The term to be defined is the definiendum (plural: definienda). The expression providing the definition is the definiens (plural: definientia). In the definition "man is a thinking animal" is "man" the definiendum while "thinking animal" is the definiens.
"A commen premise in recent work in the philosophy of science is that the meaning of a concept is theory-dependent (Tuomela, 1973, 113-132). Although definitions are required, theoretical frames, other concepts and their relationships give added content to the concept. Not only definiens but the entire theoretical context signifies the definiendum. In order to be useful, a concept must function within a theoretical frame of reference" (Vakkari, 1996, 210).
Standard encyclopedias contain many kinds of definitions. Yagisawa (1995), for example, discuss the following kinds of definitions:
Other kinds include:
We shall not discuss all kinds of definitions here (see specific entries). The philosophy behind a given way to define a term is of course deeply rooted in epistemological assumptions. The logical positivists, for example, favored the operational definitions (not mentioned by Yagisawa, 1995). The most famous kind of definition is “definition by species and genus” constructed by Aristotle.
The rest of this article discusses definitions based on pragmatism and speech act theory.
It is well known, that definitions are not true or false, but more or less fruitful. In a way people are free to define terms as they like, but in reality their definitions might encounter severe problems. In children’s play a chair can be defined as a table and vice versa. This works as long as the children remember and obey their own decisions and do not apply their own conventions in communication with outsiders. However, when somebody defines a term in an idiosyncratic way that definition will be neglected and not contribute to understanding, communication or advancing any practice. Instead it just confuses matter and wastes the time of other people.
It is useful to know how different people apply the terms they use. Wittgenstein’s famous use theory of meaning emphasizes this aspect of defining terms: to find out how people actually use them. Historical dictionaries such as The Oxford English Dictionary provide valuable knowledge about the etymology of a word and how different authors have used it throughout the centuries. This must be supplemented by more detailed descriptions on how the word has been used in different disciplines, for example, in Library and Information Science (LIS) and related disciplines such as computer science, the cognitive sciences, and philosophy. The actual use of terms may differ from their more formal definitions. One can say that the ordinary use of a term like information may carry other meanings than formal definitions, implying that there may be conflicting theoretical views between the explicit scientific definitions and the implicit definitions of ordinary use. Because of this, we need to compare not only different formal definitions, but also to consider the meaning of a word like information as it is used in ordinary activities of, for example, information seeking, information systems, and information services.
Studies of how a term has been used cannot, however, help us to decide how we should define it. When we use language and terms, we perform some kind of act. We want to accomplish something by this acting. Different meanings of the terms we use are more or less efficient tools in helping us accomplish what we want to accomplish. In this way, according to pragmatic philosophers such as Charles Sanders Peirce (1905), the meaning of a term is not just determined by the past, but by the future. This may be a surprising statement, but it becomes more understandable if we consider e.g. the term documents. The way they have been understood and defined may reflect the printing culture and may cause troubles for a theory of electronic communication. When researchers today define documents they have to provide a definition that makes this concept fruitful for further researh and development in electronic communication. In this way the meaning and definition of a term is depending of theoretical uses, which points to the future rather than to the past.
The meaning of terms has to be considered in the framework of the theories they are supposed to serve. In philosophy of science, Chalmers (1999) has provided an important view on the meaning of scientific concepts:
” . . . observation statements must be expressed in the language of some theory. Consequently, it is argued, the statements, and the concepts figuring in them, will be as precise and informative as the theory in whose language they are formed is precise and informative. For instance, I think it will be agreed that the Newtonian concept of mass has a more precise meaning than the concept of democracy, say. It is plausible to suggest that the reason for the relative precise meaning of the former stems from the fact that the concept plays a specific, well-defined role in a precise, closely knit theory, Newtonian mechanics. By contrast, the social theories in which the concept “democracy” occurs are vague and multifarious. If this suggested close connection between precision of meaning of a term or statement and the role played by that term or statement in a theory is valid, then the need for coherently structured theories would seem to follow directly from it.” (Chalmers, 1999, 104-105).
Chalmers also considers alternative ways to defining scientific terms, by, for example, lexical or ostensive definitions. The main problem with lexical definitions is that concepts can only be defined in terms of other concepts, the meanings of which are given. If the meanings of these latter concepts are themselves established by definition, it is clear that an infinite regress will result unless the meanings of some concepts are known by other means. A dictionary is useless unless we already know the meanings of many words. Newton could not define mass or force in terms of previously available concepts. It was necessary for him to transcend the limits of the old conceptual framework by developing a new one. The main problem with ostensive definitions is that this is difficult to sustain even in the case of an elementary notion like “apple”. It is even more implausible when it comes to the definition of something like “mass” in mechanics or “electric field” in electromagnetism or “information”, “subject” or “topicality” in information science (IS). The dependence of the meaning of concepts on the structure of the theory in which they occur — and the dependence of the precision of the former on the precision and degree of coherence of the latter — is thus made plausible by noting the limitations of some of the alternative ways in which a concept might be thought to acquire meaning.
Chalmers also points out that the typical history of a concept, whether it be “chemical element”, “atom”, “the unconscious” or whatever, involves the initial emergence of the concept as a vague idea, followed by its gradual clarification as the theory in which it plays a part takes a more precise and coherent form. He argues that Galileo was in the process of making a major contribution to the building of a new mechanics that was to prove capable of supporting detailed experimentation at a later stage. It need not be surprising that — contrary to popular myth — his efforts involved thought experiments, analogies and illustrative metaphors rather than detailed experimentation. This is quite comprehensible if it is accepted that precise experimentation can only be carried out if one has a precise theory capable of yielding predictions in the form of precise observation statements.
Following Chalmers, we will propose that the scientific definition of terms is depending on what role we give them in our theories, i.e. on what kind of methodological work they have to do for us.
Definitions are connected to information science in semantic tools such as thesauri, where the scope notes may contain definitions of descriptors. It is also important in understanding basic terms in information science, such as information, documents, subjects, knowledge organization etc.
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