This is quite a simple concept, so it needs no introduction, middle and conclusions.
It's all about the paradox between knowing and seeing.
Sometimes it's necessary to know something is there in order to see it. The biggest set of information in this group is related to cultural norms. For instance, if I didn't know it was shameful to walk naked in the street, I would probably go al fresco most of the time. If I didn't know that red traffic light meant "STOP", I would probably drive across a junction at the wrong time. If I didn't know that traffic lights exist, I might not even notice them. this is partly because their importance would also be unknown to me.
We tend to concentrate on things we think are important, and so might not see the rest. A good example of this would be the soldier who shoots a child carrying a toy gun; because the more survival-related the issue, and the more excessively focussed it is possible to become. Funnily enough, belief systems can be survival issues. I recently crossed a schizophrenic who was on a bad drugs trip, and answered his question to the effect that his friend was not anywhere near. This insulted his belief system to such a degree (he believed that his friend was there) that violence almost ensued. Fortunately most of use have far less rigid belief systems than a paranoid schizophrenic on marijuana, and so are more adaptable to surprises and variations in life. But there comes a point in all of us when the lizard brain starts to defend its territory - and its useful to be aware of where that territory has been marked out.
Now, if I went to Russia and didn't know that the "traffic light colour rule" varied geographically, I would still know about traffic lights, but would misinterpret them due to assumption of the wrong set of rules. If you've ever driven abroad, you'll know that different countries have different ways of displaying road signs which (initially, until the brain adapts to the new rules of visual context) make them almost invisible. Even ridiculous things like roundabouts - even if you know that you have to go the opposite way round a roundabout, those habit reflexes certainly can cause a few embarrassments. I remember trying to change gear (stickshift) with the window winding handle a few times too!
Knowing and seeing is related to any belief system about how the world ticks. The mediaeval belief that frogs and insects were spontaneously created from the earth was, to be sure, a lousy bit of observation, but on the other hand, they "knew" that that's how frogs are made, and so that's what they saw.
On the other hand, if I didn't know about the nervous system, I would have a great deal of difficulty working with it as a practitioner, and here we come to the tail end of the circular dragon. I would then begin to describe it in terms of concepts which I did know. And so, humours, or five elements, or even dead relatives might form the basis of my model of how various aspects of the central nervous system works, and that description would gradually evolve to be as good as my ability to observe allowed it to be. Given sufficiently honest and accurate observation, even such arcane descriptions might hold a great deal of truth in them - but codified in language and jargon which would confound a person with a different frame of reference.
Next time you meet someone who goes weak-kneed at the sight of a computer, you might remember that their brain may be wired to understand parallel relationships rather than hierarchical logic structures. The fact that such varied fundamental ways of experiencing and understanding the world can be present in a single culture with a single language makes you sit up and wonder how many ways there are to see the world. Australian Aboriginal language is presently being investigated because it reflects a non-linear, purely relationship-based model of the world, which is now hot potatoes in the world of artificial intelligence research.
Knowing about the Central Nervous System (CNS) and its existence allows me to see it, because otherwise I would have a few clues, but maybe not enough to deduce its "true" nature. On the other hand, once I know it exists, there is a huge temptation to describe all phenomena which appear to be due to the CNS, as if the CNS causes them. The knowledge makes it difficult to see, because as soon as there is a label, a knowing, then that automatically becomes a filter for the mind. More subtle, and more insidious than knowing a specific fact is the temptation to use the means of description as my model of the world. For instance, if my model of the world is chemical, then I will describe the central nervous system in chemical terms, and in so doing may miss out huge swathes of reality. If the means of description is sufficiently fundamental, the danger increases, because everything is explicable in these fundamental terms. For instance, biochemical neurotransmitters and peptides are seen as the cause of many biochemical reactions in the human body, and that biochemical mechanism is even more restricted by its demand that the interactions be wholly biochemical. And seen through a chemically filtered microscope, indeed, everything in the human body is chemical. If I were to have an electromagnetic view of body function, the chemistry might remain the same, but the interpretation would be radically different. And why restrict the filter to scientific realms? An esoteric filter might be blind to molecules, but nevertheless pick up other facets of human reality. In the end, all filters are inherently useful - because they limit reality sufficiently to make it malleable; and inherently dangerous, because we are often in ignorance of the fact that we're filtering.
I think we're particularly prone to do this in the name of cause and effect.
The other thing that seems to come easy to the human race is to only see what we want to see, and ignore the rest (which is obviously where the word ignorance comes from). If I have a nice tidy theory which maps out my world at the center of the universe, I don't want some smartass astronomer telling me that it different. And one reason for that is that I like everything to be consistent, so if I have one theory about what it means to be human, then all parts of my belief system must come to the same conclusion, mustn't they?
I've even done it with my ideas about chaotic arterial resonance, because once I have a nice comfy working model of the universe, I am tempted to fit everything into it (and have done too, and golly does it feel good - like a warm glow, y'know?). Kurt Gödel's theorem goes into the filing cabinet yet again. Kurt would probably have been kebabed a few centuries ago. Instead, he starved himself to death, because he believed that his food was poisoned. Nice little belief system, and a warning to us all - Kurt didn't eat food because poisoned food causes death - QED.
Here is a picture of Kurt and Albert in the woods.
What keeps me from locking the filing cabinet and throwing away the key is a walk through the woods. Time and again, I find myself looking at a tree, and saying "it's a beech" (lets face it, life's a beach too). As soon as it is named, something shrinks in my curiosity gland. "Oh, its only a beech", and so it becomes less easy to see, to really see all the ways the light falls through the leaves, and the way the branches seek the light spaces in graceful downcurves, and ..
So having named things, their visibility reduces, and the filter of their presence makes for things I do not know of to be less visible. Do you wonder why so many millionaires have remarkably little formal education? How would you like to be richer?
To know, or not to know, now THAT is the question.