We often talk of “memory” as though it were a single definite thing. But everyone has many kinds of memories. Some things we know seem totally detached from time, like such facts as that twelve inches make a foot or that a bull has dangerous horns. Other things we know seem linked to definite spans of time or space, like memories of places where we've lived. Still other recollections seem like souvenirs of episodes we can reexperience: Once, when visiting my grand- parents, I climbed an old apple tree.
A brain has no single, common memory system. Instead, each part of the brain has several types of memory-agencies that work in somewhat different ways, to suit particular purposes.
Why do we have so many kinds of memory? If memories are records of our mental states of earlier times, how are those records stored and kept? A popular image of memories is that they are like objects we store away in various places in the brain. But then what are those places like? How do memories get into them and come out again? And what takes place inside of the vaults in which they're stored? Are memory banks like freezer chests where time stands still, or do their contents interact? How long can our old memories remain; do some of them grow old and die, do they get weak and fade away or just get lost and never found?
We have the impression that even our long-term memories become harder to recall as time goes on, and that might lead us to suppose that they have some inherent tendency to fade away. But even that is uncertain; it could simply be because so many other memories begin to interfere with them. Most likely, some types of memory mechanisms retain the records of sensations only for seconds; we use others to adopt habits, goals, and styles that we hold only for days or weeks; and we make personal attachments that endure through many months or years. Yet suddenly, from time to time, we'll modify some memories that seemed, till then, quite permanent.
More evidence that there are many kinds of memory has come from accidental injuries to brains. One injury may cause the loss of abilities to deal with names; another injury can make you lose some capacity to recognize faces or to remember musical tunes; still other kinds of injuries can leave unchanged what you have learned in earlier times but keep you from learning anything more in some particular domain. There is evidence that long-term memories can never form at all unless their short-term antecedents are permitted to persist for certain intervals; this process can also be blocked by various drugs and injuries, and this is why some people can never recollect what happened in the minutes before a brain concussion.
Finally, it appears that there are strong limitations on how rapidly we can construct our long-term memories. Despite all the legends about prodigies, there seems to be no evidence from any well-designed experiments that any human being can continue to construct long-term memories, over any substantial interval of time, more than two or three times faster than the average person.