The Mysterious Gateway

Free from desire, one observes the miraculous; with desire, one watches the Apperture. These two, though different in terms, share the same source of mystery. Mystery within the mystery is the gateway to all the miracles of the Universe. [tranlsated by the author]

This is the opening passage of Lao Tzu's Tao Te Ching. What Lao Tzu talks about may sound highly metaphysical or mystical. But if one stops looking at Lao Tzu as a philosopher, or progenator of Taoism as a religion, but as a practical teacher of Taoist alchemy, the meanings of these seeminly vague and metaphysical words will start to make sense.

The Apperture refers to the Apperture of the Mysterious Gateway (Xuan Guan in Chinese). Free from desire, the alchemist is in a state of passivity, stillness; he is free from all mental or physical activities. This is the state of deep meditation. Then if the meditation is kept up for long enough, the Chi mecahnism within the alchemist's body begins to stir; activity comes out of complete stillness. But this action is completly natural, non-contrivant; it is not subject to the will of the alchemist. It is a bit like steam comming out of a kettle once the water is brought to boiling temperature. This alchemical process starts from the Apperture of Mysterious Gateway within the body.
But where exactly is the Apperture located, one may asks. The mysterious thing about this appeture is that it doesn't exist as a physical or physiological validity. It only reveals itself within an alchemist who's done the work to purity his mind through meditation. In other words, one cannot seek it through contrivances; it will reveal itself of its own accord when the practitioner is ready. Many later Taoist alchemists describe this apperture as the 'crucible for the elixir of life'. In the practice of Chi Kung and Tai Chi, the same thing is refered to as Dan Tian (meaning 'field of elixir). Again, it is not something one can comprehend purely through the intellect. 

Lao Tze, a great teacher

I've rendered the original words 众妙之门（meaning 'dooway to all the mysteries') into the gateway to all the miracles in the Universe because in Lao Tzu's eyes there is connection between the mystery of the human body and the mystery of the Universe. Such a connection can be found within this mysterious Apperture once it's activated within the alchemist's body through meditation work. To Lao Tzu, the human body is only a microscope of the Universe; all the mysterious of the Universe can be revealved by alchemical practice through the human body.The very impetus that caused the Big Bang to happen is embeded in each human being; that sparkle of light will shine forth and bring us home if we carry out alchemical process within ourselves by staying still (致虚极，守静笃。Push far enough towards the Void; Hold fast enough to Quietness -- Tao Te Ching, chapter 16 as translated by Arthur Waley).

Gateway to the mysteries of the Universe

Lao Tze is no clever thinker or talented philosopher who invented all his metaphysical stuff out of his head. He is a very practical teacher who wrote through his own alchemical experience and dedicated practice. As an enlightened sage, he only spoke out of compassion for the humanity in the hope of showing the way for lifting the humanity out of the wheel of life and death. One cannot possibly feel negative (that's how the world described Lao Tze's attitude toward life) about his life if he sees himself as his own avatar.