From Kahneman and Tversky:
"A person is said to employ the availability heuristic whenever he estimates frequency or probability by the ease with which instances or associations could be brought to mind"
I doubt that's news to any LessWrong readers - the availability heuristic isn't exactly cutting edge psychology. But the degree to which our minds rely on mental availability goes far beyond estimating probabilities. In a sense, every conscious thought is determined by how available it is - by whether it pops into our heads or not. And it's not something we even have the illusion of control over - if we knew where we were going, we'd already be there. (If you spend time actually looking directly at how thoughts proceed through your head, the idea of free will becomes more and more unrealistic. But I digress). What does and doesn't come to mind has an enormous impact on our mental functioning - our higher brain functions can only process what enters our working memory.
Whether it's called salience, availability, or vividness, marking certain things important relative to other things is key to the proper functioning of our brains. Schizophrenia (specifically psychosis) has been described as "a state of aberrant salience", where the brain incorrectly assigns importance to what it processes. And a quick perusal of the list of cognitive biases reveals a large number directly tied to mental availability - there's the obvious availability heuristic, but there's also the simulation heuristic (a special case of the availability heuristic), base-rate neglect (abstract probabilities aren't salient, so aren't taken into account), hyperbolic discounting (the present is more salient than the future), the conjunction fallacy (ornate, specific descriptions make something less likely but more salient), the primacy/recency bias, the false consensus effect, the halo effect, projection bias, etc etc etc. Even consciousness seems to be based on things hitting a certain availability threshold and entering working memory. And it's not particularly surprising that A) we tend to process only what we mark as important and B) our marking system is flawed. Our minds are efficient, not perfect - a "good enough" solution to the problem of finding food and avoiding lions.
That doesn't mean that availability isn't a complex system. It doesn't seem to be a fixed number that gets assigned when a memory is written - it's highly dependent on the context of the situation. A perfect example of this is priming. Simply seeing a picture is enough to make certain things more available, and that small change in availability is all that's needed to change how you vote. In state-dependent memory, information that's been absorbed while under a certain state can only be retrieved under that same state - the context of the situation is needed for activation. It's why students are told to study under the same conditions that the test will be taken, and why musicians are told not to always practice sitting in the same position, to avoid inadvertently setting up a context dependent state. And anecdotally, I notice that my mind tends to slide between memories that make me happy when I'm happy, and memories that make me upset when I'm angry (moods are thought to be important context cues). In general, the more available something is, the less context is needed to activate it, and the less available, the more context dependent it becomes. Frequency, prototypicality, and abstractness also contribute to availability. Some things are so available that they're activated in improper contexts - this is how figurative language is thought to work. But some context is always required, or our minds would be nothing a but a greatest hits of our most salient thoughts, on a continuous loop.
The problems with this approach is that availability isn't always assigned the way we'd prefer it. If I'm at a bar and want to tell a hilarious story, I can't just think of "funny stories" and activate all my great bar stories - they have to be triggered by some memory. More perniciously, it's possible (and in my experience, all too likely) to have a thought or take an action without having access to the beliefs that produced it. If, for example, I'm playing a videogame, I find it almost impossible to tell someone a sequence of buttons for something unless I'm holding the controller in my hands. Or I might avoid seeing a movie because I think it's awful, but I won't be able to recall why I think it's awful. Or I'll get into an argument with someone because he disagrees with something I think is obvious, but I won't immediately be able to summon the reasons that generated that obviousness. And this lack of availability can go beyond simple bad memory.
From Block 2008:
There is a type of brain injury which causes a syndrome known as 'visuo-spatial extinction' If the patient sees a single object on either side, the patient can identify it, but if there are objects on both sides, the patient can identify only the one on the right and claims not to see the one on the left. However as Geraint Rees has shown in two fMRI studies of one patient (known as 'GK'), when GK claims not to see a face on the left, his fusiform face area (on the right - which is fed by the left side of space) lights up almost as much as - and in overlapping areas involving the fusiform face area - when he reports seeing the face.
The brain can detect a face without passing that information to our working memory. What's more, when subjects with visuo-spatial extinction are asked to compare objects on both sides - as either 'the same' or 'different' - they're more than 88% accurate, despite not being able to 'see' the object on the left. Judgements can be made based on something that we have no cognitive access to.
In Landman et al. (2003), subjects were shown a circle of eight rectangles for a short period, then a blank screen, then the same circle where a line points to one of the rectangles, which may or may not have rotated 90 degrees. The number of correct answers suggest subjects could track about four different rectangles, in line with data suggesting that our working memory for visual perceptions is about four. Subjects were then given the same test, except the line pointing to the rectangle appeared on the blank screen, between when the first circle and the second circle of rectangles is shown. On this test, subjects were able to track between six and seven rectangles by keeping the first circle of rectangles in memory, and comparing the second circle to it (according to the subjects). They're able to do this despite the fact that they're unable to access the shape of each individual rectangle. The suggested reason for this is that our memory for visual perceptions exceeds what we're capable of fitting into working memory - that we process the information without it entering our conscious minds. It seems perfectly possible for us to know something without knowing we know it, and to believe something without having access to why we believe it.
This, of course, isn't good. If you're building complex interconnected structures of beliefs (and you are), you need to be sure the ground below you is sturdy. And there's a strong possibility that if you can't recall the reason behind something, your brain will invent one for you. The good news is that memories don't seem to be deleted - we can lose access, but the memory itself doesn't fade. The problem is one of keeping access open. One way is to simply keep your memory sharp - and the web is full of tips on that. A better way might be to leverage your mind's propensity for habituation - force yourself to trace your chain of belief down to the base, and eventually it will start to become something you do automatically. This isn't perfect either - it's not something you can do for the fast pace of day-to-day life, and it in itself is probably subject ot a whole series of biases. It might even be worth it to write your beliefs down - this method has the dual benefits of creating a hard copy for you to reference later, and increasing the availability of each belief through the act of writing and recalling. There's no ideal solution - we're limited in what new mental structures we can create, and we're forced to rely on the same basic (and imperfect) set of cognitive tools. Since availability seems to be such an integral part of the brain, forcing availability on those things we want to come to mind might be our best bet.
1: If it's hard to understand this experiment, look at the linked Block paper - it provides diagrams.
2: It can, however, be re-written. Memory seems to work like a save-as function, being saved (and distorted slightly) every time it's recalled.