This paper focuses on the mechanism by which the successful learner acquires a second language. The author postulates a core language, the common core of the speaker's native and target languages, and states that the second language becomes an extension of this common core. Whatever language-specific features are added while acquiring the second language are extensions of the common system underlying the two systems. In successful second language learners, latent language structure is linked with latent psychological structure, whereas in the ordinary learner there is a separation between these two structures. The successful language learner achieves a prelinguistic identification that results in successful communication in two codes. The language learner, in seeking genetic equivalence, discovers a deep interlingual identification between native and target language forms. In discussing the grammar-translation method, the situation-structural method, and the notional-rhetorical approach, the latter is seen as primary since it establishes a closer link between the latent psychological and latent language structures. By assuming that function is prior to form and that the whole is prior to the parts, this method facilitates the realization of conceptual and behavioral potentials of language in a properly linked manner. (Author/JK)