When Dogen returned to Japan from Song dynasty China in 1227, having trained under a Chinese Chan master and received transmission in what would become the Soto lineage, he did not wait long to put his understanding of practice into writing. The Fukanzazengi, "Universally Recommended Instructions for Zazen," is his statement of the seated meditation he had built his entire teaching around, addressed deliberately to everyone rather than to advanced monastics alone. The word "universally" in its title is doing real work: Dogen wanted this practice available to anyone willing to sit down and try it, not reserved for a monastic elite.
The Tenpuku-bon is Dogen's own handwritten copy of that text, made in 1233 or 1234, a few years into his teaching career and the same period in which he founded the temple that would eventually become Eiheiji. It survives there today, designated a Japanese National Treasure, one of the very few pieces of calligraphy confirmed to be in Dogen's own hand rather than a later copyist's. For a school built explicitly on the authority of direct transmission from teacher to student, having the founder's own instructions for its central practice, in his own handwriting, preserved at the temple he himself established, is about as close to an unbroken thread back to the source as a religious tradition can hold onto.