Children master grammar from speech too messy to have taught it to them

Noam Chomsky pointed out that children hear only a limited, often broken stream of speech, yet reliably learn grammatical rules no one explicitly taught them. He argued this 'poverty of the stimulus' only makes sense if humans are born with some innate scaffolding for language, not blank slates shaped purely by experience.

— Noam Chomsky, Aspects of the Theory of Syntax — 1965