The Scripture of the Mind Seal says, “Once the returning wind mixes together, and all chaos be brought into harmony for hundred days, the work will be achieved.” In general, setting up the foundation requires a hundred days before you have real light. At this point you are still working with the light of the eyes, not the fire of spirit, not the fire of essence, not the torch of wisdom. When you have turned around the light for a hundred days, vital energy will naturally be sufficient for true yang to rise spontaneously, and there is true fire existing in water. If you carry on the practice this way, you will naturally achieve intercourse and formation of the embryo. You are then in the heaven of unknowing, and the child thus develops. If you entertain any conceptual view at all, it is immediately a misleading path.
《心印经》云：“回风混合，百曰功灵。”总之立基百日，方有真光。如子辈尚是目光，非神火也，非性光也，非慧智炬烛也。回之百日，则精气自足，真阳自生，水中自有真火，以此持行，自然交媾，自然结胎，吾方在不识不知之天，而婴儿自成矣。若略作意见，便是外道。
A hundred days of setting up the foundation does not literally mean a hundred days; one day of setting up the foundation does not mean one day. One breath of setting up the foundation does not refer to respiration. The Chinese character for breath (息) is made up of the characters for Self (自) and mind (心). The Self (自) and the mind (心) sustaining it, is the true breath (息), which also refers to the primordial spirit – the original energy and vitality. Rising and descending, parting and joining, all start from the mind; being and non-being, emptiness and fullness, are all in the mind. To realize one true breath, it takes a lifetime’s work, not just a hundred days; however, a-hundred-day is one breath.
百日立基，非百日也。一日立基，非一日也。一息立基，非呼吸之谓也。息者自心也，自心为息，元神也，元气也，元精也。升降离合，悉从心起，有无虚实，咸在念中。一息一生持，何止百日，然百日亦一息也。
A hundred days is merely a way to indicate the importance of dedicated effort. Engaging the true breath during the day, you benefit from it at night. Engaging the true breath at night, you benefit from it during the day. A hundred days of setting up the foundation is a priceless teaching. These sayings of the realized ones all tell the truth of the human body; these sayings of the skillful teachers all speak truthfully to students. This is the mystery of mysteries, which is inexplicable; you will know it once you see your own true essence. Therefore, students must seek the teaching of a true teacher. Everything that emerges naturally from the true essence proves simply true.
百曰只在得力，昼间得力，夜中受用，夜中得力，昼间受用。 百日立基，玉旨耳。上真言语，无不与人身应。真师言语，无不与学人应。此是玄中玄，不可解者也。见性乃知，所以学人，必求真师授记，任性发出，一一皆验。
Buddha Mind 