phenomenon classification uses plants as a corresponding main class. It is not only a matter of formal definition. Indeed, for the class botany some facets can be defined, such as Methods or History, which have no sense for the class plants: clearly  dissection, a possibile focus in the facet Methods, is a facet of botany, not of plants themselves, as plants alone do not dissect anything. The class plants will rather have facets like Organs, Growth stages, Diseases, Habitat, etc. The Web catalogue of a gardening shop could use facets of plants, but a Web directory of botanical resources could use facets of botany: some of the facets in the two sets can coincide (like Organs), while others can be different (like Methods).
To describe this dependence of facets on their basic class, a useful model is predicate logic. Predicate logic describes the elements of a language in terms of predicates and their arguments. Each predicate can provide for a few arguments:

moving [P] of object [A1] towards destination [A2] from provenance [A3] by means [A4]

so that a sentence like “I go to Vienna from Pavia by train” is translated as “moving me Vienna Pavia train”. Arguments A1, A2 etc. can be identified by their position (so that Vienna will be understood as the destination because it is the second argument) or, in case arguments are not all expressed mandatorily, by a special marker. As “argument markers”, natural languages use prepositions, like towards, or cases, like’s.
Predicate logic is used to structure various kinds of languages, like programming languages or logical artificial languages such as Loglan and Lojban (Cowan, 1997), and I suggest that it can be usefully applied to indexing languages as well. Indeed, in a faceted classification scheme, any class can be considered as a predicate, and provided with a set of potential arguments which are its facets, giving a facet formula:

plants [P]  with organ [A1]  at growth stage [A2]  affected by disease [A3]
living in habitat [A4]

In this way, virtually any phenomenon, not only disciplinary main classes, can provide for facets. Disciplines can be seen just as a special kind of phenomena (Gnoli, 2005), providing for facets like Methods or History. The function of argument markers can be carried out by facet indicators, like punctuation marks in Colon Classification or letters in FATKS. Predicate logic also allows for nested predicates, which correspond to Ranganathan’s “rounds” and “levels”.
Allowing any class to be a predicate with potential arguments agrees with the emphasis on relations and dynamics, rather than on “substance”, recently claimed in philosophical trends such as process ontology (Seibt, 2004). The word phenomenon is neutral enough to include both substancial and relational aspects. A phenomenon can be thought as defined by a set of relationships, both internal and with other phenomena, of which the most relevant and typical are expressed by its facets. This idea is close to that of system, which was also considered by the CRG as a tool for framing classification schemes: indeed, according to Bunge (1979) a system consists of a composition, an environment, and a structure. 
Though arguments of a predicate can be defined freely, experience with logical languages suggests that it is useful to state some conventional sequences for the “place structure” of predicates conveying analogous meanings, like “moving”, “going”, “traveling” etc.: “The places tend to appear in decreasing order of psychological saliency or importance. There in as implication within the place structure of klama, for example, that lo klama (the one going) will be talked about more often, and is thus more important, than lo se klama (the destination), which is in turn more important than lo xe klama (the means of transport)” (Cowan, 1997, sect. 12.16, 295). Clearly, this is equivalent to what is called citation order in bibliographic classification.
This brings us to the identification of semantical categories of the kind used in traditional faceted classification, like Parts, Materials, Operations, etc.: they can be introduced by a constant facet indicator, working as an approximation of their syntactical role. For example, all dynamical aspects, like “at growth stage...” or “by locomotion mechanism...”, can be represented by Process facets, and be introduced by a colon. The following section deals with the status of categories in a phenomenon scheme based on integrative levels: readers more interested in technical applications may want to skip it.

