仁王護國般若波羅蜜多經疏 1.37

1.37

解品名者，言序者，因由也，謂說般若起之由致。言品者，類也，文義彙聚各自區分。言第者，次之居也。言一者，數之始也。此經一部總有八品，此品居初故稱第一。

Regarding explaining the chapter name: the word "preface" (xu) means cause and origin, referring to the causes and reasons for the arising of the preaching of prajñā. The word "chapter" (pin) means category, where texts and meanings gather together into distinct divisions. The word "ordinal" (di) is the location of sequence. The word "one" (yi) is the beginning of numbers. This sūtra as a whole has eight chapters in all; this chapter is at the beginning, and therefore it is called the first.