The logical problem of second language acquisition：interpretation and reflection

Is there the logical problem of second language acquisition? If yes, what's the difference between the logical problem in second language acquisition and the one in first language acquisition? The arguments from universal grammar and connectionism have their own good points and weak points, but till now neither universal grammar nor connectionism could successfully solve this problem.The logical problem of second language acquisition remains an unsolvable brainteaser.To solve it relies on the further development of scientific technology and research, especially multi-disciplines,cooperation.