The previous two chapters have discussed two attractive features of second-order conditioning: its assistance in the measurement of learning and its help in determining the circumstances that produce learning. In the present chapter I describe a third feature: its help in understanding the content of learning. In many ways the central issue in the study of associative learning is that of understanding what is learned. What is associated with what? When we say that the organism forms an association between two events in the world, that assertion is elliptical for the more precise statement that it associates the internal representations of those events. The issue is then which aspects of those events the organism incorporates into its representations. Which features of rather complex occurrences are actually associated with each other?