What is the logical problem of foreign language learning?

The linguistic data to which children are exposed appear to be insufficient to determine, by themselves, the linguistic knowledge that children eventually attain. The gap between available experience and attained competence forms what has been called the logical problem of language acquisition. The approach to a solution, which has been followed in linguistic theory over the past two decades, is to suggest that the gap is bridged by an innate Universal Grammar: a system of knowledge of what a human language can be and innate domain-specific procedures for arriving at a grammar. The classic statement is that of Chomsky: