However, a larger-scale change could cause more disruption and result in wasted time and confusion. So, as we evolved more ways to think, we also evolved machinery for more quickly returning to previous contexts. In everyday life we simply say that we’re using our ‘short-term memories’— but any good theory of how that might work must answer questions like these:
How long do recent records persist, and how do we make room for new ones? There must be more than one answer to that because various parts of the brain must work in somewhat different ways. Some memories may be permanent, while others may rapidly fade away, unless they happen to get “refreshed.” Such records also would get erased if stored in a ‘place’ that has a limited size—because then each new item would have to replace some records that are already there. Indeed, this is how modern computers work: whenever data is created or retrieved, it is first stored in a cache—a device that has been designed to be especially quickly accessible. Then, whenever such a cache gets full, its oldest records get erased—although some of them may have been copied to larger, more permanent memory boxes.
How do some memories become permanent? There is evidence that it takes hours or days for what we call short-term memories to be converted to longer-term ones. Older theories about this assumed that frequent repetitions made the original record more permanent. However, it seems more likely to me that new memories are briefly maintained in resources that act like a computer’s cache—and then, over time, more permanent versions are created in other regions of our brains. See §8-4.
In any case, some memories seem to last for the rest of one’s life. However, this could be an illusion because they might need ‘refreshment’ from time to time. Thus, when you recall a childhood memory, you often also have the sense of having remembered the same thing previously; this makes it hard for you to know whether you have retrieved an original record or merely a later copy of it. Worse yet, there now is ample evidence that those records can be changed while they’re being refreshed.[146]
How do we retrieve old memories? We all know that our memories often fail—as when you try to recall some important details but find that their records have disappeared, or that at least we cannot retrieve them right now. Now clearly, if no trace of that record remains, further search would be a futile quest. Nevertheless, we frequently manage to find some clues that we can use to reconstruct more of those memories. Here is a very old theory of this:
St. Augustine: “But what happens when the memory itself loses something, as when we forget anything and try to recall it? … Perhaps the whole of it had not slipped out of memory; but a part was retained by which the other lost part was sought for, because the memory realized that it was not working so smoothly as usual, hence, it demanded the restoration of what was missing. For example, suppose we see or think of some man we know, and, having forgotten his name, try to recall it—but some other thing presents itself, which was not previously associated with him; then this is rejected, until something comes into the mind which better conforms with our knowledge.”
So if you can link a few of those fragments together, you may be able to reconstruct a good deal more—
“... by gathering together those things that the memory already contains but in an indiscriminate and confused way, and now putting them together [so that] where they formerly lay hidden, scattered, or neglected, they now come easily to present themselves to the mind which is now familiar with them.”[147]
Augustine soon turned to other concerns, and concluded this discussion of memory by plaintively asking, “Who will work this out in the future?” But more than a thousand years were to pass before further progress on theories of memories.
How many thoughts can you think at once?
How many feelings can you feel at once? How many different things can you simultaneously ‘pay attention’ to? How many contexts can be active at once in your context-box? To what extent can you be aware of how many mental activities?
The answers to such questions depend on what we mean by ‘aware’ and ‘attention.’ We usually think of ‘attention’ as positive, and highly regard those persons who are able to ‘concentrate’ on some particular thing, without getting distracted by other things. However, we could also see “attention” as negative—because not all our resources can function at once—so there always is a limit to the range of things we can think about at the same time. Nevertheless, we can train ourselves to overcome at least some of those built-in constraints. [See §§Attention.]
In any case, in our high level thinking we can only maintain a few different ‘trains of thought’ before we start to become confused. However, at our lower reactive levels, we carry on hundreds of different activities. Imagine that you are walking and talking among your friends while carrying a glass of wine:[148]
Your grasping resources keep hold of the cup.
Your balancing systems keep the liquid from spilling.
Your visual systems recognize things in your path.
Your locomotion systems steer you around those obstacles.
All this happens while you talk, and none of it seems to require much thought. Yet dozens of processes must be at work to keep that fluid from spilling out—while hundreds of other systems work to move your body around. Yet few of these processes ‘enter your mind’ as you roam about the room—presumably because they use resources that work in separate realms that scarcely ever come into conflict with what you are usually “thinking about.”
It is much the same with language and speech. You rarely have even the faintest sense of what selects your normal response to the words of your friends, or which ideas you choose to express—nor of how any of your processes work to group your words into phrases so that each gets smoothly connected to the next. All this seems so simple and natural that you never wonder how your context-box keeps track of what you have already said—as well as to whom you have mentioned them.
What limits the number of contexts that a person can quickly turn on and off? One very simple theory would be that our context-box has a limited size, so there is only a certain amount of room in which to store such information. A better conjecture would be that each of our well-developed realms acquires a context box of its own. Then, some processes in each of those realms could do work on their own, without getting into conflicts until when they have to compete for the same resources.
For example, it’s easy to both walk and talk because these use such different sets of resources. However, it is much harder to both speak and write (or to listen and read) simultaneously, because both tasks will compete for the same language-resources. I suspect such conflicts get even worse when you think about what you’re thinking about, because every such act will change what is in the context box that keeps track of what you were thinking about.
146
The construction of long-term memories appears to involve special kinds of sleep, in ways that are not yet understood. It also appears that different kinds of memories (e.g., about autobiographical events, about other kinds of episodes, about what are called ‘declarative’ facts, and about perceptual and motor events) are each stored in somewhat different ways and in different locations in the brain.
147
Section 19.10 of