3: The ontological levels of general categories

The general scheme of integrative levels, as was drafted by the CRG with reference to several philosophers, ideally reflects an evolutionary sequence from more primitive and simple phenomena to more and more organized and complex ones: particles, atoms, molecules, crystals, cells, organisms, societies, institutions, cultures, etc. (Foskett, 1970).
We have seen how each facet can refer to a standard general category, like Properties or Operations. These categories are usually taken as a-priori. However, in the perspective of integrative levels, many of them can refer to the specific level where they first appear, rather than being postulated as prior to any knowledge and valid throughout the whole scheme. This idea is also present in the ontology of Nicolai Hartmann (1952).
Categories like Material, Beneficiary, Product and By-product (used e.g. in BC2) all imply some technological activity carried out with some purpose, at least in a broad sense. This can only apply from the level of complex organisms, like a bird building a nest, onwards. For phenomena at lower ontological levels, they have no sense: molecules or rocks are not products and have no beneficiary, unless one wants to introduce some metaphysical statement in classification. Even some human activities, like mental processes or language, have no material. Similarly, the categories Operation and Agent imply some voluntary activity by a living being, which can apply to human and animal behaviour but not to plants or simpler phenomena (Gnoli & Poli, 2004).
The Process (or Energy in Ranganathan’s terms) category has a more general status, as it can be connected with any dynamical phenomenon, i.e. anything showing some change in time. Electrons exchange between atoms can be described as a Process. Energy is a very universal concept; even matter is connected with energy by relativity theory. At present, it looks as one of the most general concepts in human knowledge; maybe only information, taken in its physico-mathematical sense, can be said to be prior to it.
Space and Time may look as the most universal categories, as they are kept unchanged in most classification schemes: they play as universal coordinates onto which any subject can be mapped, like in Kant’s a-priori categories. While in practice this is often effective, it can be remembered that in physics space and time are not absolute concepts; in quantum mechanics, space-time is an aspect of gravitation. Therefore, even these categories appear at a given moment – though early it is – in the cosmic evolution, and some phenomena exist, like certain forms of energy, for which they don’t apply. As noticed by Paul Davies, questions such as “what was there before the Big Bang?” are nonsense, as time itself began with the Big Bang. Therefore, using Space and Time as absolute categories can be problematic in classifying high energy physics and cosmology.
All the categories examined until now refer to more or less general aspects of the physical world. We are left with Entity, Kind, Part and Property (or Personality and Matter/property in Ranganathan’s terms). Here we really seem to enter a universal realm, that of the logic by which any knowledge is organized. 
Property (also called Attribute) is a very basic category which is difficult to do without in any knowledge organization language. In mathematical terms, properties are operations having as a result “true” or “false”, so they refer to the basic concepts of relation and reality. Entities and their Kinds are dealt by set theory, which is also fundamental in mathematics. Finally, entities and relations look as two basic notions which cannot be avoided in any language.

4: Free classification by integrative levels

Despite of being invoked and studied, the phenomenon approach has not yet given birth to any complete bibliographic classification scheme. This also seems to be due to reasons internal to the CRG, where the disciplinary approach eventually prevailed and produced BC2. The ISKO Italian chapter has been applying nondisciplinary classification within the Integrative Level Classification (ILC) research project. The project aims at experimenting with phenomenon classification in several small bibliographical samples in various domains, each consisting of some hundreds records, and to exploit its structure and notation by search interfaces (ISKO Italia, 2004; Hong, 2005).
ILC classmarks are built following the method already drafted by the CRG: notations for the phenomena occurring in a subject are combined, by simply listing them in inverted order of integrative levels. Hence “Italian vineyards” can be analyzed into cultivation : grapes : Italy and expressed by the corresponding notation S Mpfdg Kl^ei. Such technique, already used by Brisch (Kyle, 1956), was referred to as free classification (Gardin, 1965; Gnoli & Hong, submitted). The first experiments have shown that free classification can work quite well for broad domains, like the whole culture and environment of a rural area (Gnoli & Merli, 2005). 
However, when documents are more specialized and their subjects become more complex, simple juxtaposition of all the component phenomena can yield combinations both cumbersome and ambiguous. To take a case really encountered in the tests, the expression villages : cultivation : words : grapes : Oltrepò pavese, where phenomena are listed freely in inverted order of integrative levels, is not a satisfying synthesis of a book entitled “the dialect of the village of Portalbera, and wine terminology in Oltrepò pavese”. Indeed, when several component phenomena occur, the relations holding between them (cultivation of grapes, words referring to cultivation) and their order should be expressed more specifically, in order to yield the sense of the combination. These functions are usually carried out by facets and their place structure: therefore, for detailed indexing in a specific domain, facets are needed. 

5: The place of definition of facets

On the basis of the predicate logic approach, we can mean facets as the relations typical of a phenomenon with other phenomena: plants typically have organs (roots, stem, leaves, etc.), are in some growth stage (seed, bud, adult, etc.), can be affected by some disease (smut, peronospora, oidium, etc.), live in some habitat (desert, meadow, forest, etc.); cars typically have some bodywork (saloon, station wagon, coupé, etc.), use some fuel (petrol, diesel oil, methane, etc.), are made by some firm, etc. These facets can be expressed by categories (organs are Parts of plants, firms are Agents of cars).
In Austin’s draft of an integrative level classification, relations between  phenomena could be combined by using operators, which expressed more precisely the syntactical relations holding between them. Austin speaks of a freely faceted classification scheme (though it can be discussed whether his operators resemble more to classical facets or to phase relationships); later, his draft evolved into the PRECIS subject heading system. He also argued that this kind of technique would be suited for machine processing of subjects but not for shelving, as it would produce very complex and long notations (Austin, 1976; 1979). However, I suggest that notation can be shortened by using extra-defined foci, in the way described below.
Let’s take the integrative level of cultivation (i.e. the phenomenon consisting of all the agricultural activities performed by humans and their products), S. One obvious facet of it, S6, is the cultivated species (the ILC test schemes use digits as facet indicators). Now, what is cultivated is always a plant – it cannot be a lake or a skyscraper: therefore, notation for the foci of S6 can be borrowed from the subclasses of plants Mp, like in the “divide like” instructions of traditional classification manuals, instead of being enumerated again in the schedule. As the broad part Mp of notation is implied in the formal definition of facet S6 (written in square brackets in the test schedules, and stored in a special field in the database), it is omitted, and only notation for its subclass “grapes” is expressed:

