It is said that when Gautama Buddha had his first Buddhist lecture at Mrgadava, he preached the Theory of Four Philosopies and the Eight Noble Truth. And among them I think the Theory of Four Philosophies are very important, because the Theory of Four Philosophies are fundamental basis of Gautama Buddha's Buddhist thoughts.
The reason, why the Theory of Four Philosophies are so important, comes from that Buddhism is just Realistic Philosophy. If Buddhism were not Realistic Philosophy, it is not necessary for us to explain Buddhism on the basis of the four philosophies. Because if Buddhism were not realistic philosophy, it is never necessary for Buddhism to be explained by four philosophies. But in the case of Buddhism it is just the Realistic Philosophy, and so if we want to explain Buddhism utilizing only one kind of philosophy, it is perfectly impossible for us to clarify the meaning of Buddhism at all. Because the Reality itself is the very inclusive and complicated philosophy. In other words Reality seems to be limitless, and it is also very complicated. Therefore if we want to realize the true meaning of Reality utilizing only one simple philosophical basis, it might be completely impossible for us to clarify such a kind of Realistic Buddhism on the basis of only one philosophy.
Therefore Gautama Buddha found that if we want to explain what this world is, or what this Universe is, it is necessary for us to utilize the four kinds of philosophies one by one. The four philosophies are (1) Philosophy of Pain, or Duhkha-satya, (2) Philosophy of Accumulation, or Samudaya-satya, (3) Philosophy of Self-Regulation, or Nirodha-satya, and (4) Philosophy of Morals, or Marga-satya.
(1) The Philosophy of Pain is another name of Idealism. Because idea always suggests perfect situation of each idea, but such kind of perfect situation of idea does never exist in the real world, and so if we pursue such a kind of perfect idea, we are alway disapointed by the real facts that idea can never be realised on the earth at all. Therefore when we have found the real situation that idealistic philosophies are always pursuing the ideal or perfect situation of concepts, which can never be realized on the earth at all. And so idealism has been repreasented by the name of "philosophy of pain" in the Ancient India.
(2) The Philosophy of Accumulation can be interpreted as Materialistic philosophy today. Because in Ancient India there had been Materialistic Philosophers, who were called the Six Non-Buddhist Thinkers a little before Gautama Buddha's had been born, and they denied the real existence of idea, but they believed in the real existence of Matter. They believed in the real existence of Matter as accumulation of molecules, and so we can interprete that the Six Non-Buddhist Thinkers might be Materialistic philosophers, who were thinking that Matter might be accumulation of molecules in Ancient India.
(3) But in the case of Gautama Buddha, he noticed that Idealism is just a philosophy, which has been considered by only human brain, and so Idealism can never be the Truth.
And at the same time Materialism has been produced by relying upon human perception, and so whether Materialism, in which we have believed the real existence of Matter, might be very unreliable, because whether it might be not sure that what we perceived actually existing, or not.
Therefore Gautama Buddha denied the real existence of both idea, and matter, and he insisted the existence of the Real Action at the present moment. He thought that it was not sure whether idea really exists, or not. Because the existence of idea might be only motion of our brain cells in our Brain, and so the real existence of idea might be doubtful.
However at the same time he doubted whether Matter really existed on the earth, or not. Because Matter were just sense stimuli on the sense organs.
And because of such a reason Gautama Buddha established the Philosophy of Action as the third philosophy of Buddhism. He thought that both Idea and Matter could never be Real Entities at all, and the Real Entity must be our Human Action at the present moment.
And furthermore he thought that our Action at the present moment was just the same as the Universe itself.
(4) Therefore we can say that our balanced Action at the present moment is just the Truth, and the Universe at the present moment is also just the Truth. In other words we are just acting as the Truth at the present moment, and the whole Universe is also moving as the Truth at the present moment.
And by practicing Zazen when we can make our ANS balanced, and then we can realize just the Truth at the present moment, and so we can do Action at the present moment, and we have become the Truth itself. And furthermore the Whole Universe can become the Truth, and so the Reality itself is just the Truth actually.