Our mind in Zazen may be compared to the sky ... We are open, clear, spacious, boundless, like the clear blue sky ... Our attention is focused on everything and nothing, just as the sky covers all the world without discrimination ... Thoughts, like clouds, may come and go.
Clouds drift in and out, that is natural. However, we bring our attention again and again to the open, blue sky between, allowing the clouds of thought to drift away. More clouds will come, and so we repeat the process endlessly, once more and once more bringing our attention back to the blue sky ... to the open spaces between thoughts. It is no harder than opening the eyes on a summer's day, looking up at the clear blue sky, letting clouds drift in and out ... not having a care in the world, not having a goal to achieve, no other place to be.
However, this is important to bear in mind:
Although we seek to appreciate the blue, open sky between the clouds, we do not resist the clouds of thought that drift through our mind. We are not disturbed by them, we do not actively chase them out, neither do we welcome them, focus on them, play with them or stir them up. We allow them to pass, and return our focus once more to the quiet blue. Again and again.
As in the real sky, both blue expanse and clouds are at home there. We should reject neither, not think the blue somehow "truer" than the clouds. In fact, some days will be very cloudy, some days totally blue ... both are fine. We never say "this cloudy day is not good because there is no blue sky today". When the sky is blue and empty, let it be so. When the sky is cloudy, our mind filled with thoughts, let it be so. You see, even when hidden by clouds, the blue is there all along. Both the blue sky and the clouds are the sky ... do not seek to break up the sky by rejecting any part of it. (In other words, do not think one good and the other bad). Though we reject neither, we allow the clouds to drift from mind and return our attention again and again to the blue. Throughout, we are awake, aware and alert, conscious and present ... we are not in some mysterious or extreme state.
The clouds of thought and the clear blue are not two, are simultaneously functioning and whole ... a single sky. This is our way in 'Just Sitting' Shikantaza Zazen.
Master Dogen called that 'thinking not thinking' or 'non-thinking' ...