In order for a mind to think, it has to juggle fragments of its mental states. Suppose you want to rearrange the furniture inside a room you know. Your attention keeps shifting, first to one corner, then to another, next to the center of the room, and then, perhaps, to how the light falls on some object on a shelf. Different ideas and images interrupt each other. At one moment it seems as though your entire mind were focused on one small detail; at another moment you might dwell on why you are thinking about that room in the first place; then you might find yourself comparing or contrasting two different rearrangements of that scene: If that couch were over here, there would be room for guests to chat — but no, that would block the path so that they wouldn't be able to enter.
How do our various agencies keep track of imaginary changes in scenes? Where do the different versions go when out of mind, and how do we get them back again? They must be stored as memories.
But what do we mean by that? Some readers may be surprised to learn that biologists still have no well-established theory of what happens in our brains when memories are formed. Psychologists, however, do agree that there must be at least two different mechanisms. We appear to have long-term memories, which can persist for days or years or all one's life. We also have short-term memories, which last only for seconds or minutes. In the next few sections we'll talk mostly about the uses of these transient traces of our recent thoughts. For example, whenever we get stuck in the course of solving a problem, we need to be able to backtrack, modify our strategy, and try again. To do this we need those short-term memories, if only not to repeat the same mistake.
How much do we remember? Sometimes we surprise ourselves by remembering things we didn't know we knew. Could this mean that we remember everything? Some older theories in psychology have supposed this to be true, and there are many legends of persons having fabulous abilities. For example, we often hear about people with photographic memories that enable them to quickly memorize all the fine details of a complicated picture or a page of text in a few seconds. So far as I can tell, all of these tales are unfounded myths, and only professional magicians or charlatans can produce such demonstrations.
In any case, I suspect we never really remember very much about a particular experience. Instead, our various agencies selectively decide, unconsciously, to transfer only certain states into their long-term memories — perhaps because they have been classified as useful, dangerous, unusual, or significant in other respects. It would be of little use for us simply to maintain vast stores of unclassified memories if, every time we needed one, we had to search through all of them. Nor would it be of any use for them all to flood at once into our agencies. Instead, each of us must develop fruitful and effective ways to organize our memories — but how that's done is inaccessible to consciousness. What barriers keep us from knowing such things? The next few sections sketch out some theories, both about how our memory-systems work and why we can't find this out directly by examining our own thoughts.