俱舍論記 4.31

4.31

「論曰至且有五品」者，心之所有故名心所。應言心所有，略故但言心所，猶如我所。於心所中，廣即更有不定等法，略而言之故言且五。

"The treatise says ... for now there are five categories": that which belongs to mind (citta) is therefore called a mental factor (caitta). One should say "belonging to mind" (citta-saṃprayukta / cittasya bhāva); being abbreviated, it is simply called mental factor (caitta), just like "belonging to self" (ātmīya). Among mental factors, when expanded there are furthermore indeterminate and other dharmas; spoken of in brief, it therefore says "for now there are five".