Even when we practice zazen we are not in a state in which thoughts do not appear. Various thoughts float into our mind. However, if we chase after them, though we happen to be sitting in the zazen posture, we are just thinking. At such times you tell yourself, ‘I am practicing zazen, this is not the time to be thinking,’ correct your posture and return to zazen. This is called ‘waking up from absentmindedness.’
There are times when we become sleepy. During these times, return to zazen, telling yourself, ‘I am practicing zazen, this is not the time to be dozing,’ and correct your posture. This is called ‘waking up from darkness.’
Waking up from both absentmindedness and darkness, returning to your practice countless times is zazen. That is to say, shikantaza is arousing the mind of ‘practice-enlightenment’ over and over; it is the meaning of zazen, or the experience of raw life.
Kosho Uchiyama Roshi, in “Zazen That Amounts to Nothing” in the Spring 2011 issue of Buddhadharma: The Practitioner’s Quarterly
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