In the opening pages of the work, Abe interprets a well-known Zen saying: Before one studies Zen, mountains are mountains and rivers are rivers; after one attains some insight, mountains are not mountains and rivers are not rivers; and when one becomes enlightened, mountains are really mountains and rivers are really rivers. For Abe, this short passage elucidates Mahayana Buddhism’s central doctrine of absolute nothingness, or emptiness (shunyata). Abe’s commentary on this passage explains the philosophical basis from which he compares Zen with Western thought and criticizes Western philosophy and theology.
Abe explains that before studying Zen, an individual affirms the existence of mountains and rivers, differentiates between them, and, most significantly, objectifies them—mountains are mountains and rivers are rivers. Objectification entails the positing of mountains and rivers as realities external to the internal subject, which Abe terms the ego-self. From the Zen standpoint, this duality between the subject (ego-self) and object (in this case, mountains and rivers) obstructs the realization of one’s true self, or Buddha-nature. To inquire into the self in an objectified way throws the questioner into an infinite regression of subjects and objects. When one asks, “Who am I?” or “Who sees these mountains and rivers?” the very subject that perceives objects becomes an object. One can take this further by inquiring, “Who is asking these questions?” ad infinitum and never arrive at an awakening to the self, one’s Buddha-nature.
Release from this infinite regression entails a full existential realization that Buddha-nature is beyond the realm of objectifying thought, which frees one from the dichotomy of subject and object. From the Zen standpoint, this emptying of duality is the perception of nondifferentiation. The second phrase in the saying—mountains are not mountains and rivers are not rivers—refers to the realization that comes after the student of Zen gains some insight.
A subtle duality, however, still exists in this stage of nondifferentiation: a division between the state of differentiation and nondifferentiation. The realization of Buddha-nature requires a further negation, the negation of nondifferentiation. Abe calls this negation of negation absolute negation and contends that this absolute negation is, logically, an absolute affirmation. This paradox of the oneness of affirmation and negation, or what Abe calls the logic of is/is not, is the essence of Buddha-nature.