坐禪三昧經 1.58

1.58

He answers: "Because the other methods of contemplation are loose and difficult to lose, whereas the method of counting the breath is tight and easily shifts away. It is like tending cattle: because cattle are difficult to lose, guarding them involves little trouble; whereas tending a monkey is easy to lose, so guarding it involves much trouble. This is just like that! When counting breaths, the mind's counting must not turn to other thoughts even for a brief moment; if it turns to other thoughts for a brief moment, the count is lost. For this reason, to initially sever discursive thoughts, one should count the breath."