Those interested in the study of neidan practice may have come across the term, pure yang or pure light in Daoist texts. The neidan adept having filled his or her body with pure yang has probably attained enlightenment.
Enlightenment not in the sense of some ‘loose’ teachings or wordings whereby disciples were enlightened after thinking through difficult koans or getting whacked by a cane or a stick, but a state attained through an extended and continual practice of dual cultivation. (Tao Te Ching, Dhammapada, Twofold entrance to Tao)
The person enlightened through dual cultivation would probably have attained Immortality or Buddhahood. According to Buddha in the Leng Yen, the immortal still needs to continuously cultivate before he reaches higher stages of immortality or Nirvana.
How do neidan adepts reach this state of pure yang or pure light? One can perhaps use simple arithmetic to explain the process.
To achieve purity cultivators need to cultivate virtues to rectify the heart/mind (Hsin) to become good again before attaining a pure state of mind. To reach a yang state the Circulation of the Light meditation allows the light and energy (Qi) to circulate the entire body to melt down the slag or yin state within. (Secret of the Golden Flower) Once yin or darkness has been overcome, only yang or light remains within.
Therefore the arithmetic adds up that through continual dual cultivation, the neidan adept can perhaps reach this state of pure yang or pure light.
Although the analysis sounds simple, the practice is not. It takes a long time and therefore a far journey. According to later Daoist texts, we have to be the right person(s) to attain Tao.