There is no getting around it: for achieving the initial entrance into satori [the experience of awakening], nothing can excel a direct and expeditious assault fired by intense, vigorous, urgent desire. People who engage in practice a little bit at a time when the thought occurs to them will not achieve kensho [self-realization] even if they continue doing it for thirty or forty years. As time passes, their efforts physically exhaust them, drain them of the necessary spirit and strength they need to subdue the illusory, passion-ridden thoughts that crowd into their minds. In the end, they are reduced to fingering rosaries and tearfully reciting Nembutsu, a pastime that brings them no more relief than one of those ready-made toothache medicines sold in the streets.
From Beating the Cloth Drum: Letters of Zen Master Hakuin, translated by Norman Waddell, page 73.