To give an overview it is necessary to say that the notion of recursion have been attributed from linguists to human linguistic faculty in order to solve the problem of the Poverty of the Stimulus. The Problem of the Poverty of the Stimulus is the fact that children, since a very young age, can generate sentences that they have never heard before and that, therefore, do not come from their external input. Considering that, it has been assumed that knwoledgde of language is tacit and partially innate/internal therefore we talk about ‘I(internal/innate)-language’. This I language follows the principle of recursion because it uses rules which apply recursively in order to generate an array of infinite structures which convey an infinite array of meanings. Therefore, we explain the ‘recursivity’ of language trough rules which apply recursively allowing children to generate sentences recursively.
The notion of Recursion is so important to the study of language because it explains the human competence of generating (i) infinite sentences i.e. we can always add modifiers to constituents to make the sentence longer (ii) an infinite number of different sentences embedded in another sentence.
In this essay I am going to focus in particular on both (i) and (ii).
A rule applies recursively when the output of the rule contains the input of the rule.
(i) For example, in X-Bar Syntax there is a recursive rule that allows X-Bar to duplicate itself.
In this case the X is substituted with N -in case of noun phrase- and with Adj –in case of Adjective phrase. Every A bar is attached to an N bar which represents the name to which the adjective refers. It seems therefore clear that the rule applies recursively since N-bar can duplicate itself and therefore the output of the rule contains the input.
S(entence) = NP + VP
V(erb) P(hrase) = V + (CP)/(NP)
C(omplement) P(hrase) = C + S
(1) [S [NP David] [VP [V believed] [NP the rumour]]]
(2a) [S [NP The man] [VP [V thought] [CP that David believed the rumour]]]
(2b) [S [NP The man] [VP [V thought] [CP [C that][S David believed the rumour S] CP]]]
(2c) [S [NP The man] [VP [V thought] [CP [C that] [S [NP David] [VP [V believed] [NP the rumour]]]]]]
This example shows that producing a sentence we produce a verb phrase which can be composed by a complement phrase i.e. a phrase which complements either a verb or a noun. In turn the complement phrase is composed by a complementizer and a sentence. Therefore, in the end, a sentence can generate another sentence. The output contains the input, which means that the rules apply recursively.
If we go further we can also demonstrate that the sentence can continue, generating another sentence, which generates another sentence, which generates another sentence, and so on. If we consider the previous example we can still create a longer sentence adding another complement phrase to the noun ‘story’ –while in the example 2 the complement phrase was added to the verb ‘to think’.
(3) [S [NP Robert] [VP [V understood] [CP [C that][ S [ NP Ben] [VP [V said] [CP [C that] [S [NP The man] [VP [V thought] [CP [C that] [S [NP David] [VP [V believed] [NP [NP the rumour] [CP [C that] [S [NP John] [VP [V said] [CP [C that] [S [NP Peter] [VP [V killed ] [NP a man] VP] S] CP] VP] S] CP] NP] VP] S ] CP] VP] S] CP]VP] S]CP] VP] S]
Example (1) was only one sentence. In the example (2) there are two sentences i.e. ‘The man thought’ and ‘David believed the rumour’. This last sentence, which is part of the complement phrase, is embedded inside the previous sentence. In the example (3) there are 6 sentences i.e. ‘Robert understood’, ‘ Ben said’,‘the man thought’, ‘David believed the rumour’ ‘John said’ and ‘Peter killed a man’. I have used the example (3) to show that it’s possible to add sentences either after the original sentence or before it. However, this last way is the easiest in order to create an infinite number of complement phrases embedded in the original sentence.
In conclusion the examples I have used show that the notion of recursion refers not only to the rules which apply recursively but also to the content of the sentences generated by these rules.
Adger, D 2003, Core Syntax, Oxford University Press, New York
Robert N. St. Clair, X-Bar Theory, University of Louisville