Chomsky argues that the underlying logic, or deep structure, of all languages is the same and that human mastery of it is genetically determined, not learned. Those aspects of language that humans have to study are termed surface structures.

Surface structure refers to the actual words people use and what’s readily apparent about them, whereas deep structure refers to the information that underlies a sentence and gives it meaning. Another way of seeing this distinction is by considering sentences that are grammatically correct but totally devoid of meaning.

For example, consider the sentence, “Dark purple ideas eat angrily.” It is perfectly correct in terms of grammar but has no meaning whatsoever.

In view of such facts, Chomsky and others have argued that we can never understand the true nature of spoken language by focusing only on words and grammatical rules. Rather, we must search for underlying meaning and the ways in which people translate, or transform, this into overt speech.

* * *

Sources:
Gale encyclopedia of Psychology 
Psychology, Robert Baron (Click for free eBook preview)