成唯識論了義燈 2.27

2.27

諸門分別中，一多分別者，四多五一，以我及意，依一人說名之為一。若約多人即多數故，空、時、方三一切共有，更無多體名之為一。

Among the distinctions across various categories, in the distinction of one and many: four are many and five are one. Because self and manas are spoken of as one based on a single person, but when considered with respect to many persons, they are of multiple number; the three of space, time, and direction are common to all and have no further multiple essences, hence they are named 'one'.