I have to admit, I am somewhat disappointed that nobody has ever asked that. I can't imagine that it is transparent, so I suppose that means that nobody cares. Oh well, so be it. I will explain anyway.
In my view, this is the one key, central insight: all distinctions are arbitrary. Armed with this insight, everything else becomes transparent to you. It has deep philosophical, scientific, and religious implications, and I will address these in that order.
"He who breaks a thing to find out what it is has left the path of wisdom." -- J.R.R. Tolkein (Gandalf to Saruman)
In his excellent book, Zen and the Art of Motorcycle Maintainance, Robert Pirsig talks a lot about the "Ghost of Reason," by which he means rational analysis. In the art of rational analysis, the idea is to take your subject and cut it up into smaller and smaller pieces, and give each one a name. Aristotle was the original master of this art. He could cut the simplest thing, like the "good", into thousands of little bits and give each one a name and then go on for hundreds of pages about the properties and status of each bit. This is great, if you like that sort of thing, and it is a very powerful intellectual tool. But it has its limits, as we will see below.
Phaedrus, the character in Zen, reaches a major turning point in his philosophical development when he realizes that all distinctions are arbitrary. This means that you can make each cut of the analytical knife any way you choose. You can define your terms however you like, cut up your subject along whatever dividing lines you prefer. This gives the analyst enormous power and flexibility.
However, it must be remembered that the distinctions are arbitrary. If I am the one doing the analysis, I am free to cut up my subject however I like. So if you are listening to an analysis, you must remember that the distinctions being made are arbitrary. You are free to make your own analysis, making your own distinctions as you see fit. This is known as thinking for yourself. It is probably a good idea. Who controls your distinctions controls your thoughts.
In particular, the deepest insights often come from simply choosing an unconventional analysis, a different way of cutting up the subject. In Zen, Pirsig talks about what happens when you choose to distinguish things based on an undefined concept he calls Quality. Analysis by Quality gives very different results than other analyses, and is very interesting. It is not my purpose to explore this here, but rather to point out that once you understand the arbitraryness of distinctions, you are free to create unconventional analyses which often lead to deep insights about the subject.
It is also important to keep in mind that distinctions do not constitute proof. People often try to convince you with arguments based on arbitrary distinctions; you are always free to disregard these distinctions, make your own instead, and demolish the line of argument.
But there is another aspect of this which cuts even deeper. If all distinctions are arbitrary, that means that they are a product of the act of rational analysis itself. In other words, distinctions do not exist in "the world itself," they exist only in the mind of the analyst. The world itself is undifferentiated. All distinctions are therefore illusory, as well as arbitrary. They are all in your head. This has deep scientific and religious implications, as we will see below.
"I believe in the fundamental interconnectedness of all things." -- Douglas Adams (Dirk Gently)
Science was originally based on an epistemology called "objectivity," which assumes that it is possible to separate the observer from the system to be observed. While philosophically this was always understood to be an imperfect model, it was taken so far that it became the basis of an entire worldview. Or rather, to be precise, it was used as a justification for a worldview which was already in place. This is the worldview in which we cut the world up into distinct, solid physical objects and give each one a name and a set of properties (color, shape, etc.).
While this sort of analysis is obviously very useful, it has also demonstrated its own limitations. When we started messing around with Quantum Mechanics, it was very quickly realized that there is no way to separate the observer from the system to be observed. Indeed, the act of observation itself seems to be what gives the system its properties. In some very real sense, the system does not even exist until it is observed. Furthermore, ideas about particle/wave duality questioned the whole idea of distinct physical objects (particles). From a wave-mechanics perspective, particles do not even exist, but are an artifact of the way you choose to measure (observe) the system. This reinforces, from a scientific standpoint, the idea that all distinctions are illusory, and called into question the whole epistemology of objectivity, which ironically was what was used to come up with Quantum Mechanics in the first place.
