For those not yet familiar with the concept of the koan, it is a question or short story which relates to the essence of reality. The problem from there is indeed sorting out he details of that perception of reality. The following problem from there is the perception of the koan, where koans have developed a reputation of being obscure riddles, traps of words, and related puzzles. In actually, the koan is indeed simply a question about or description of basic essential reality, where only someone who does perceive basic essential reality will answer the question based only on that experience, and without adding any random suppositions or bits of random reasoning.
The apparent subtlety comes from the nature of an individual's level of perception. If the answering individual does not have the necessary level of experience, the result is going to be a sort of best guess or attempt to reason with logic, where the level of experience needed will never be reached by mere reason.
Continuing from this, the main complication that an individual can usually have is the assumption or declaration that "I am This Person, I am such an age, have done particular things, of course I know reality." That isn't enough. To see why this isn't enough, let us consider actual reality, and by contrast what is merely considered to be actual reality.
For actual reality, let us consider it to be the entirety of everything that Just Is, with no embellishments, meanings, additions, symbolic references, Nothing. Let us consider this ultimate realty to be the Objective Universe, or O.U. Now let us consider that some individual has the fantasy that that a particular car is very important, or that doing certain actions or having objects with a particular brand name is very important---only in this case, such a person is claiming that the fantasy isn't a fantasy, that it is absolutely reality, and How Dare You Notice Otherwise?!?!?!!!!. Let us consider that the same person can then do a complete change of mind and demand that all such big shiny objects or practices to get attention must be denied or destroyed, where they must be denied or destroyed because they are considered to be important for the wrong reasons, and again, How Dare You Notice Otherwise?!?!?!!!. Let us consider that whether such a view demands the big shiny, or demands to destroy it, such a view is a Subjective Universe, or S.U. Also note that there is the O.U. but there is a S.U. . . . because every individual has that individual's personal fantasies and other occurrences of non reality that are distinct from the next person, therefore for pretty much every individual, there is going to be an individual S.U.
The subtlety and the confusion then comes in with all those times when the S.U. is seen as the O.U. There is a recurring reference to Buddhism stating that all is an illusion, and then someone picks up a rock and notes that the rock is a pretty solid illusion. The observation about the rock is quite correct, but that's not the illusion being referred to. Instead, there is this matter of everyone creating their own view of what they think everything is, which here has already been noted as the S.U., where the confusion comes from each individual S.U. becoming the individual to individual "universal" illusion where occasionally the individuals test their subjective views against everyone else's just to make certain that everyone does indeed have the same S.U. . . . . which is then claimed to be the O.U., because of course absolutely everyone knows what everyone else knows.
Now, the easy part of spotting someone's S.U. is indeed spotting any claims that the big shiny must be, or not be. That sort of claim is what Buddhism refers to as attachment. In all instances, in the O.U, there is only the individual. To get the addition claim of necessity of the big shiny, whether for or against, that's where the S.U. gets created by the individual so that the individual can make those claims. Out here in the O.U, no one cares about the big shiny, this brand name, that kind of car.
The difficult part of sorting the S.U. from the O.U. is that is that the S.U. is formed from very early on, is psychologically based, and on an extremely large scale is based on such concepts as down is where things go when you let go of them, usually. On a much smaller and personal scale, the much more subtle psychological influences come in with such things like cats being the embodiment of evil, so that all of them must be destroyed . . . . Or, in the O.U., when you were around two years old, you cornered a cat and were batting at it, and the cat clawed the hell out of you to get away, and you hated all of that, at the time, because later, you don't remember it . . . but your family does remember it, but they don't talk about it, where they don't like cats either, they just shoo cats away at all opportunities, especially the ones where you are around, because the cat clawed you, which they remember, but they don't remind you of it . . . and you in turn, are the one who got clawed, and also your family keeps shooing cats away, so you particularly despise cats, but this hatred is never discussed, it's just reenforced, so, "naturally" . . . Very subtly, but totally subjectively, in your S.U., which you think of as the O.U., "Everyone" hates cats.
The difficulty from there is indeed the observation and expressing of what is O.U. and not S.U., particularly because of the subtleties inherent in the S.U.. Methods of scientific observation have gotten to the quantum level of "we cannot really know, we can just point at"----to state in Really general terms---where a large number of S.U.s have taken that to mean "we can not really know, therefore there is no absolute, and everything is just like everything else". Such a belief gives us the combined origin and abject and total failure of Postmodernism.
From the Buddhist view, there is indeed the O.U. and the O.U. can indeed be apprehended in its entirety by an individual. However, particularly because of the level of subtlety inherent in everyone's S.U., particularly because of the name your quantum effect, only each individual can work to perceive that which is the individual S.U. as opposed to the O.U., only each individual can see for him or herself that "All (that is layered upon the O.U. by the personal psychology of the individual) is illusion", and then, having perceived the nature of the illusion that the S.U. provides, go beyond it, and begin to perceive the O.U.., in its entirety, with no trace of any S.U.
Soooo . . . Does this mean that Buddhism opposes the apparent conclusion of the quantum theorists, in that Buddhism says that an individual can indeed perceive all that is exactly as it is, and the quantum types say that absolutely nothing can be known with certainty? No. Not at all.
