You will never get to the irreducible definition of anything because you will never be able to explain why you want to explain, and so on. The system will gobble itself up.
The Godhead is never an object of its own knowledge. Just as a knife doesn't cut itself, fire doesn't burn itself, light doesn't illuminate itself. It's always an endless mystery to itself.
Irrevocable commitment to any religion is not only intellectual suicide; it is positive unfaith because it closes the mind to any new vision of the world. Faith is, above all, openness - an act of trust in the unknown.
Essentially Satori is a sudden experience, and it is often described as a "turning over" of the mind, just as a pair of scales will suddenly turn over when a sufficient amount of material has been poured into one pan to overbalance the weight in the other. Hence it is an experience which generally occurs after a long and concentrated effort to discover the meaning of Zen.
What is the next step, the practical application? —I will answer that the absolutely vital thing is to consolidate your understanding, to become capable of enjoyment, of living in the present, and of the discipline which this involves. Without this you have nothing to give.
The clash between science and religion has not shown that religion is false and science is true. It has shown that all systems of definition are relative to various purposes, and that none of them actually “grasp” reality.
That's a waste of time. If you really understand Zen... you can use any book. You could use the Bible. You could use Alice in Wonderland. You could use the dictionary, because... the sound of the rain needs no translation.
For the world is an ever-elusive and ever-disappointing mirage only from the standpoint of someone standing aside from it—as if it were quite other than himself—and then trying to grasp it. But a third response is possible. Not withdrawal, not stewardship on the hypothesis of a future reward, but the fullest collaboration with the world as a harmonious system of contained conflicts—based on the realization that the only real "I" is the whole endless process.
Really, the fundamental, ultimate mystery -- the only thing you need to know to understand the deepest metaphysical secrets -- is this: that for every outside there is an inside and for every inside there is an outside, and although they are different, they go together.