Much of eastern spiritualism and zen involves the idea that reality doesn't really exist. Its all a joke, and nothing really matters. We are but insiginifigant motes of cosmic dust. I need not go into exaustive detail with this, read some Ramm Dass or your favority book of Zen Koans. Realization of nothingness is important as the first foundation of being. Before there was man, before there was night and day, before there was light, there was darkness.
More applicably, in the cognitive sense, nothing you know can be known for certain as true. There is no truth. Everything you know is only known as true by relation to other things that are known. Concepts exist because of other concepts to relate to. Each is a collection of smaller concepts and ideas that make up the larger idea. Each of these concepts in turn contain other smaller ideas. Break it down.
What is the smallest idea that can be held? If every idea is a combination of other ideas, then what basis do we have for our own thinking? Computers have bits. What do humans have? We have sensory input. The lowest level knowlege we can know is to translate external input to a name. This color is blue. This feeling is heat. And these, ultimately, are arbuitrary names given to random stimuli.
Choose something you believe is true. Why do you believe this is so? If it is just because you do, then there is no basis for this belief at all. There is no reason to support that it is true, other than your choice that it is. That is not sufficient for this examination. We are looking for REAL truth. Is there any to be found?
If there are reasons you believe in this thing, examine each of them. Each reason is a belief in and of itself. Each reason then must be questioned in the same manner. Why is A so? Because of B and C. Why is B so? Because of D and E and so on. Eventually, one reaches a point where the question of why cannot be answered.
The lowest reasons are only, "that it is observed to be so." At the bottom, we do not have proof, only observations. All the laws fo physics and science are based on carefull observation and theory. Laws are only tested theories to explain observation. We have no actual proof that we are correct, and we are often proven wrong. New observations challenge existing systems. Mechanics give way to Relativity, gives way to Quantum Mechanics. Even the most sure that we know is only true until proven false. True only until observed as otherwise.
An example of deconstruction: I believe that one should not put one's hand on the stove. This is a silly rule. Why is it so? Well, because if you put your hand on the stove, it will be burned. Why is that? Because the stove is hot? Is the stove hot? How do you know the stove is hot? Because if you put your hand on the stove, you will observe that the stove is hot. Why? Because the nerves in your hand will detect the heat, and transfer that information to your brain where the impulses are translated into knowlege. An excess of stimulation to that degree means that if not removed, the flesh will be damaged. Reflexive actions are called to remove the hand from danger.
Ok, that's all well and good, but how do I know that the nerves in my hand are accurately telling me that the stove is hot enough to burn? Well, you don't. That's where you must assume that they are correct. At the bottom of every tree is an asumption based on observation. In this case, the assumption is that your nerves are able to correctly detect heat. Your senses do not lie, do they?
Lets jump to the meaty example you know this is leading up to: I believe in God. Why? Because I was told it was so. Why? It is written in the bible. What is the bible? A book. Who wrote this book? Long ago, men, though they were translating the words of God. How do you know these were the words of God and not just the words of men told to be the words of God? Well, you don't. You have to take that on faith. You have to assume it to be true.
But I've expierenced spiritual things! I've seen magic work! I know God and spirituality is real! Ok, on what do you base these things? Your expierences? How do you know that what you have percieved was real spirituality or magic and not just the fancy of your own subconscious? What proof do you have to show for this? Is there in fact scientificly documented observations from multiple reliable sources that can be verified as real occurances that cannot be explained by the current theories we have? Well, no.
Csn you prove the existance of the soul, of spirit? Is there any tangiable evidence to show that this exists? You've been told it exists, but how do the people who told you know? What basis is there for this belief? What observation was made that an independent existance is perpetuated after death? Well? None. The best we have is Jesus, and the only recorded history we can really rely on for that is the words of an impartial roman historian at the time: "There was a holy man named Jesus. He was put to death by crucification. Some claim to have seen him live after this event."
Ultimately, even the best "proof" of the existance of the soul, the afterlife, and God is still taken on faith. Unless you have seen him youself, all that you know is that the people who wrote the gosples claimed to have seen him. There is no proof, there is no evidence. We believe because we assume it to be true. Even if you did see him, can you know he was real and not the product of your imagination? Can you trust your senses, you creature who enters into dreams and worlds which do not exist when deprived of your normal waking senses? You creature, who can dream while awake, can you be sure this vision was not a dream?
Ultimately, we know nothing. Even the highest things we believe as true with all our hearts are still only believed as true. We can't prove it. They are based on an assumption. Perhaps that assumption is that your senses can be trusted, or that someone else's senses can be trusted, or that the person who told you this has reliable evidence to back it up. Of course, unless you can see this evidence, you can never know for sure.
Doubt is the engine of the destruction of all ideas. Held up to examination, nothing can survive it. Eventually, all things fall to a reliance on sensory input. Nothing can be proved ultimately as true. There is nothing in our world, even the best of our science, that is ultimately certainly true beyond a shadow of a doubt.
There is no truth. There is nothing. You know nothing. Nothing can be trusted. Everything is subject to question, doubt, and deconstruction. You will never know for certain if anything is true. Nothing has ever been proven as independently ultimately and objectively true.
We are lost in the dark, in a room without walls. We will never know where we are, what we are, or who we are. If we let go of everything we cannot be sure of, we have nothing. We are nothing. There is nothing. This is the only thing that can ultiately be known: That we don't know.
What is below 1? Zero. What was your name before you were born? What was before the Big Bang? In the beginning there was the Word and the Word was with God and the Word was God. But what was before that?
There is no truth. This is the first and only thing that is True. This is the first foundation.