The ‘external’ alchemy of the Taoist tradition involved the search for a ‘pill’ or ‘elixir’ of immortality, the most important element of which was a mercury compound (cinnabar).
This passage defines the elixir’s dual historical existence—external (chemical) and internal (meditative)—and establishes how Hakuin’s instruction redirects the immortality quest from physical ingestion to cultivation of ki-energy in the lower tanden.
, Wild Ivy: The Spiritual Autobiography of Zen Master Hakuin, 1999thesis