3. The Mentalist Attitude
According to Chomsky, language is creative and behaviourism is totally unable to explain creativity. He argues that the comparison of the sentences a speaker has heard (the input) with the sentences a speaker produces (the output) shows differences between them. That is to say, the output contains sentences the speaker has never heard before. On the basis of this evidence, one can deduce that there is “something” between the input and the output. Chomsky calls it the Language Acquisition Device (LAD)
Input ----------------- LAD -------------------- output
The LAD is an inborn capacity (a genetic mechanism or apparatus) which is present in the brain right from the beginning and which enables children (by the age of 3 to 4) to extract the rules of language from speech when they are exposed to it and to use them productively. Animals do not possess this capacity. For this reason, their learning of language-like behaviour stops at a definite stage even if they are exposed to it.
Language acquisition takes place not as a result of imitation (stimulus + response) but as a result of the functioning of the LAD. In fact, what happens is that the child, when exposed to adult language, tries mentally to form hypotheses about its rules, then he tests the validity of these rules continuously and adapts them until he internally masters the abstract system of rules that adults have as part of their competence. So, language acquisition is part of the maturational process.
This view about language learning is called mentalism. It is based on the premise that human beings possess minds.