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In disciplines such as philosophy and knowledge representation, the type–token distinction is a distinction that separates a concept from the objects which are particular instances of the concept. For example, the particular bicycle in your garage is a token of the type of thing known as "The bicycle". Whereas, the bicycle in your garage is in a particular place at a particular time, that is not true of "the bicycle" as used in the sentence: "The bicycle has become more popular recently". In logic, the distinction, used to clarify the meaning of symbols of formal languages, is known as Pierce's type-token distinction.
Types are often understood ontologically as being concepts. They do not exist anywhere in particular because they are not physical objects. Types may have many tokens. However, types are not directly producible as tokens are. You may, for instance, show someone the bicycle in your garage, but you cannot show someone "The bicycle". Tokens always exist at a particular place and time and may be shown to exist as a concrete physical object.
It can be quite useful to distinguish between an abstract "type" of thing, and the various physical "tokens" or examples of that thing. This type-token distinction is illustrated by way of examples. If we say that two people "have the same car", we may mean that they have the same type of car (e.g. the same make and model), or the same particular token of the car (e.g. they share a single vehicle). This distinction is useful in other ways, during discussion of language. In the phrase "Grendel is Grendel is Grendel is Grendel", there are only two types of words ("Grendel" and "is") but there are seven tokens (four "Grendel" and three "is" tokens).
There is a related distinction very closely connected with the type-token distinction. This distinction is the distinction between an object, or type of object, and an occurrence of it. In this sense, an occurrence is not necessarily a token. Quine discovered this distinction. However, he only gave what he called an "artificial, but convenient and adequate definition" as "an occurrence of x in y is an initial segment of y ending in x". Quine's proposed "definition", known as The Prefix Proposal, has not received the attention it deserves, but at least one counter-proposal has been formulated.
If we consider for example the famous sentence: "A rose is a rose is a rose". We may equally correctly state that there are eight or three words in the sentence. There are, in fact, three word types in the sentence: "rose", "is" and "a". There are eight word tokens in a token copy of the line. The line itself is a type. There are not eight word types in the line. It contains (as stated) only the three word types, 'a,' 'is' and 'rose,' each of which is unique. So what do we call what there are eight of? They are occurrences of words. There are three occurrences of the word type 'a,' two of 'is' and three of 'rose'.
The need to distinguish tokens of types from occurrences of types arises, not just in linguistics, but whenever types of things have other types of things occurring in them. Reflection on the simple case of occurrences of numerals is often helpful.
The defining criteria which a typographic print has to fulfill is that of the type identity of the various letter forms which make up the printed text. In other words: each letter form which appears in the text has to be shown as a particular instance ("token") of one and the same type which contains a reverse image of the printed letter.
- Quine, Quiddities
- See the PhysicsWiki article "Occurrences of numerals"
- Stanford Encyclopedia of Philosophy, Types and Tokens
- Brekle, Herbert E.: Die Prüfeninger Weiheinschrift von 1119. Eine paläographisch-typographische Untersuchung, Scriptorium Verlag für Kultur und Wissenschaft, Regensburg 2005, ISBN 3-937527-06-0, p. 23
- Baggin J., and Fosl, P. (2003) The Philosopher's Toolkit. Blackwell: 171-73. ISBN 978-0-631-22874-5.
- Peper F., Lee J., Adachi S.,Isokawa T. (2004) Token-Based Computing on Nanometer Scales, Proceeding of the ToBaCo 2004 Workshop on Token Based Computing, Vol.1 pp. 1–18.