摩訶止觀 4.15

4.15

不缺戒者，即是持於性戒乃至四重，清淨守護如愛明珠。若毀犯者，如器已缺，無所堪用；佛法邊人，非沙門釋子，失比丘法，故稱為「缺」。

The unblemished precept means upholding natural morality up to the four grave offenses (pārājika), purely guarding them as one would cherish a bright pearl. If one violates them, one is like a vessel that is already broken and of no use; one is on the periphery of the Buddha's Dharma, not a śramaṇa son of the Śākya, and has lost the status of a monk; hence it is called 'blemished'.