This well-documented slippage between theory and practice illustrates a particular kind of applied linguistic problem. It also emphasizes the importance of considering more closely an issue which is at the heart of all applied linguistic enquiry: what it means to learn, to know, and to use a language. To examine this problem and to extend our discussion of areas other than language teaching, we will benefit from closer assessment of the theory from which CLT derives, and it is to this that we turn in the next chapter.