Chomsky proposed that language was not a system established by training and experience, but the result of an innate human capacity to understand the formal principles underlying linguistic structures. Pointing out that young children were able to produce an endless number and range of sentences as a result of merely listening to adults talking, he argued that this indicated an understanding of the grammatical rules underlying ordinary sentences, an understanding that had to be innate.

Chomsky distinguished between the actual words and sounds heard, which he called "surface structures", and the mechanisms whereby a sentence's underlying meaning was grasped, which he called "deep structures". People were able to form sentences by generating surface-structure words from deep-structure rules: sets of "grammatical transformations" that, according to Chomsky, are basically the same in all languages because they correspond to innate, genetically transmitted mental structures in human beings.