The rules of the grammar will also need the crucial property of recursion. Recursive (“repeatable any number of times”) rules have the capacity to be applied more than 98 The Study of Language once in generating a structure. For example, we can have one prepositional phrase describing location (on the table) in the sentence The gun was on the table. We can also repeat this type of phrase, using different words (near the window), for as long as the sentence still makes sense (in the bedroom). So, in order to generate a sentence such as The gun was on the table near the window in the bedroom, we must be able to repeat the rule that creates a prepositional phrase over and over again.

We must also be able to put sentences inside other sentences. For example, when we produce a sentence such as Cathy knew that Mary helped George, we do so with the sentence Mary helped George inside it. And those two sentences can be generated inside another sentence such as John believed that Cathy knew that Mary helped George. In principle, there is no end to the recursion that would produce ever longer versions of complex sentences with this structure.

Basically, the grammar will have to capture the fact that a sentence can have another sentence inside it or that a phrase can be repeated as often as required. We should note that recursion of this type is not only a feature of grammar, but can also be an essential part of a theory of cosmic structure.