Sins of Memory
Most people's conception of memory is that it is an accurate recording machine that takes down events as they appear and simply keeps them until further notice. According to this view, memories never deteriorate, can easily be summoned at will, keep themselves separate like documents in a filing cabinet, don't interfere with each other, are the same however and whenever and wherever you recall them, are not biased by subsequent experience, and don't jump out to frighten you. Even if not all of these ideas are accepted by any one person, many of them will be.
Unfortunately, memories are nowhere near as faithful and angelic as these ideas would suggest. There is a traitor in your skull.
If you're having memory problems, you're probably at the receiving end of one or more of the seven sins of memory, a set of concepts about memory that Daniel Schacter described in his book The Seven Sins of Memory. It is probably no exaggeration to say that, as far as memory goes, we are all steeped in sin.
The seven can be divided into sins of omission, which are failures to record or to recall a memory for some reason, and sins of commission, which are failures to keep the memory faithful. Sins of omission are usually what we first think of when we think of memory problems.
Sin of Absent-Mindedness - Lost your keys? The odds are that you weren't paying attention when you set them down, being too busy memorizing other things. Sins of absent-mindedness are when key information fails to be recorded because attention wasn't sufficient to enable it. The resulting memory is fragmented.
Sin of Blocking - It's on the tip of your tongue. Unfortunately, the memory that was stored ended up getting in the way of another memory, meaning that there'll be a temporary delay while your nerve impulses try and race for the correct memory code. Your tongue, in the meantime, is kept waiting.
Sin of Transience - Your body is fighting against the Second Law of Thermodynamics every day, including your brain. Not to mention those memories are heaping up faster by the month, criss-crossing, writing over each other, fighting for space in that limited organ within your head. Transience is simply that more recent memories are easier to recall than older ones, and it's not hard to imagine why.
However, another set of memory sins belong to another category, one that's generally not recognized and which would strike serious doubt into the minds of those who face them. They are the sins of commission, the point at which one's judgement is called into question, the sins that require a traitor in your skull.
Sin of Bias - You actually make one-sided requests from your memory. People who want to look back to the good old days can see the distant cherry trees and almost smell the blossoms as they walk along the grassy path. Unfortunately, they don't notice the craggy cliff beside the path. They're too busy cherry-picking.
Sin of Misattribution - This is to fuse reality with fantasy such that things you only imagined were true are confused with what really happened. The strange thing is that you don't notice the mismatch because it looks like just another faithful memory. You assign a memory to the wrong source, such as remembering something you read in one book, could have sworn was written in that book, and it turns out you either read it in another book, found it on an online article, or had it told to you by a friend who had read the book.
Sin of Persistence - Don't think about a polar bear. Don't think about a tall pink giraffe. Don't think about a shuffling corpse walking slowly towards you. If you end up thinking of nothing else for a while afterwards, or if it jumps out at you again over the next few days or weeks, you've just been caught out by this sin. As you can guess, it works best when the persistent memory pulls on your emotions. He's right behind you.
Sin of Suggestibility - The reason hypnosis is no longer used as a valid means of gaining evidence. The reason leading or loaded questioning is frowned upon in courts. The reason your story might be slightly different each time you tell it. External clues may draw attention to certain aspects of the story, as though it was swelling, obscuring other aspects that don't get as much attention. If you're very good, you can get a person to tell the same story in a different way later just by asking differently-worded questions. And, of course, if a little now, then a little next time, and a little more next time, and so on...
The aching part? These sins might be side effects of how our memory normally works. To understand this, you have to realize that memory is not all about recalling events. It's about forming a picture of the self that is consistent - in a sense, fooling yourself the better to fool others. The relevance to discussions of religion and people's experiences (say, in a synagogue for the first time when converting) should be apparent, as religious people often recall personal events as a means of justifying their position - their social position as well as their intellectual one.
The sin of suggestibility, for instance, might be taking external clues the better to calibrate your story with the prevailing social mood. We shape our responses according to others' feedback all the time, and it would be less of a cognitive load to simply modify the existing memory than to create two, three, or more versions of it. The brain did have to build itself up from the cell, and in the meantime the genes building it needed bodies that could survive first, get their facts right second. It would be surprising for evolutionary reasons if it was perfect.
So, the next time someone pushing a claim relies on their memory to make a point, keep these sins in mind. I recommend the book The Seven Sins of Memory by Daniel Schacter if you can find a copy. And keep an eye out for your own memory transgressions. The easiest sins to commit are the ones you don't know about.