Children's brains are a blank slate at birth
According to this view posited by early linguists, philosophers, and behavioral psychologists, children are not born with any ingrained language knowledge.
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The blank slate, or tabula rasa, theory of child language acquisition stems originally from behavioralist psychology. This hypothesis was first posited by John Locke in 1690 and was popularized in the mid-20th century by B.F. Skinner.
Children are not born with any linguistic information in their brains. In fact, they start off with a completely blank slate onto which experiences, words, and phrases are "written onto." Children learn through exposure to language around them as well as reinforcement from others. Like all acts in behaviorist psychology, language is learned as a response to a stimulus. It is a process of imitation where children attempt to copy what they hear. Children also learn through feedback, with a positive response encouraging their use of an utterance and negative feedback discouraging it. Through these methods, children learn the rules of language as they grow.
This theory does not offer enough of an explanation as to why children typically acquire language as easily and quickly as they do. Also, if they learned purely through reinforcement and environmental factors, then people would only understand and be able to say words and phrases that they had already heard, but this isn't the case. Both children and adults are able to produce new sentences to which they have never been exposed. Also, for many young children, language is not explicitly taught--at least, not in detail--and yet they learn it well all the same. In short, the blank slate theory cannot adequately account for the patterns in child language acquisition that researchers have documented for decades.
Rejecting the premises
Chomsky's famous review of the behaviorist Skinner's article on child language acquisition: https://chomsky.info/1967____/