This famous quote from Canadian Marshall McLuhan has already formed the basis of many modern approaches to teaching and learning, such as Teaching as a Subversive Activity by Neil Postman and Charles Weingartner, and is at the centre of the way I structure my classes.
Basically, it states that we learn by doing, and that the way we learn is of vital importance – not just what we learn. This may sound like common sense, but I have worked with many frustrated clients who didn't understand why all their reading and grammar work wasn't improving their speaking skills. They were just getting better at reading and doing grammar exercises! It is true that we need to study the basics first, but then we need practice what we know as much as possible, we simply need to do it.
It's like learning to drive a car: no amount of reading about how to drive can ever be a substitute for actually driving. Reading books and watching movies are a great supplement to language learning, but we must also speak and write as much as we can, really use the language we’re learning – especially if our goal is to do more than just understand it