The fundamental principle that underlies the Natural Method is that a student learns a second language in the same general manner in which they originally learned their native language.
Four basic language structures are:
Reading
Writing
And the fifth, related, albeit advanced, activity is grammar. (It's not a first activity: if you give a two-year old kid a grammar book, and that's the way she learns the language, she'll be 50 or 60 years old before she can speak).