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Classification is a term used both about the process to classify (distinguishing and distribution kinds of "things" into different groups) and about the resulting set of classes, as well as the assignment of elements to pre-established classes. To classify in the broad meaning given above is a fundamental concept and a part of almost all kinds of activities, and it is an interdisciplinary field of study. Among the most important contributing disciplines are philosophy, biology, knowledge organization, psychology, statistics and mathematics.

Definitions

Frederick Suppe[1] distinguished two senses of classification: a broad meaning, which he called "conceptual classification" and a narrow meaning, which he called "systematic classification".

About conceptual classification Suppe (1989, 292) wrote: "Classification is intrinsic to the use of language, hence to most if not all communication. Whenever we use nominative phrases we are classifying the designated subject as being importantly similar to other entities bearing the same designation; that is, we classify them together. Similarly the use of predicative phrases classifies actions or properties as being of a particular kind. We call this conceptual classification, since it refers to the classification involved in conceptualizing our experiences and surroundings"

About systematic classification Suppe (1989, 292) wrote: "A second, narrower sense of classification is the systematic classification involved in the design and utilization of taxonomic schemes such as the biological classification of animals and plants by genus and species.

During the history of science and philosophy many definitions and theories of classification have been put forward. A chronological list of definitions appears in Birger (2017).[2]

Basic units

Hull (1998)[3] suggested "The fundamental elements of any classification are its theoretical commitments, basic units and the criteria for ordering these basic units into a classification".

The basic units in a classification system are classes (or sometimes clades or related terms, cf. Hjørland (2017, Section 2) [1].

There is a widespread opinion in knowledge organization and related fields that such classes corresponds to concepts. We can, for example, classify ‘waterfowls’ into the classes “ducks”, “geese”, and “swans”; we can also say, however, that the concept “waterfowl” is a generic broader term in relation to the concepts “ducks”, “geese”, and “swans”. This example demonstrates the close relationship between classification theory and concept theory. A main opponent of concepts as units is Barry Smith (cf. Smith 2004).[4] Arp, Smith and Spear (2015, 5ff)[5] discuss ontologies and criticize the conceptualist understanding. The book writes (7): “The code assigned to France, for example, is ISO 3166 – 2:FR and the code is assigned to France itself — to the country that is otherwise referred to as Frankreich or Ranska. It is not assigned to the concept of France (whatever that might be).” Smith's alternative to concepts as units is based on a realist orientation, when scientists make successful claims about the types of entities that exist in reality, they are referring to objectively existing entities which realist philosophers call universals or natural kinds. Smith's main argument - with which many followers of the concept theory agree - seems to be that classes cannot be determined by introspective methods, but must be based on scientific and scholarly research. Whether units are called concepts or universals, the problem is to decide when a thing (say a "blackbird") should be considered a natural class. In the case of blackbirds, for example, recent DNA analysis have reconsidered the concept (or universal) "blackbird" and found that what was formerly considered one species (with subspecies) are in reality many different species, which just have chosen similar characteristics to adopt to their ecological niches (Fjeldså 2013, 141).[6]

An important argument for considering concepts the basis of classification is that concepts are subject to change and that they changes when scientific revolutions occur. Our concepts of many birds, for example, have changed with recent development in DNA anal

About conceptual classification Suppe (1989, 292) wrote: "Classification is intrinsic to the use of language, hence to most if not all communication. Whenever we use nominative phrases we are classifying the designated subject as being importantly similar to other entities bearing the same designation; that is, we classify them together. Similarly the use of predicative phrases classifies actions or properties as being of a particular kind. We call this conceptual classification, since it refers to the classification involved in conceptualizing our experiences and surroundings"

About systematic classification Suppe (1989, 292) wrote: "A second, narrower sense of classification is the systematic classification involved in the design and utilization of taxonomic schemes such as the biological classification of animals and plants by genus and species.

During the history of science and philosophy many definitions and theories of classification have been put forward. A chronological list of definitions appears in Birger (2017).[2]

Hull (1998)[3] suggested "The fundamental elements of any classification are its theoretical commitments, basic units and the criteria for ordering these basic units into a classification".

