The Epistemological Lifeboat

Epistemology and Philosophy of Science for Information Scientists

Birger Hjørland & Jeppe Nicolaisen (eds.)

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Scientific classification and taxonomy

All sciences classify their objects. Astronomy classify celestial bodies such as planets, stars and galaxies, botany classify plants, chemistry the chemicals, medicine classify the illnesses, psychology classify mental processes, library and information science classify documents and systems of knowledge organization, religious studies classify the religions, etc. etc. 

Such classifications are not performed just in order to create an aesthetic effect or something like this. Classifications are constructed in order to work efficiently. Illnesses and drugs are, for example, classified in order to help patients recover from their illness. Classification is not something extra put on the top of scientific work, but is deeply integrated in scientific work. If a new group of chemical substances are demonstrated to help cure a certain disease, they are classified as drug (e.g. as antidepressives, tranquillizers or antiinflammatory drugs). Although not all examples are as clear as such medical examples, classifications are fundamentally pragmatic constructions why the pragmatic understanding of classifications is extremely important. 

There is a close connection between the development of scientific concepts and classifications. When astronomy recognizes the different nature of stars and planets, for example, they reflected this in both their concepts and their classifications of celestial bodies. 

The periodical system of chemical elements stands out as model in scientific classification. It is a very informative and authoritative structure with immense practical importance. Biological classification (often termed taxonomy) is an area in which the importance of classification is also broadly accepted. 

“In the behavioral and social sciences, hundreds of classifications are published every year. Noteworthy examples are Bloom's taxonomy of educational objectives (Krathwohl et al. 1964), as well as the DSM (Diagnostic and Statistical Manual of Mental Disorders) and ICD (International Classification of Diseases) classification systems used in psychology and psychiatry. None of these systems have been formally derived, however. Instead, they were generated based on `experience.' The resulting classes are so heterogeneous that they acknowledge many exceptions. Also, a phenomenon called `comorbidity' shows that these classification systems are not optimal yet. It refers to the simultaneous existence of two or more disturbances in the same patient. If comorbidity is the rule rather than the exception, then the classification system loses plausibility and practicability.” (Feger, 2001, p. 1968) 

What research methods are being used to construe scientific classifications? The answer is, that there are many. One family of methods is statistical methods such as cluster analysis and factor analysis. These are directly “methods of classification”. Often, however, classifications are arrived at using other kinds of methods and often more indirect methods. Frank C. Keil illuminates this:

“The history of all natural sciences documents the discovery that certain entities that share immediate properties nonetheless belong to different kinds. Biology offers a great many examples, such as the discoveries that dolphins and whales are not fish but mammals, that the bat is not a kind of bird, that the glass “snake” is in fact a kind of lizard with only vestigial limbs beneath its skin. In the plant kingdom it has been found, for example, that some “vegetables” are really fruits and that some “leaves” are not really leaves. From the realm of minerals and elements have come the discoveries, among others, that mercury is a metal and that water is a compound- 

In almost all these cases the discoveries follow a similar course. Certain entities are initially classified as members of a kind because they share many salient properties with other bona fida members of that kind and because their membership is in accordance with current theories. This classification may be accepted for centuries until some new insight leads to a realization that the entities share other, more fundamentally important properties with a different kind not with their apparent kind.

Sometimes it is discovered that although the fundamental properties of the entities are not those of their apparent kind, they do not seem to be those of any other familiar kind either. In such cases a new theoretical structure must develop that provides a meaningful system of classification. 

There are many profound questions about when a discovery will have a major impact on a scheme of classification, but certainly a major factor is whether that discovery is made in the context of a coherent causal theory in which the discovered properties are not only meaningful but central” (Keil, 1989, p. 159).

The choice of scientific methods is related to epistemological views. In Biology, for example, there are three major schools connected to classification: “There are two popular theories of taxonomy based on these evolutionary principles: evolutionary taxonomy and cladism (or phylogenetic systematics) and one based on statistical similarities between groups (phenetics).

