The highest vehicle is the ineffable Way of supreme ultimate reality. Here cosmic space is the cauldron, the absolute is the furnace. Clear serenity is the foundation of the elixir pill, nondoing is the matrix of the elixir pill.
Essence and life are the lead and mercury. Concentration and insight are water and fire, controlling desire and anger is the mixing of water and fire. Unification of essence and sense is the combining of metal and wood. Cleaning the mind is bathing. Maintaining sincerity and settling the will is stabilization.
Discipline, concentration, and insight are the three essentials. The centre is the mysterious pass. Clarifying the mind is miraculous experience. Seeing the essence of mind is crystallisation. Merging of the three bases into one is the spiritual embryo. Unification of essence and life is the completion of the elixir pill. Having a body outside the body is release from the matrix. Breaking through space is perfect attainment.
The subtlety of this supreme vehicle can be practiced by the most developed people. When accomplishment is fulfilled, character is well-developed and one directly transcends to completion all at once; physically and spiritually sublimated, one merges with the Tao in reality.
–Thomas Cleary, The Book of Balance and Harmony