Yes, as Chuck says. Bowing before Zazen, we are also bowing to those sitting with us in the room (or, in this Sangha, everywhere), to the room ... to all space and time.
That being said, we do not necessarily think anything when bowing ... although I usually feel in my heart that "Great Gratitude" I sometimes mention.
I usually say this ...
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We have some "bowing practice", and discussion of the many ways of seeing and "just bowing" bowing, in preparation for our annual retreat ... where there are times of prostrating (Raihai), done in a series or three (Sanpai) ...
Many Westerners don't care for it, because it is not part of our culture generally. We see it as humiliating, embarrassing, somehow "idol worshipping" or undemocratic. I am often asked to whom or what we are bowing ... Is it to some thing, god, place like Mecca, person or effigy?
I answer by saying that there is nothing that's true that is omitted from our bow. We might consider that we're simply bowing to the whole universe, and to ourself and the other people around us … after all, 'All is One'! The hands, palms upwards, are raised in a gesture traditionally symbolic of lifting the Buddha's feet over one's head, but that truly means lifting all things of the universe over one's head. It's appropriate to cultivate an attitude of emptying, letting go, receptivity and gratitude in our bows.
If there is some physical or personal reason not to prostrate, a simple Gassho can be substituted. However, there is greatness in the humility of the prostration.
No less, are we raising something up or ... seen another way ... is the whole world raising us up at the same time?
The Korean Zen folks are very big with the Prostrations, often recommending
at least 108 each day.
http://london-zen-centre.weebly.com/...strations.html
It is a powerful physical Practice. These days, I usually practice a deep Gassho during our Zazenkai and such. However, I engage in Prostrations also, during our more formal monthly Zazenkai and like times.
Many Tibetans (many Christians pilgrims too) will travel for hundreds of miles, prostrating with each step ...
Gassho, J