Phenetics is a classification based on the statistical similarities between organisms. All characters are given an equal weight and by measuring large number of characters, it was hoped that a stable classification based on overal similarities between organisms would be reached. This kind of taxonomy has received a great interest with the development of computers were later largely abandonned because phenetic classifications were arbitrary and unstable. However, as molecular techniques became popular and more refined, phenetics enjoyed a resurgence. The sequence of amino-acids in any protein, or the sequence of nucleic acids in the DNA provides a large numbers of equally weighted characters suitable for phenetic analysis. A similarity between organims could be calculated on the bases of the changes or non changes in its proteins or DNA structure.” (Anonymous, 2003). Phenetics is thus a school that is closer related to classical empiricism compared to the other schools of biological classification. Ereshefsky (2000) is a major treatise on the philosophy of biological classification recommending an historical approach.

Another philosophical problem is important: 
“Systematists have rediscovered a problem long familiar to philosophers. How can one know that a particular chunk of metal is gold unless one knows what gold is, and how can one know what gold is without inspecting some samples of gold? But if one does not know what gold is, one cannot decide what to inspect….” (Hull, 998). An answer based on “pragmatic realism” to Hull’s problem may be that different methods may be used until we arrive at a theory that satisfies our demands and meets reasonable consensus among researchers. We define our concepts tentatively and revise our theories and conceptual systems when needed. As criteria we use both the coherence of our theories, our observations, and – in the end- pragmatic criteria. 


Adams, W. Y. (2001). Classification and typology. (Archaeological systematics). IN: International Encyclopedia of the Social and Behavioral Sciences. (Vol. 3, pp. 1962-1966). 

Anonymous [2003]. Classification, taxonomy and phylogeny of animals. 

Bailey, K. D. (1994). Typologies and taxonomies: An introduction to classification techniques. London: Sage. 

Bryant, R. (2001). Discovery and Decision: Exploring the Metaphysics and Epistemology of Scientific Classification. Madison, NJ : Fairleigh Dickinson University Press.

Dean, J. (1979) Controversy over classification: A case study from the history of botany. In: Barnes B, Shapin S (eds.) Natural Order: Historical Studies of Scientific Culture. Sage, Beverly Hills, CA, pp. 211-30.

Ereshefsky, M. (2000). The Poverty of the Linnaean Hierarchy : A Philosophical Study of Biological Taxonomy. Cambridge: Cambridge University Press.

Feger, H. (2001). Classification: Conceptions in the social sciences. IN: International Encyclopedia of the Social and Behavioral Sciences. (Vol. 3, pp. 1966-1973)

Hjørland, B. (1998). The Classification of Psychology: A Case Study in the Classification of a Knowledge Field. Knowledge Organization, 24(4), 162-201.

Hull, D.L. (1998). Taxonomy. IN: Routledge Encyclopedia of Philosophy, Version 1.0, London: Routledge.

Keil, F. C. (1989). Concepts, kinds, and cognitive development. Cambridge, Massachusetts: The MIT Press.

Krathwohl, D. R.; Bloom, B. S. & Masia, B. B. (1964) Taxonomy of Educational Objectives. Longman, London.

Krauth, J. (1981). Techniques of classification in psychology I: factor analysis, facet analyses, multidimensional scaling, latent structure analysis. International Classification, 8(3), 126-132.

Krauth, J. (1982). Techniques of classification in psychology 2: cluster analysis, typal analysis, configural frequency analysis, discriminant analysis, regression analysis. International Classification, 9(1), 1-10.

Kuiken, Don; T. Cameron Wild & Don Schopflocher (1992): Positivist Conceptions of Induction and the Rejection of Classificatory Methods in Psychological Research. (Pp. 47-56 In: Tolman, C. W. (ed.): Positivism in Psychology. Historical and Contemporary Problems. New York & Berlin: Springer-Verlag).

Schuh, R. T. (1999). Biological Systematics: Principles and Applications. Cornell University Press.

Szostak, R. (2004). Classifying science, Phenomena, data, theory, method, practice. Berlin: Springer.


See also: Classification of the Sciences


Entry added: June 3, 2005
Last update: November 28, 2005