The first paragraph of "Genjo Koan" in Shobogenzo, is as follows.
"When all dharmas are [seen as] the Buddha Dharma, then there is delusion and realization, there are practice and act, there is life and death, and there are buddhas and ordinary people. (1)
When the myriad dharmas are each not of the self, there is no delusion and no realization, no buddhas and no ordinary people, and no life and no death. (2)
The Buddha's truth is originally transcendent over abundance and scarcity, and so there is life and death, there is delusion and realization, and there are ordinary people and buddhas. (3)
And though it is like this, it is only that flowers, while loved, fall; and weeds while hated, flourish. (4)
And by reading them, we can notice that those four stentences are manifesting the four kinds of different thinking methods separately. For example, in the case of (1), it proclaims that when all things and phenomena (all dharma) are considered on the basis of idealistic Buddhist thinking method (Buddha Dharma), there can be the distinctions between delusion and realization, between practice and act, between life and death, and between buddhas and ordinary people.
But in the second case of (2), when we perceive everything on the basis of a perfectly materialistic viewopint ("each are not of the self), there is no distinction between delusion and realization, buddhas and ordinary people, or life and death at all. In other words when we do not have any subjective criteria of meaning, delusion and realization, buddhas and ordinary people, or life and death can not have any difference between them on the basis of material substance alone.
Cases (1) and (2) are representative of Idealistic philosophies and Materialistic philosophies, both belonging to the category of intellectual philosophies, and such, they are very common types of philosophies which are meant to be understood by words. But at the same time, the intellectual philosophy of (1) and (2) are completely, and dimentionally, different from the practical philosophies of (3) and (4).
I think that when we research the many kinds of philosophies in the world, perhaps Buddhism is the only philosophy that includes the two kinds of dimentionally different basis, might be only Buddhism. And I think that only such a kind of philosophy, which includes dimensionally different bases, has been successful at hitting the target of the Truth. In other words, if Buddhism did not have the theory of the four philosophies, not even Buddhism could ever have the power to hit the target of the Truth for the first time in human history. And Gautama Buddha found the Truth in the 5th or 4th Century BC. Someone might insist that the theory of the four philosophies were found by Master Dogen. But thinking about the historical situations, it is impossible for us to think of the existence of Nagarjuna without Gautama Buddha, and it is impossible for us to think about the existence of Master Dogen without the prior existence of Gautama Buddha.
Therefore Master Dogen proclaimed that "The Buddha's truth is originally transcendent over abundance and scarcity," which suggests that the Buddhist Truth transcends the relative comparison of mental strength or material volume, and the expression of (3) belongs to the area of act, or reality. In such situations Master Dogen could proclaim that the real existence can exists really, the real extinction can exist really, the real delusion can exist really, the real enlightenment can exist really, the real common people can exsist really, and the real Buddhas, who has got the Truth, can exist really. And this is the basis of Buddhist Realism.
Therefore in the phase of (4), leaving from the phase of philosophy, the Reality itself is described directly. That is the description of "And though it is like this, it is only that flowers, while loved, fall; and weeds while hated, flourish." This sentence suggests that Reality is just Reality. Furthermore, Master Dogen used the structure of the Four Philosophies in the four nouns, which are arranged by the Four Philosophies, and he arrainged four sentences in the order of the Four Philosophies. And even paragraghs are arranged following the order of Four Philosophies in Shobogenzo. Therefore we have to think that the system of the Four Philosophies is just the structure of the Universe, and it is completely impossible for us to think about the Rule of the Universe without the Four Philosophies.
At the same time even in the Euro-American Civilization, there is a special method of consideration, called Dialectic, in the Modern Age, called the Dialectic. The Dialectic is relevant to the structure of the Four Philosophies, and I think that this historical fact in the Euro-American suggests that even in the Euro-American Civilization, the Age of Realism has already begun.
It is said that when Gautama Buddha wanted to have his first Buddhist lecture, he first planned to have the lecture of the Four Philosophies. But at the same time, he hesitated so much to give the lecture of the Four Philosophies, because the Four Philosophies in Buddhism seemed to be too difficult to understand even to Gautama Buddha himself.
Therefore we, the people in the 21 st Century, having begun to understand the meaning of the Four Philosophies by relying upon the benevolence of Gautama Buddha's benevolence, and relying upon the existence of the Four Philosophies, may experience the very happy condition of the excellent Euro-American Civilization meeting with Buddhism.