On the other hand, as with Oryoki eating (a wonderful example of "Kata"), there is a beauty in the fixed form that one literally can lose ones' 'self' in. So, "Kata" is also a
.... an immovable set of rules that govern what is and what is not accepted as acceptable behavior or thinking in japan ... In reality, there are many “Ways” to do most things in Japan, although each group will have a tendency to claim that its pattern is “the Way.” As a medical researcher who has participated in procedures and experiments at many dozens of Japanese hospitals, universities and the like, I know that no two groups ever will follow exactly the same patterns. Each, however, will have a tendency to explain that its way is “the Way,” usually because the most senior person in the group will have come to that conclusion after having learned it to be the thinking of some other person ... that the senior person respects. (Also, one must be very careful in suggesting that a competing group might have a better way which contradicts the opinion of a senior member of group). Every group in every culture does this, but what is unusual in Japan is the inflexible, almost mechanical way the system operates. The emphasis on proper “Kata” (Boye de Menthe has a wonderful, hard to find little book on this) in Japanese society is reminiscent of any conservative, tradition based culture, though unique in the way is has developed to permit a functioning, industrial society.
Instead of worshiping the Lotus Posture, I believe that the position is best which lets us forget about the position (and the rest of the body) during sitting. If one sits as if there was nothing to add or take away from the sit, nothing lacking, no other place to go, as the fulfillment of all time and space in that moment ... then such is
The basic principle is that balance of body is hand-in-glove with balance of mind, one nurturing the other, truly just body-mind. For millenia, the Full Lotus, and to a lessor extent, the Half Lotus have been considered positions of great poise and balance. The Burmese position is also very balanced. The lifting of the rump, straightening of the back with slight curvature of the lower back, the stability of the legs with good circulation, the comfortable head position ... all lend themselves to our forgetting about the body during Zazen. Once mastered, they are intended as incredibly comfortable and stable positions ... not torture
My teacher, Nishijima, is against Seiza and, even more so, chairs. They do not provide such balance in his view, and furthermore, were not the tradition in the Zen schools. Now, the official "Soto" school line (for Westerners, at least) is that chairs, Seiza and Burmese are acceptable ... if not ideal.
Nishijima makes the valid point that many Westerners give up on the Lotus postures for lack of trying, lack of giving it time and stretching. He is right. He may be a little stubborn in not yielding on this issue to people's needs who have legitimate physical issues, and I sometimes think so (this is a very Japanese attitude). But most westerners give up much too easily.
So, you should try many ways and make up your own mind. However, if you are physically capable of Lotus or Half Lotus (or Burmese), that is the best I think.
The philosophy around Treeleaf Sangha about sitting is that everyone should try out for themselves, and adjust, the fine points of sitting Lotus (Full and Half) and Burmese. Seiza and chair sitting, is tolerated as maybe necessary in some cases, if there is a true and uncorrectable physical need.
So, please tinker away with the Full/Half Lotus and Burmese in minor ways. You will know when you are balanced because, quite simply, you will feel balanced, and generally comfortable in sitting for long stretches day after day. The proof is in the pudding.