Mp plants
...
Mpfdg grapes
...
S cultivation
S6 [Mp] cultivation of species...
S6fdg cultivation of grapes
	
The use of extra-defined foci throughout a classification scheme should produce a notation reasonably short, and at the same time expressive, therefore exploitable by a machine information retrieval system in order to produce relevant results and to display them in helpful sequences.
Extra-defined foci will usually be special, that is they will be borrowed from a special class, like plants in the example above. Sometimes, however, they can be general, that is they will be borrowed from any other class of the general scheme; in the latter case, of course, their notation must be copied entirely. Examples of general extra-defined foci are perceived objects as a facet of perception, meanings as a facet of language, subjects as a facet of documents; indeed, any phenomenon in the known universe can in principle be perceived, or spoken about, or dealt with:

T artefacts

Ne perception
Ne7 [] perception of object...
Ne7mp perception of plants
Ne7t perception of artefacts

Finally, some other foci need to be defined only within the facet itself, instead of being borrowed from other classes. This happens e.g. in music with facet “played by instrument”: indeed, its foci are instruments, like guitar, fiddle, bagpipe, etc., which only exist as such in the context of music, and not as independent phenomena at different integrative levels; in Farradane’s terms, their “place of unique definition” is the facet itself. Another example are organs of animals: there is no arm or liver other than as a facet of some animal. These can be called context-defined foci:

Xi music
Xi6 [:] music played by instrument...
Xi6k music played by stringed instruments
Xi6n music played by wind instruments

To summarize it, a facet can have

[A-Z]  special extra-defined foci
[]    general extra-defined foci
[:]   context-defined foci

In disciplinary classifications, facets apply to all the subclasses included in the hierarchical tree of a discipline. In phenomenon classification, in a similar way, facets can apply to any subclass of the phenomenon for which they are defined: if Mq6 means “organs of animals”, and Mqvo are “birds”, then Mqvo6 will automatically mean “organs of birds”. However, in some cases conflicts may occur:

X arts
X5 arts produced by technique...

Xi music
Xi5 music with rhythm...

Xiv compositions
Xiv5 ??

Does Xiv5 mean “compositions produced by technique...”, or “compositions with rhythm...”? A reasonable solution seems to be that the latter be valid, on the basis of a principle of cascading facets: that is, the definition of a facet at a more specific class prevails over that of a more generic class, like it happens in cascading style sheets for websites.

6: Ongoing work

The test bibliographical samples are currently under development in the framework of the ILC research project. The bibliography on local culture and another on faceted classification literature itself allow us to perform test searches through Web interfaces (ISKO Italia, 2004). The former bibliography is indexed mainly by free classification, with some experiments of facets, while the latter is indexed mainly by freely faceted classification. 
Bibliographic references are stored in a MySQL database, and the Web search interfaces are developed in PHP language. More PHP code is currently being written in order to manage facets and foci of the three kinds described above, and to exploit them consistently in processing user queries (Gnoli & Hong, submitted). Another planned tool is a Web assistant for easy construction of classmarks through the selection and combination of terms from the schedule database: this could be useful to show users in a practical way how the whole system work, without forcing them to dive into theoretical explanations. 
Each special domain to be indexed can be connected to the rough general scheme of integrative levels through the definition of a preferred class. Facing practical classification problems in fields of knowledge at different integrative levels will help us to check and to tune the solutions suggested by the general model.
As can be seen, the ILC project involves several aspects: application of the theory of integrative levels to the arrangement of classes; syntactical issues in the definition of facets; building a notation suitable to be exploited in a digital environment; developing usable interfaces; etc. Apart from specific problems and details, the whole experimentation is expected to show the possibility of nondisciplinary classification, at one time making full use of facet analysis, and freeing classifiers and users from the limitations of the disciplinary approach.

Acknowledgements

Many thanks go to Rick Szostak and Lorena Zuccolo for their helpful suggestions to improve previous drafts. This paper builds on experience made with Hong Mei, Gabriele Merli and the other people in the ILC project, and on discussions with And Rosta about the syntax of logical languages.

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