Type–token distinction

The sentence "they drive the same car" is ambiguous. Do they drive the same type of car (the same model) or the same instance of a car type (a single vehicle)? Clarity requires us to distinguish words that represent abstract types from words that represent objects that embody or exemplify types. The type–token distinction separates types (abstract descriptive concepts) from tokens (objects that instantiate concepts).

For example: "bicycle" represents a type: the concept of a bicycle; whereas "my bicycle" represents a token of that type: an object that instantiates that type. In the sentence "the bicycle is becoming more popular" the word "bicycle" represents a type that is a concept; whereas in the sentence "the bicycle is in the garage" the word "bicycle" represents a token: a particular object.

(The distinction in computer programming between classes and objects is related, though in this context, "class" sometimes refers to a set of objects (with class-level attribute or operations) rather than a description of an object in the set, as "type" would.)

The words type, concept, property, quality, feature and attribute (all used in describing things) tend to be used with different verbs. E.g. Suppose a rose bush is defined as a plant that is "thorny", "flowering" and "bushy". You might say a rose bush instantiates these three types, or embodies these three concepts, or exhibits these three properties, or possesses these three qualities, features or attributes.

Property types (e.g. "height in metres" or "thorny") are often understood ontologically as concepts. Property instances (e.g. height = 1.74) are sometimes understood as measured values, and sometimes understood as sensations or observations of reality.

Some types exist as descriptions of objects, but not as tangible physical objects. One can show someone a particular bicycle, but cannot show someone, explicitly, the type "bicycle", as in "the bicycle is popular.". Such use of typologically similar yet different semantic properties appear in mental and documented models, and are often referenced in everyday conversation.

In typography, the type–token distinction is used to determine the presence of a text printed by movable type:[1]

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.

There are only 26 letters in the English alphabet and yet there are more than 26 letters in this sentence. Moreover, every time a child writes the alphabet 26 new letters have been created.

The word 'letters' was used three times in the above paragraph, each time in a different meaning. The word 'letters' is one of many words having "type–token ambiguity". This section disambiguates 'letters' by separating the three senses using terminology standard in logic today. The key distinctions were first made by the American logician-philosopher Charles Sanders Peirce in 1906 using terminology that he established.[2]

The letters that are created by writing are physical objects that can be destroyed by various means: these are letter TOKENS or letter INSCRIPTIONS. The 26 letters of the alphabet are letter TYPES or letter FORMS.

Peirce's type–token distinction, also applies to words, sentences, paragraphs, and so on: to anything in a universe of discourse of character-string theory, or concatenation theory. There is only one word type spelled el-ee-tee-tee-ee-ar,[3] namely, 'letter'; but every time that word type is written, a new word token has been created.

Some logicians consider a word type to be the class of its tokens. Other logicians counter that the word type has a permanence and constancy not found in the class of its tokens. The type remains the same while the class of its tokens is continually gaining new members and losing old members.

The word type 'letter' uses only four letter types: el, ee, tee, and ar. Nevertheless, it uses ee twice and tee twice. In standard terminology, the word type 'letter' has six letter OCCURRENCES and the letter type ee OCCURS twice in the word type 'letter'. Whenever a word type is inscribed, the number of letter tokens created equals the number of letter occurrences in the word type.

Peirce's original words are the following. "A common mode of estimating the amount of matter in a ... printed book is to count the number of words. There will ordinarily be about twenty 'thes' on a page, and, of course, they count as twenty words. In another sense of the word 'word,' however, there is but one word 'the' in the English language; and it is impossible that this word should lie visibly on a page, or be heard in any voice .... Such a ... Form, I propose to term a Type. A Single ... Object ... such as this or that word on a single line of a single page of a single copy of a book, I will venture to call a Token. .... In order that a Type may be used, it has to be embodied in a Token which shall be a sign of the Type, and thereby of the object the Type signifies." – Peirce 1906, Ogden-Richards, 1923, 280-1.

These distinctions are subtle but solid and easy to master. This section ends using the new terminology to disambiguate the first paragraph.

There are 26 letter types in the English alphabet and yet there are more than 26 letter occurrences in this sentence type. Moreover, every time a child writes the alphabet 26 new letter tokens have been created.

• Linda Wetzel. "Types and Tokens". In Zalta, Edward N. (ed.). Stanford Encyclopedia of Philosophy.