Mohe Zhiguan did not begin as a book. It began as a series of lectures, delivered across a single summer retreat in the year 594 at the Temple of the Jade Spring in Jingzhou Prefecture, where the monk Zhiyi spent the season teaching a systematic method of meditation and doctrine he had spent decades developing. Nobody in that room was writing a school's founding scripture on purpose. They were attending a teaching retreat.
What turned the lectures into a text was the work of one listener in particular. Zhiyi's disciple Guanding took notes across the retreat and, afterward, organized and compiled them into a finished written work spanning seven chapters across ten fascicles, a task that required real editorial judgment about how to render a live, responsive teaching session into a coherent, readable treatise. Without Guanding's compilation, Zhiyi's 594 lectures would likely have survived, if at all, only as fragments or secondhand summaries, the way most oral teaching from this period has come down to us.
The result stands today as one of Tiantai Buddhism's Three Great Works, alongside Zhiyi's Fahua Xuanyi and his line by line commentary on the Lotus Sutra, the Fahua Wenju. It is a useful reminder that a school's foundational text is not always written by its founder sitting alone at a desk. Often enough, it is built jointly, a teacher's spoken words and a student's careful record of them, neither one sufficient without the other.