The final nails in the coffin of objectivity were, in my view, driven by a pair of theorems which have yet to be fully incorporated into mainstream consciousness. These are Bell's Theorem, and Godel's Theorem.
Bell's Theorem basically demolishes a position known as "local realism". Realism holds that objects in the world (particles) exist, and have real properties which exist independent of the observer. Locality holds that no influence can propagate faster than the speed of light. Bell's Theorem uses Quantum Mechanics to predict that one or the other of these must be false. Recent experiments have supported Quantum Mechanics against local realism. If you believe the experiments (which some do not), you must abandon either locality or realism. Now, if you abandon locality, you must admit that everything is causally connected in a way which is immediate and unmitigated, that is to say, everything influences everything else all at the same time. This means that the Universe is a single, whole integrated system, and any distinctions among its subsystems are arbitrary. In particular, the distinction between observer and observed is arbitrary and unreal, thus demolishing objectivity. If you abandon realism, then objects don't even exist outside the context of the act of observation, also demolishing objectivity. So objectivity is pretty much toast.
As if that weren't enough, we also have Godel's theorem. This is usually stated as, "you cannot have a complete logical system". This is pure math, and so perhaps it belongs in the philosophy section, but it relates most directly to science so I put it here. Another way to put it is to say that, given a logical system of sufficient complexity to be interesting (for instance, the current logical system of modern mathimatics), there will be truths which cannot be proven from the axioms which generated the system.
So now introduce fuzzy logic, and chaos theory (which is deterministic but not predictable), and so forth, and the old "world-machine" view of 19th century science pretty much goes out the window. Things are much more interesting than we previously thought.
It is also worth mentioning that, during all this scientific uproar, the very foundations of science were also called into question, this time by the work of Thomas Kuhn, a sociologist. If you have not read Kuhn (for instance, On the Structure of Scientific Revolutions), you really should. He is the one who coined the term "paradigm," and described the process he called "paradigm shift." Basically, what Kuhn showed is that the process of choosing between competing scientific theories is not a purely rational process, and indeed it cannot be made purely rational. There is an irreducable element of subjectivity in the process. The reason for this is that it does not go like: theory implies predictions, which can be falsified by experiment, thus falsifying the theory, as was previously thought. Instead, it goes like: theory plus auxilliary assumptions implies predictions, which can be falsified by experiment, but you will never know whether it was the theory or the assumptions which were falsified. Another way to put this is to say that, no matter how much evidence you have against a particular theory, adherants of that theory can always defend it by introducing further assumptions ad infinitum. At some point, however, you just have to say, "this is rediculous, let's get a new theory". This point is the irreducable element of subjectivity. Rational analysis and logic will never tell you when this point has been reached, you just have to make a subjective judgement call for yourself. This realization (and the term "paradigm shift" itself) is just a small part of a much larger paradigm shift which is taking place in all aspects of our culture, as described in Fritjoff Capra's The Turning Point. I will discuss this at greater length in other journal entries.
Basically, what this all means to me is that science, rationality and logic have effectively demonstrated their own limitations. They are still useful tools, but they cannot provide us with a complete understanding of the Universe, as some had hoped that they would. This leads us into religion.
"The Tao which can be named is not the eternal Tao." --Lao Tsu, Tao Te Ching
There exists something which cannot be defined. Lao Tsu called it the Tao, Pirsig called it Quality, Christian Mystics call it God, and it has been called by many other names. I can easily show that all these concepts must refer to the same thing. There can only be one undefinied entity in a logical philosophical system. Suppose that you had two such entities. Then, you must be able to tell me the difference between them. But any difference is a distinction, in other words, you must define for me what the two entities are in order to distinguish them, otherwise I cannot tell the difference. But then you have defined them, so they are not undefined. Therefore, there can only be one undefined entity, so all of these people are talking about the same thing. The position that there is an undefined entity is called "mysticism", so I am a mystic.
