瑜伽論記 5.129

5.129

又有四種為欲住心乃至觀察諸法者，此解四神足，義有兩重：一以定體為神，作用難測故，欲等為因生定名足，約彼四足取所生定，第二依定功德名神，難測量故，欲勤等四所生定體與彼所生功德為足。

Furthermore, regarding '[there are four kinds] for the sake of causing the mind to abide... up to examining dharmas': this explains the four bases of psychic power. There are two layers of meaning: First, the essence of concentration is taken as the psychic [power], because its functioning is difficult to fathom; zeal and so forth are causes that produce concentration, hence they are called the bases—taking the concentration produced with reference to those four bases. Second, the merits that rely on concentration are called psychic [power], because they are difficult to measure; the essence of concentration produced by the four, namely zeal, diligence, and so forth, serves as the base for those produced merits.