¶ Go-shintai
Shintai is an object of worship in which the spirit of a deity is believed to reside. A symbol or medium of the spirit of a deity. Go-shintai is a sacred shintai (go- is a honorific).
¶ Shintai in Wikipedia
In spite of what their name may suggest, shintai are not themselves part of kami, but rather just temporary repositories which make them accessible to human beings for worship. Shintai are also of necessity yorishiro, that is objects by their very nature capable of attracting kami.
The most common shintai are man-made objects like mirrors, swords, jewels (for example comma-shaped stones called magatama), gohei (wands used during religious rites), and sculptures of kami called shinzō (神像), but they can be also natural objects such as rocks (shinishi, 神石), mountains (shintai-zan, 神体山), trees (shinboku, 神木), and waterfalls (shintaki, 神滝) Before the forcible separation of kami and Buddhas of 1868 (shinbutsu bunri) a shintai could even be the statue of a Buddhist deity.
The founding of a new shrine requires the presence of either a pre-existing, naturally occurring shintai (for example a rock or waterfall housing a local kami), or of an artificial one, which must therefore be procured or made to the purpose.
See more in Wikipedia.
¶ Goshintai in Takuan
Goshintai is mentioned in the second book of Takuan’s adventures, where Soliang the monk arrives to the Blue Mountain Monastery to locate some magical implements which Xiwanmu, the Goddess of the West, commandeered him to find.
He comes too late, as the implements have already been stolen, but let’s not get ahead of ourselves. Here is a piece of evidence of the theft:
“I would like to see this sacred relic,” continued the monk. “At least to have a single glimpse!”
The abbot did not refuse the monk, and personally escorted him to the goshintai shrine, where a necklace was hung in a conspicuous place, gleaming in the light of oil lamps.
“What’s there?” the monk asked, looking at the bulbs of thin tea glass laid out in a honeycomb-like pyramid.
“This is another sacred relic,” the abbot said. “It was given to us by the Ruler of the Under Realm, Yanwang Umma-ö, when he visited the monastery at the very dawn of time.”