Philosophical and social backgrounds of the “kyousei” (living together or symbiosis) concept were reviewed. Based on the review, we conclude that the most appropriate and useful perspective is commensalism rather than mutualism, coexistence, and parasitism, for discussing what relationship between human being and environmental water should be. The best condition of water or hydrological cycle cannot be determined scientifically. Therefore, we have to decide a desirable one by mutual consent among the interested stakeholders and to sustain the condition without failure from various points of view. In the process of decision making, hydrology is required to play the following roles: 1) to provide correct and scientific knowledge by making a diagnosis of actual water environment, 2) to suggest and organize the desirable relationship between human being and water through discussion from aspect of Fuudo, and 3) to help sustainable water management in reasonable way to evaluate gains and losses for each stakeholder.
