Read the following passage and answer the item that follows. Your answer to this item should be based on the passage only.

Only by learning a second language properly, far enough to argue in it and to be embarrassed in it, does a speaker discover which of his habits of thought were his own and which were merely his language's. Courtesy, obligation, even the way a promise is framed, turn out to be handled differently elsewhere, and yet to be recognisable everywhere once the local dressing is removed. The traveller who learns only enough to order a meal never meets this. What the fluent speaker gains is not vocabulary but a sense of which parts of his own thinking travel and which do not.

According to the passage, learning a second language properly teaches a speaker to distinguish between