The basic units in a classification system are classes (or sometimes clades or related terms, cf. Hjørland (2017, Section 2) [1].

There is a widespread opinion in knowledge organization and related fields that such cla

The basic units in a classification system are classes (or sometimes clades or related terms, cf. Hjørland (2017, Section 2) [1].

There is a widespread opinion in knowledge organization and related fields that such classes corresponds to concepts. We can, for example, classify ‘waterfowls’ into the classes “ducks”, “geese”, and “swans”; we can also say, however, that the concept “waterfowl” is a generic broader term in relation to the concepts “ducks”, “geese”, and “swans”. This example demonstrates the close relationship between classification theory and concept theory. A main opponent of concepts as units is Barry Smith (cf. Smith 2004).[4] Arp, Smith and Spear (2015, 5ff)[5] discuss ontologies and criticize the conceptualist understanding. The book writes (7): “The code assigned to France, for example, is ISO 3166 – 2:FR and the code is assigned to France itself — to the country that is otherwise referred to as Frankreich or Ranska. It is not assigned to the concept of France (whatever that might be).” Smith's alternative to concepts as units is based on a realist orientation, when scientists make successful claims about the types of entities that exist in reality, they are referring to objectively existing entities which realist philosophers call universals or natural kinds. Smith's main argument - with which many followers of the concept theory agree - seems to be that classes cannot be determined by introspective methods, but must be based on scientific and scholarly research. Whether units are called concepts or universals, the problem is to decide when a thing (say a "blackbird") should be considered a natural class. In the case of blackbirds, for example, recent DNA analysis have reconsidered the concept (or universal) "blackbird" and found that what was formerly considered one species (with subspecies) are in reality many different species, which just have chosen similar characteristics to adopt to their ecological niches (Fjeldså 2013, 141).[6]

An important argument for considering concepts the basis of classification is that concepts are subject to change and that they changes when scientific revolutions occur. Our concepts of many birds, for example, have changed with recent development in DNA analysis and the influence of the cladistic paradigm - and have demanded new classifications. Smith's example of France demands an explanation. First, France is not a general concept, but an individual concept. Next, the legal definition of France is determined by the conventions that France has made with other countries. It is still a concept, however, as Leclercq (1978) demonstrates with the corresponding concept Europe.[7]

Hull (1998) continued: "Two fundamentally different sorts of classification are those that reflect structural organization and those that are systematically related to historical development." What is referred to is that in biological classification the anatomical traits of organisms is one kind of classification, the classification in relation to the evolution of species is another (in the section below, we expand these two fundamental sorts of classification to four). Hull adds that in biological classification, evolution supplies the theoretical orientation.

Ereshefsky (2000)[8] presented and discussed three general philosophical schools of classification: "essentialism, cluster analysis, and historical classification. Essentialism sorts entities according to their essential natures. Cluster analysis divides entities into groups whose members share a cluster of similar traits, though none of those traits are essential. The historical approach classifies entities according, to their causal relations rather than their intrinsic qualitative features."

These three categories may, however, be considered parts of broader philosophies. Four main approaches to classification may be distinguished: (1) logical and rationalist approaches including "essentialism"; (2) empiricist approaches including cluster analysis (It is important to notice that empiricism is not the same as empirical study, but a certain ideal of doing e

These three categories may, however, be considered parts of broader philosophies. Four main approaches to classification may be distinguished: (1) logical and rationalist approaches including "essentialism"; (2) empiricist approaches including cluster analysis (It is important to notice that empiricism is not the same as empirical study, but a certain ideal of doing empirical studies. With the exception of the logical approaches they all are based on empirical studies, but are basing their studies on different philosophical principles). (3) Historical and hermeneutical approaches including Ereshefsky's "historical classification" and (4) Pragmatic, functionalist and teleological approaches (not covered by Ereshefsky). In addition there are combined approaches (e.g., the so-called evolutionary taxonomy", which mixes historical and empiricist principles).

"Empiricism alone is not enough: a healthy advance in taxonomy depends on a sound theoretical foundation" (Mayr 1968, 548)[12]

Historical and hermeneutical approaches