What are the perfect interpenetration of principle and phenomena, and the perfect interpenetration of phenomena with phenomena?

The highest realm of Buddhist practice and realization is the Buddha's perfect integration of principle and phenomena, and the perfect integration of all phenomena—where all phenomena are unobstructed and all accord with principle; principle can manifest phenomena, and phenomena can manifest principle, with principle and phenomena never falling out of step. To achieve this, one must begin from the very first seeing of the Way in Buddhist practice and cultivation, and end with the final seeing of the Way and attainment of buddhahood. The Way seen gradually deepens from beginning to end—that is, the principle seen becomes ever more profound and subtle, and phenomena likewise become ever more perfectly integrated and complete. From the moment of the first seeing of the Way, phenomena follow principle in their conduct: wherever principle goes, phenomena follow, never falling out of step, until the Buddha's final seeing of the Way—awakening upon seeing the morning star—when principle is perfected and all phenomena are instantly perfected along with it. Neither principle nor phenomena need any further cultivation, and He becomes the supreme and venerable Dharma King.

Seeing the Way is divided into seeing it in principle and seeing it in phenomena. What is the difference between seeing in principle and seeing in phenomena? Seeing in principle is going from principle to principle: on the level of the mind consciousness, one fully understands and clearly comprehends the principle, but phenomena do not follow along—at this time phenomena and principle are out of step. Because the manas has not seen it, the samādhi state in which meditative concentration and wisdom are complete and perfectly integrated does not appear, and phenomena do not follow along with it. Phenomena are controlled by the manas; the mind consciousness cannot make phenomena follow along and be perfectly integrated.

As for seeing the Way in phenomena, this is investigating and realizing principle through phenomena. When one sees the Way in this way, phenomena necessarily accord with principle, phenomena necessarily follow principle, and principle and phenomena can necessarily merge together without falling out of step. Only from this point can one be transformed root and branch, with the mind washed clean and the countenance renewed. Outwardly, the person is still the same person, but in terms of mental conduct, the old ways of acting are no longer present; one's thinking and character are completely renewed, the mind is empty of self, the conditioned manifests the unconditioned, and the unconditioned in turn serves the conditioned—empty yet not empty, not empty yet empty.

Therefore, true seeing of the Way is seeing and entering into principle after thorough investigation through phenomena—it is the realization of principle in which phenomena follow principle and phenomena manifest principle, with principle and phenomena undivided and meditative absorption (samādhi) following along. When principle and phenomena merge, they are not necessarily perfectly integrated, nor necessarily unobstructed, because the principle is not yet thorough; when the principle is thorough, one becomes a Buddha. Seeing principle through phenomena is realization; if one does not see principle through phenomena, it is not realization. If principle remains merely principle and does not merge with phenomena, that principle cannot resolve matters; a principle that cannot resolve matters has no practical use. Therefore, merely knowing principle, without seeing and manifesting principle through phenomena, is not genuine realization.