In actuality, the quantum theorists, et al, simply state that reality can only be perceived to the limit of the observing instruments, particularly when the observing instruments wind up disturbing that which is observed in that moment of observation. In addition, the assorted theorists also state that these results must be communicable so that they can be tested and confirmed by other, independent observers. From the point of view of Buddhism, the view is that the O.U. can indeed be apprehended in its totality, without any intervening S.U., such a condition being the nature of enlightenment, which is very nice, However . . . when someone is enlightened, that information cannot be transmitted, because any transmission that is not the observation of direct, perpetually changing reality as it is becomes subjective at the moment that it is locked into the telling . . . especially if the telling involves language in any form, as language is itself an utterly subjective creation which is indeed shared by many people, but is always shared in different, subjective forms.
Thus, on one end of perception there is the flower sermon, stated to be the founding of Zen Buddhism, where Gautama was sitting with a group of disciples one day, and at one moment simply and wordlessly held up a flower. One disciple smiled, because after all, there was no need to subjectively comment on the O.U., the O.U. Just Is . . . The other disciples were left scratching their heads, commenting Yes, it's a flower, what's the (subjective) point? . . . Subjectively, there was and is no point, as the direct perception and apprehension of the O.U. is the issue, and any discussion, including this, is manufactured and not what Just Is. ---Note, the pitfall here is to then disappear into one's navel while chanting "everything that is, just is." Yes, one can do that, where is a good way to disappear into a different S.U. and be lost. The only way to the direct perception of the O.U. is to actively engage and explore and study the world in all of its details, so as to better perceive what is the S.U. and what is the O.U. . . .
At the same time, there is still the problem of how to get past the carefully built up layers of the S.U. especially when one really may not remember where all the assorted parts come from. Such prying apart of S.U. From O.U.---to finally get to the main point-- is where koans can come in.
All the koans come from the same body of experience, and the point is experience, not knowledge, because there are no unchanging facts and thus knowledge to work with, just direct, ever changing experience of ongoing reality. Also, as they have come from the same experience of direct and ever changing reality, any new answer that any enlightened person will come up with at some moment is going to be a match for any other answer from someone also enlightened. Also, as koans come from that same body of experience, that too serves as a pointer to the O.U.
But at the same time, how does that pointer work? Well, take a koan, look at it, sit---and that itself is where zazen, seated meditation, comes in. Just sit, watch the thoughts go by, use the awareness of the koan as a goad, as a guide, where again, there is no logic to apply---where again, koans have been described as puzzles to solve, but no, that would involve logic.
So, keeping that in mind, consider as a much simpler example, that at some point rather a few years ago, or centuries ago, one master asks another "How difficult is your walk?" The second master is recorded as replying that "My ten foot staff really helps to focus." and over time there is rather an amount of detailed commentary that discusses and claims to explain the conversation. Over time, various reviewers comment on different types of ground to walk on and how difficult the different kinds of ground are, comment on possible lengths of the staff, possible ways to hold the staff while walking along.
A while later, one fellow who became enlightened during a rainstorm comments that the option of slipping on the wet surface really focused his concentration, and that comment alone then inspires many others to do much walking through pools and across ice. Someone doing even more commentary starts noting possible versions of dual canes or crutches as examples of the original masters and their discussion about the difficulty of walking
Much later, someone else comes along, and announces that all the discussions of all the canes, crutches, assorted hand grips are all the product of people who have no experience and are just trying to guess and extrapolate. The answer that he---or she--- provides to the long before predecessors is, in fact "I need no staff, I balance just fine."
Now consider that what all these masters are discussing, based on their own personal experience, and based only on their experience, is the act and direct experience of walking along a tightrope that is 30 feet off the ground. "How difficult is your walk?" asks one master who has climbed up, strolled along the cable, and climbed back down again. "My ten foot staff really helps to focus." replies another master who has climbed up with the staff, who used the staff as a balance pole to aid his balance as he walked along the cable, and then climbed back down again.
In time there is additional commentary from people who can only think of walking across the ground, and so they discuss types of ground and types of walking cane, and crutches. They're doing excellent amounts of engineering development, but mere puzzle solving is not being enlightened and is not thus practicing nothing at all more than one's moment to moment experience.
In time there is yet another master who climbed up during a rainstorm, recognized that the cable was being particularly slippery, and paid very close attention as he walked across. And finally, there is the later master who has perfect balance, and doesn't need a staff to help in balancing. And all of these masters tell of nothing more than their direct experience of walking a tightrope, where all the non masters complain that all the tightrope koans are obscure, and confusing, and puzzling, and contradictory, and they, the masters and the koans, just don't follow everyday logic and reason.
Of course they don't follow reason, reason isn't involved. All such masters and koans are, however, totally logical---logic always acknowledges the direct experience of direct reality.
Thus, of not walking a tightrope, but of only being only in the moment, from one moment to the next;
A monk told Joshu: `I have just entered the monastery. Please teach me.' Joshu asked: `Have you eaten your rice porridge?' The monk replied: `I have eaten.' Joshu said: `Then you had better wash your bowl.' At that moment the monk was enlightened.