 If logic is to be free of issues that would complicate telling valid from non-valid arguments by form, detecting ambiguity is essential to logical representation of natural language arguments. Frege noted this to be the main defect of natural language and a real obstacle to trying to formalize it (as opposed to just using the formal language without translation from natural language). We typically are more optimistic on this point than Frege; but the long history of dispute over such issues as the pragmatic-semantics distinction and skepticism over the viability of semantic theory in general stand as challenges.