This is brushed in archaic seal script, a special kind of calligraphy that Mokurai was well known for using. The second half of the couplet is, "Oceans are wide, final home for all rivers." The world (of buddha-nature) is in perfect harmony and all things are in good order, so why should we interfere?
Mokurai ("Silent Thunder") became a monk at an early age and trained under many of the top Zen masters of the day. He became abbot of Kyoto's Kennin-ji, one of the oldest and most important Zen temples, in 1892 at the age of thirty-nine. Noted for his humor and unconventional behavior, Mokurai was one of the most popular Zen masters of the early twentieth century, and he was also a skilled and prolific Zen artist.