Thinking is just the process of gathering together these scattered facts from our intellectual memory and combining them in various ways. The Latin for to think' is cogito'. As a piece of etymology this is probably pretty bogus, but as a philosophical doctrine it makes some sense. Thinking means gathering up scattered bits of intellectual memory and combining and recombining them. Notice that this gives you another parallel between sense memory and intellectual memory.
Imagination is to sense memory as thinking is to intellectual memory.
Augustine is not terribly interested in imagination, but he does allude to it. In an analogous way, we can combine the objects of intellectual memory so as to think of new things that we have not thought before. Then the important question arises. Here he answers no. Then of course he is.