Type–token distinction
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In philosophy and knowledge representation, the type-token distinction is a distinction that separates an abstract 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 is used to clarify the meaning of symbols of formal languages.
Types are often understood ontologically as being abstract objects. 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 brand 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 "yellow is yellow is yellow is yellow", there are only two types of words ("yellow" and "is") but there are seven tokens (four "yellow" and three "is" tokens).
Occurrences
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 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."
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." However, although there are eight word tokens in a token copy of the line, there aren't any tokens at all in the line itself. 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.
See also
