This photo was taken December 2005 at Ryoanji, a Zen temple in Kyoto. The little fountain is a tsukubai, a basin for visitors to purify themselves by washing their hands and rinsing their mouths before entering the temple, or before a tea ceremony.
With this particular tsukubai, the square area holding the water represents the individual self, and the circle of the entire tsukubai represents the Absolute (see dharmakaya). On the sides of the basin are four Chinese characters representing "myself": "just" or "only"; "sufficient" or "complete"; and "to know." When put together with the "I" square in the middle, the inscription reads "I only know I am complete and sufficient as I am." This arrangement of characters is attributed to Eihei Dogen.
Photo Credit: datigz / flickr.com, Creative Commons License
Ah, Ryuanji! I visited that place in 2005 when the snow was falling. Very cool. I should dig out some old pictures of the place. Yeah, those tsukubai are found in Japanese Shinto temples too. It’s part of the old (read pre-Buddhist) notion of pollution, and cleansing yourself before entering a temple or sacred ground.