I cannot present evidence that this entity exists. It is by nature not subject to proof of this sort. I can only say that I have experienced it directly, and so have you. Pirsig would say that anyone who recognizes the existence of Quality must admit to the existence of the undefined. How can you define Quality? You cannot, and yet you know that it exists, and you recognize it when you see it. The way I see it, experience itself is the same thing as Quality, the Tao, or God. The very forefront of awareness, the crest of the wave of consciousness, is undefinable and unexplainable. It is the awareness of the Universe, seeing itself through you. It is Quality, it is the Tao, it is God.
The first, most fundamental, and most arbitrary distinction we are taught to make is the distinction between ourselves and the rest of the Universe. We draw an imaginary surface at the outer edge of our epidermis (a totally arbitrary surface, which only exists in our minds), and we say that that which is inside this surface is "me", and that which is outside it is not. This distinction, while useful, is arbitrary and illusory. What this means is that "you" are a figment of your own imagination. I am not saying that you do not exist, but rather that, except for in your own mind, you do not exist as a separate entity, distinct from the rest of the Universe. To see this, all you have to do is realize that the rest of the Universe is affecting you all the time, and you are affecting it. You cannot separate yourself from it. You are a part of it, and it a part of you. You are a subsystem of the Whole System, and the dividing line between a system and its subsystems is arbitrary. It exists only in the mind of the observer. If the observer (you) is the subsystem in question, then the observer itself, as a distinct entity, is a figment of its own imagination. Einstein once said something very wise to this effect: "A human being is part of the whole, called by us "universe," limited in time and space. He experiences himself, his thoughts and feelings as something separated from the rest - a kind of optical delusion of his consciousness. This delusion is a prison, restricting us to our personal desires and to affection for a few persons close to us. Our task must be to free ourselves from our prison by widening our circle of compassion to embrace all humanity and the whole of nature in its beauty."
However, we are so deeply conditioned to this illusion that it is very difficult to disillusion ourselves of it. Thus, I feel that the objective of religious practice is reintegration. Reintegration of ourselves with each other, with the rest of the Universe, and with God Itself. The goal is the destruction of all distinctions, including the one which separates you from everything else. This is called becoming One with the Universe (in Zen and other Buddhist practices), and it is not easy. I have had glimpses of it, under the influence of hallucinogens, and it is beautiful and terrifying. Beautiful in a way which cannot be described, and terrifying because to accept it fully would mean the death of what we call the "self". It is not the same as death, of course, but it does mean the death of the "self" as a separate entity, isolated from all the rest.
Once you get past the initial terror, though, the realization is profoundly joyous. You can never be alone, for you are always part of the Whole. The Universe cannot be "out to get you", it actually is you. This leads to the realization of omnibenevolence. God is on our side. It cannot be otherwise, since we are not really separate from It. The separation was an artifact of our consciousness in the first place. This leads to the Christian idea of Salvation, the reuniting of the soul with God. We were never separated in the first place, but we thought we were, which leads us to existential crisis, suffering, and eventual destruction (since the "self" as a separate entity cannot be perminant). Salvation, on the other hand, leads to eternal joy.
This also leads directly into the Great Commandment of Jesus, which is to love God with all your being, and to love your neighbor as yourself. Well, once you realize the arbitraryness of all distinctions, you cannot help but love God and your neighbor, since you realize that there is really no distinction between them and you. You love it all, God and the Universe and other people and creatures and yourself, wholly and unconditionally.
However, this realization is very hard to sustain. In everyday life, we are so deeply enmeshed in the illusion of our separation that we cannot get over it, we take it for granted in so many ways we cannot even begin to unravel them. It forms part of the basic fabric of our existance. This is why we need a consistent, disciplined religious practice, to break us free from the tyranny of illusion. In my view, this should involve some form of meditation, prayer, and a consistent practice of kindness and compassion towards other beings. In time, this practice will help us develop empathy, which is the first step towards full reintegration.