Question Six:

Q. [Someone asks], Among the four different postures (walking, standing, sitting, lying down), why does Buddhism encourage entering realization through meditation only in sitting?

A. It is not possible to thoroughly comprehend the path by which the buddhas from the past, one after another, have been practicing and entering realization. If you seek a reason, you must know that it is only because [sitting] is what has been used by Buddhist practitioners, and beyond this you do not need to search. However, the Ancestor [Nagarjuna] praises it, saying “Zazen is the dharma gate of ease and delight (nirvana).” So, couldn’t we infer that it is because [sitting] is the most stable and peaceful of the four postures? Moreover, this is not the path of practice of one or two buddhas, but all buddhas and ancestors follow this path.

From: Talk on the Wholehearted Practice of the Way -Kosho Uchiyama (with Shohaku Okumura, Taigen Daniel Leighton)