The contribution to L2-acquisition which comes from UG is conceptually distinct from that which comes from L1 (or from L1 and L2 jointly), but it is difficult to tease the two apart. The workings of deep, core principles (e.g., locality and subjacency) are so massively evident in L1 and L2 as to be of questionable use in the search for the contribution which is purely of UG.