In disciplines such as logic, linguistics, metalogic, typography, and computer programming, the type–token distinction is a distinction that separates a descriptive concept from objects that instantiate the concept, seen as particular instances of it. For example, the sentence "the bicycle is in the garage" refers to a token of the type named "bicycle", while the sentence "the bicycle is becoming more popular" refers to the type.
This distinction in computer programming between classes and objects is similar, though in this context, "class" may refer to a set of objects (with class-level attribute or operations) rather than a description of an object in the set.
The words type, concept, property, quality, feature and attribute are all used in describing things. Some verbs fit some of these words better than others. E.g. You might say a rose bush is a plant that instantiates the type(s), or embodies the concept(s), or exhibits the properties, or possesses the qualities, features or attributes “thorny”, “flowering” and “bushy”. The term "property" is used ambiguously to mean property type (height in feet) and/or property instance (1.74). The term "concept" is probably used more often for the property type (height in feet) than the property instance.
Types like "thorny" are often understood ontologically as concepts. Types exist in descriptions of objects, but not as tangible physical objects. A type may have many tokens. However, types are not directly producible as tokens are. One can show someone a particular bicycle, but cannot show someone the type "bicycle", as in "the bicycle is popular." It is often presumed that tokens exist in space and time as concrete physical objects. But tokens of the types "thought", "tennis match", "government" and "act of kindness" don't fit this presumption.
Clarity requires us to distinguish between abstract "types" and the "tokens" or things that embody or exemplify types. If we hear that two people "have the same car", we may conclude 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). The distinction is useful in other ways, during discussion of language.
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.
Considering the 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.
- Charles Sanders Peirce's type–token distinction
- Formalism (philosophy)
- Class (philosophy)
- Type theory
- Type physicalism
- Quine, Quiddities
- See the WorldHeritage 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.
- The Stanford Encyclopedia of Philosophy: "Types and Tokens" by Linda Wetzel.